2018 ACC/HRS/NASCI/SCAI/SCCT Expert Consensus Document … · Physicians who either order or conduct such proced-ures need to: 1. Know the magnitude of a patient’s risk associated

J O U R N A L O F T H E A M E R I C A N C O L L E G E O F C A R D I O L O G Y V O L . 7 1 , N O . 2 4 , 2 0 1 8

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P U B L I S H E D B Y E L S E V I E R

EXPERT CONSENSUS DOCUMENT

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2018 ACC/HRS/NASCI/SCAI/SCCTExpert Consensus Document onOptimal Use of Ionizing Radiationin Cardiovascular Imaging:
Best Practices for Safety and Effectiveness A Report of the American College of Cardiology Task Force onExpert Consensus Decision Pathways
Developed in Collaboration With Mended Hearts

Writing John W. Hirshfeld, JR, MD, FACC, FSCAI, Chair
CommitteeMembers
Victor A. Ferrari, MD, FACC, Co-Chair

Frank M. Bengel, MD*Lisa Bergersen, MD, MPH, FACCCharles E. Chambers, MD, FACC, MSCAIyAndrew J. Einstein, MD, PHD, FACCMark J. Eisenberg, MD, MPH, FACCMark A. Fogel, MD, FACCThomas C. Gerber, MD, FACCDavid E. Haines, MD, FACCzWarren K. Laskey, MD, MPH, FACC, FSCAIMarian C. Limacher, MD, FACCKenneth J. Nichols, PHDxDaniel A. Pryma, MDGilbert L. Raff, MD, FACCk

0

This document was approved by the American College of Cardiology Clinica

of the Heart Rhythm Society (HRS), North American Society for Cardiova

terventions (SCAI), and Society of Cardiovascular Computed Tomography (S

The American College of Cardiology Foundation requests that this docume

Chambers CE, Einstein AJ, Eisenberg MJ, Fogel MA, Gerber TC, Haines DE, La

Stillman AE, Thomas SA, Tsai TT, Wagner LK, Wann LS. 2018 ACC/HRS/N

radiation in cardiovascular imaging: best practices for safety and effectiven

Expert Consensus Documents. J Am Coll Cardiol 2018;71:e283–351.

Copies: This document is available on the World Wide Web site of the Am

please contact Elsevier Reprint Department via fax (212) 633-3820 or e-mail

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business/policies/copyright/permissions).

Geoffrey D. Rubin, MD, MBA, FNASCI{Donnette Smith,#Arthur E. Stillman, MD, PHD, FNASCISuma A. Thomas, MD, MBA, FACCThomas T. Tsai, MD, MSC, FACCLouis K. Wagner, PHDL. Samuel Wann, MD, MACC

*Society of Nuclear Medicine and Molecular Imaging Representative.

ySociety for Cardiovascular Angiography and Interventions

Representative. zHeart Rhythm Society Representative. xAmerican Society

of Nuclear Cardiology Representative. kSociety for Cardiovascular

Computed Tomography Representative. {North American Society for

Cardiovascular Imaging Representative. #Mended Hearts Representative.

Authors with no symbol by their name were included to provide

additional content expertise.

https://doi.org/10.1016/j.jacc.2018.02.016

l Policy Approval Committee in November 2017 and by the approval bodies

scular Imaging (NASCI), Society for Cardiovascular Angiography and In-

CCT) in January 2018.

nt be cited as follows: Hirshfeld JW Jr, Ferrari VA, Bengel FM, Bergersen L,

skeyWK, Limacher MC, Nichols KJ, Pryma DA, Raff GL, Rubin GD, Smith D,

ASCI/SCAI/SCCT expert consensus document on optimal use of ionizing

ess: a report of the American College of Cardiology Task Force on Clinical

erican College of Cardiology (www.acc.org). For copies of this document,

([email protected]).

/or distribution of this document are not permitted without the express

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Hirshfeld, Jr. et al. J A C C V O L . 7 1 , N O . 2 4 , 2 0 1 8

ECD on Optimal Use of Ionizing Radiation in CV Imaging J U N E 1 9 , 2 0 1 8 : e 2 8 3 – 3 5 1

e284

ACC Task Forceon ExpertConsensusDecisionPathways**

Adrian F. Hernandez, MD, MHS, FACCWilliam Hucker, MD, PHD

James L. Januzzi, JR, MD, FACCLuis C. Afonso, MBBS, FACCBrendan Everett, MD, FACC

Hani Jneid, MD, FACCDharam Kumbhani, MD, SM, FACCJoseph Edward Marine, MD, FACC

Pamela Bowe Morris, MD, FACCRobert N. Piana, MD, FACCKarol E. Watson, MD, FACCBarbara S. Wiggins, PHARMD, AACC

**Formerly named ACC Task Force on Clinical Expert Consensus

Documents.

TABLE OF CONTENTS

PREAMBLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e286

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . e287

1.1. Document Development Process andMethodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . e287

1.1.1. Writing Committee Organization . . . . . . . . e287

1.1.2. Document Development and Approval . . . e287

2. PURPOSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e288

2.1. Document Purpose . . . . . . . . . . . . . . . . . . . . . . . e288

2.2. The Radiation Safety Issue . . . . . . . . . . . . . . . . e288

2.3. The Need for Physician Radiation SafetyEducation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e288

2.4. Appropriateness of Medical Radiation . . . . . . . e288

3. CURRENT TRENDS IN PATIENT AND MEDICAL

PERSONNEL RADIATION EXPOSURE FROM

CARDIOVASCULAR PROCEDURES . . . . . . . . . . . . e289

3.1. Trends in Patient and Medical PersonnelRadiation Exposure . . . . . . . . . . . . . . . . . . . . . . . e289

3.2. Potential Consequences of Patient andMedical Personnel Radiation Exposure . . . . . . . e290

4. THE MANY MEASURES OF RADIATION . . . . . . . . e291

4.1. Radiation Exposure and Dose Metrics . . . . . . . e291

4.2. Challenges in Relating Radiation Exposure andDose to Risk of Detrimental Effects . . . . . . . . . e292

4.3. Types of Ionizing Radiation Used inMedical Imaging . . . . . . . . . . . . . . . . . . . . . . . . . e292

4.3.1. X-Rays and Gamma Rays . . . . . . . . . . . . e292

4.3.2. Positrons . . . . . . . . . . . . . . . . . . . . . . . . . . e293

4.4. Relationships Between Exposure andAbsorbed Dose . . . . . . . . . . . . . . . . . . . . . . . . . . e293

4.4.1. Exposure From External Beams . . . . . . . e293

4.4.2. Exposure From Radionuclides . . . . . . . . e294

4.5. Estimating Effective Dose . . . . . . . . . . . . . . . . . e294

4.6. Synopsis of Measures of Radiation Exposureand Dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e295

5. HOW RADIATION CAN HARM PEOPLE . . . . . . . . . e295

5.1. Mechanism of Radiation-InducedBiological Effects . . . . . . . . . . . . . . . . . . . . . . . . e295

5.2. Types of Radiation-Induced Health Effects . . . e295

5.2.1. Tissue Reactions (Formerly CalledDeterministic Effects) . . . . . . . . . . . . . . . . e295

5.2.2. Stochastic Effects: Cancer . . . . . . . . . . . . e296

5.2.3. Stochastic Effects: Heritable Effectsin Offspring . . . . . . . . . . . . . . . . . . . . . . . . e297

5.3. Tissue Reactions: Dose-Effect Relationships . . e297

5.3.1. Skin Injury . . . . . . . . . . . . . . . . . . . . . . . . e297

5.3.2. Bone Injury . . . . . . . . . . . . . . . . . . . . . . . . e298

5.3.3. Eye Injury: Cataracts . . . . . . . . . . . . . . . . e298

5.3.4. Tissue Reactions: Managing SkinInjuries . . . . . . . . . . . . . . . . . . . . . . . . . . . e299

5.4. Stochastic Effects: Radiation-Induced Cancer . e299

5.4.1. Stochastic Effects: AttributionChallenges . . . . . . . . . . . . . . . . . . . . . . . . e299

5.4.2. Stochastic Effects: Risk Metrics . . . . . . . e299

5.4.3. Stochastic Risk: Dose-RiskRelationships . . . . . . . . . . . . . . . . . . . . . . e300

5.4.4. Incremental Cancer Risk Attributable toRadiation Exposure for OccupationallyExposed Healthcare Workers . . . . . . . . . e303

6. MODALITY-SPECIFIC RADIATION EXPOSURE

DELIVERY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e306

6.1. General Principles . . . . . . . . . . . . . . . . . . . . . . . . e306

6.1.1. Characteristics of Medical DiagnosticRadiation . . . . . . . . . . . . . . . . . . . . . . . . . . e306

6.1.2. Tools Used to Estimate Absorbed Dose . e306

6.2. X-Ray Fluoroscopy . . . . . . . . . . . . . . . . . . . . . . . e306

6.2.1. X-Ray Fluoroscopy Subject andOperator Dose Issues . . . . . . . . . . . . . . . . e306

6.2.2. Basics of Operation of an X-RayCinefluorographic Unit . . . . . . . . . . . . . . e307

J A C C V O L . 7 1 , N O . 2 4 , 2 0 1 8 Hirshfeld, Jr. et al.J U N E 1 9 , 2 0 1 8 : e 2 8 3 – 3 5 1 ECD on Optimal Use of Ionizing Radiation in CV Imaging

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6.2.3. Measures and Determinants of Subject andOperator Exposure . . . . . . . . . . . . . . . . . . e308

6.2.4. Measures and Determinants of PhysicianOperator and Healthcare WorkerOccupational Exposure . . . . . . . . . . . . . . e310

6.3. X-Ray CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e311

6.3.1. X-Ray CT Subject and Operator DoseIssues . . . . . . . . . . . . . . . . . . . . . . . . . . . . e311

6.3.2. Basics of Operation of an X-Ray CT Unit . . e311

6.3.3. X-Ray CT Measures of SubjectExposure . . . . . . . . . . . . . . . . . . . . . . . . . . e312

6.3.4. X-Ray CT Measures of Effective Dose . . e313

6.3.5. X-Ray CT Dose Alert Monitoring . . . . . . e314

6.4. Patient and Personnel Exposure in NuclearCardiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e314

6.4.1. Patient Exposure in NuclearCardiology . . . . . . . . . . . . . . . . . . . . . . . . e314

6.4.2. Personnel Exposure in NuclearCardiology . . . . . . . . . . . . . . . . . . . . . . . . e316

7. MODALITY-SPECIFIC DOSE REDUCTION

STRATEGIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e316

7.1. General Principles . . . . . . . . . . . . . . . . . . . . . . . . e316

7.1.1. Case Selection . . . . . . . . . . . . . . . . . . . . . . e317

7.1.2. Dose-Determining Variables . . . . . . . . . . . e317

7.1.3. Image Quality Issues . . . . . . . . . . . . . . . . e317


7.2.1. General Principles . . . . . . . . . . . . . . . . . . e319

7.2.2. Digital X-Ray System OperatingModes . . . . . . . . . . . . . . . . . . . . . . . . . . . . e319

7.2.3. X-Ray System Calibration, Operation,and Dose . . . . . . . . . . . . . . . . . . . . . . . . . . e319

7.2.4. Determinants of Total Dose for anExposure . . . . . . . . . . . . . . . . . . . . . . . . . . e320

7.2.5. Procedures and Practices to MinimizePatient and Personnel Exposure . . . . . . . e321

7.2.6. Pregnant Occupationally ExposedWorkers . . . . . . . . . . . . . . . . . . . . . . . . . . . e323

7.2.7. Alternative Imaging Techniques . . . . . . . e323

7.2.8. Summary Checklist for Dose-Sparing inX-Ray Fluoroscopy . . . . . . . . . . . . . . . . . . e323
Checklist of Dose-Sparing Practices forX-Ray Fluoroscopy . . . . . . . . . . . . . . . . . . . e323
7.3. X-Ray CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e324

7.3.1. X-Ray CT General Principles . . . . . . . . . . e324

7.3.2. Equipment Quality and Calibration . . . . e324

7.3.3. Variables That Affect Patient Dose forX-Ray CT . . . . . . . . . . . . . . . . . . . . . . . . . . e324

7.3.4. Summary Checklist of Dose-SparingPractices for X-Ray CT . . . . . . . . . . . . . . . e327

7.4. Nuclear Cardiology Techniques . . . . . . . . . . . . . e327

7.4.1. Nuclear Cardiology General Principles . . e327

7.4.2. Nuclear Cardiology Equipment Quality,Calibration, and Maintenance . . . . . . . . . e327

7.4.3. Nuclear Cardiology Spatial Resolution andImage Detector Dose . . . . . . . . . . . . . . . . e328

7.4.4. Procedures and Practices to MinimizePatient Exposure . . . . . . . . . . . . . . . . . . . e328

7.4.5. Procedures and Practices to ProtectOccupationallyExposedHealthcareWorkersin Nuclear Cardiology Facilities . . . . . . . . e330

7.4.6. Summary Checklist of Dose-SparingPractices for Nuclear Cardiology . . . . . . . e330

7.5. Summary of Dose Minimization Strategies inX-Ray Fluoroscopy, X-Ray CT, andCardiovascular Nuclear Scintigraphy . . . . . . . . . e331

8. MODALITY-SPECIFIC OPERATOR EDUCATION

AND CERTIFICATION . . . . . . . . . . . . . . . . . . . . . . . e331

8.1. General Principles . . . . . . . . . . . . . . . . . . . . . . . e331

8.1.1. Regulatory Authority . . . . . . . . . . . . . . . . e331

8.1.2. Professional Society Guideline andPosition Statements . . . . . . . . . . . . . . . . . e331


8.2.1. Physician Responsibilities . . . . . . . . . . . . e332

8.2.2. Operator Training/EducationRecommendations and Requirements . . e332

8.3. X-Ray CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e332


8.3.2. Society-Developed Operator Training/Education Requirements . . . . . . . . . . . . . e333

8.4. Nuclear Cardiology Techniques . . . . . . . . . . . . . e333


8.4.2. Summary of Current RegulatoryRequirements . . . . . . . . . . . . . . . . . . . . . . e333

8.4.3. Operator Training/EducationRequirements . . . . . . . . . . . . . . . . . . . . . . e334

9. QUALITY ASSURANCE . . . . . . . . . . . . . . . . . . . . . . e334

9.1. Introduction and General Principles . . . . . . . . . e334


9.2.1. X-Ray Fluoroscopy Regulatory Issues andSocietal Policy Statements . . . . . . . . . . . e335

9.2.2. X-Ray Fluoroscopic RadiologicalEquipment Quality and Calibration . . . . e335

9.2.3. X-Ray Fluoroscopic Imaging ProtocolSelection Practices . . . . . . . . . . . . . . . . . . e335

9.2.4. X-Ray Fluoroscopic Operator andPersonnel Conduct . . . . . . . . . . . . . . . . . . e335

9.2.5. X-Ray Fluoroscopic Patient RadiationExposure Monitoring . . . . . . . . . . . . . . . . e336



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9.2.6. Effectiveness of Programs to MinimizePatient Radiation Exposure in X-RayFluoroscopy . . . . . . . . . . . . . . . . . . . . . . . e336

9.3. X-Ray CT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e336

9.3.1. X-Ray CT Regulatory Issues and SocietalPosition Statements . . . . . . . . . . . . . . . . . e336

9.3.2. X-Ray CT Equipment Quality andCalibration . . . . . . . . . . . . . . . . . . . . . . . . e337

9.3.3. X-Ray CT Imaging Protocol Selection . . . e337

9.3.4. X-Ray CT Patient Radiation ExposureMonitoring . . . . . . . . . . . . . . . . . . . . . . . . e338

9.4. Nuclear Cardiology . . . . . . . . . . . . . . . . . . . . . . . e338

9.4.1. Nuclear Cardiology Regulatory Issues . . e338

9.4.2. Nuclear Scintigraphy Equipment Qualityand Calibration . . . . . . . . . . . . . . . . . . . . e338

9.4.3. Nuclear Scintigraphy AttenuationCorrection Equipment Quality andCalibration . . . . . . . . . . . . . . . . . . . . . . . . e339

9.4.4. Nuclear Cardiology Patient RadiationExposure Monitoring . . . . . . . . . . . . . . . . e339

9.4.5. Nuclear Cardiology Clinical PersonnelExposure Protection and Monitoring . . . e339

10. PATIENT AND MEDICAL PERSONNEL RADIATION

DOSE MONITORING AND TRACKING:

PROGRAMMATIC AND INDIVIDUAL

CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . e340

10.1. Requirements for Dose Monitoring andTracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e340

10.2. Program-Level Dose Monitoring and Tracking . . e340

10.3. Patient-Level Dose Monitoring and Tracking . . e340

11. SUMMARY, CONCLUSIONS, AND

RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . e341

11.1. The Issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e341

11.1.1. Patient Participation in Clinical ImagingDecisions . . . . . . . . . . . . . . . . . . . . . . . . . . e341

11.2. Clinical Value of Radiation-Based ImagingStudies and Radiation-Guided TherapeuticProcedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e341

11.3. Individual Patient Risk and Population Impact(Including Occupationally Exposed Workers) . . e342

11.4. The ALARA Principle . . . . . . . . . . . . . . . . . . . . . e342

11.5. The Potential to Minimize Exposure toPatients and Personnel . . . . . . . . . . . . . . . . . . . e342

11.5.1. Imaging Modality Choice . . . . . . . . . . . . e342

11.5.2. Procedure Conduct Choice . . . . . . . . . . . e342

11.5.3. Protecting Occupationally ExposedWorkers . . . . . . . . . . . . . . . . . . . . . . . . . . e342

11.6. Physician Responsibilities to MinimizePatient Exposure . . . . . . . . . . . . . . . . . . . . . . . . e342

11.6.1. Case Selection . . . . . . . . . . . . . . . . . . . . . e342

11.6.2. Procedure Conduct . . . . . . . . . . . . . . . . . e343

11.6.3. Facility Management . . . . . . . . . . . . . . . e343

11.6.4. Imaging Equipment Renovation andReplacement . . . . . . . . . . . . . . . . . . . . . . e343

11.7. Patient Radiation Dose Tracking . . . . . . . . . . . e343

11.8. Need for Quality Assurance and Training . . . . e344

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e344

APPENDIX A

Author relationships With Industry and Other Entities(Relevant): 2018 ACC/ASNC/HRS/NASCI/SCAI/SCCT/SNNMI Expert Consensus Document on Optimal Useof Ionizing Radiation in Cardiovascular Imaging:Best Practices for Safety and Effectiveness . . . . . . . e348

APPENDIX B

Peer Reviewer Information: 2018 ACC/ASNC/HRS/NASCI/SCAI/SCCT/SNNMI Expert ConsensusDocument on Optimal Use of Ionizing Radiation inCardiovascular Imaging: Best Practices for Safetyand Effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . e350

APPENDIX C

Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e351

PREAMBLE

This document has been developed as an Expert ConsensusDocument by the American College of Cardiology (ACC)in collaboration with the American Society of NuclearCardiology, Heart Rhythm Society, Mended Hearts,North American Society for Cardiovascular Imaging, Soci-ety for Cardiovascular Angiography and Interventions,Society for Cardiovascular Computed Tomography, andSociety of NuclearMedicine andMolecular Imaging. ExpertConsensus Documents are intended to inform practi-tioners, payers, and other interested parties of the opinionof ACC and document cosponsors concerning evolvingareas of clinical practice and/or technologies that arewidely available or new to the practice community. ExpertConsensus Documents are intended to provide guidancefor clinicians in areas where evidence may be limited ornew and evolving, or insufficient data exist to fully informclinical decision making. These documents therefore serveto complement clinical practice guidelines, providingpractical guidance for transforming guideline recommen-dations into clinically actionable information.

The stimulus to create this document was the recog-nition that ionizing radiation-based cardiovascular pro-cedures are being performed with increasing frequency.


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This leads to greater patient radiation exposure and,potentially, to greater exposure for clinical personnel.Although the clinical benefit of these procedures is sub-stantial, there is concern about the implications of med-ical radiation exposure both to patients and to medicalpersonnel. The ACC leadership concluded that it isimportant to provide practitioners with an educationalresource that assembles and interprets the current radi-ation knowledge base relevant to cardiovascular imagingprocedures that employ ionizing radiation. By applyingthis knowledge base, cardiovascular practitioners will beable to select and perform procedures optimally, and,accordingly, minimize radiation exposure to patients andto personnel.

This online published document is a more compre-hensive treatment of the knowledge base covered in 2print published documents published under this docu-ment’s title with subtitles “Part 1: Radiation Physics andRadiation Biology” and “Part 2: Radiological EquipmentOperation, Dose-Sparing Methodologies, Patient andMedical Personnel Protection.” In addition, this onlinedocument contains 3 sections that are not included inthe print-published documents: Modality-Specific Oper-ator Education and Certification, Quality Assurance, andPatient and Medical Personnel Radiation Dose Moni-toring and Tracking: Programmatic and IndividualConsiderations.

To avoid actual, potential, or perceived conflicts of in-terest that may arise as a result of industry relationships orpersonal interests among the writing committee, allmembers of the writing committee, as well as peer re-viewers of the document, are asked to disclose all currenthealthcare-related relationships, including those existing12 months before initiation of the writing effort. The ACCTask Force on Expert Consensus Decision Pathways(formerly the ACC Task Force on Clinical Expert ConsensusDocuments) reviews these disclosures to determine whichcompanies make products (on the market or in develop-ment) that pertain to the document under development.Based on this information, a writing committee is formedto include a majority of members with no relevant re-lationships with industry (RWI), led by a chair with norelevant RWI. Authors with relevant RWI are not permittedto draft or vote on text or recommendations pertaining totheir RWI. RWI is reviewed on all conference calls andupdated as changes occur. Author and peer reviewer RWIpertinent to this document are disclosed in Appendixes 1and 2, respectively. Additionally, to ensure complete trans-parency, authors’ comprehensive disclosure information—including RWI not pertinent to this document—isavailable online (see Online Appendix). Disclosure infor-mation for the ACC Task Force on Expert Consensus

Decision Pathways is also available online, aswell as theACCdisclosure policy for document development.

The work of the writing committee was supportedexclusively by the ACC without commercial support.Writing committee members volunteered their time tothis effort. Conference calls of the writing committeewere confidential and were attended only by committeemembers and ACC staff.

James L. Januzzi, Jr., MD, FACCChair, ACC Task Force on

Expert Consensus Decision Pathways

1. INTRODUCTION

1.1. Document Development Process and Methodology

1.1.1. Writing Committee Organization

The writing committee consisted of a broad range ofmembers representing 9 societies and the following areasof expertise: interventional cardiology, general cardiol-ogy, pediatric cardiology, nuclear cardiology, nuclearmedicine, clinical electrophysiology, cardiovascularcomputed tomography (CT), cardiovascular imaging, andthe consumer patient perspective. Both a radiation safetybiologist and a physicist were included on the writingcommittee. This writing committee met the College’sdisclosure requirements for relationships with industry(RWI) as described in the Preamble.

1.1.2. Document Development and Approval

The writing committee convened by conference call ande-mail to finalize the document outline, develop theinitial draft, revise the draft per committee feedback, andultimately approved the document for external peer re-view. All participating organizations participated in peerreview, resulting in 21 reviewers representing 299 com-ments. Comments were reviewed and addressed by thewriting committee. A member of the ACC Task Force onExpert Consensus Decision Pathways served as leadreviewer to ensure that all comments were addressedadequately. Both the writing committee and the task forceapproved the final document to be sent for ACC ClinicalPolicy Approval Committee. This Committee reviewed thedocument, including all peer review comments andwriting committee responses, and approved the docu-ment in November 2017. The Heart Rhythm Society (HRS),North American Society for Cardiovascular Imaging(NASCI), Society for Cardiovascular Angiography and In-terventions (SCAI), and Society of CardiovascularComputed Tomography (SCCT) endorsed the document inJanuary 2018. This document is considered current until


http://www.acc.org/guidelines/about-guidelines-and-clinical-documents/guidelines-and-documents-task-forces

http://www.acc.org/guidelines/about-guidelines-and-clinical-documents/relationships-with-industry-policy

http://www.acc.org/guidelines/about-guidelines-and-clinical-documents/relationships-with-industry-policy



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the Task Force on Expert Consensus Decision Pathwaysrevises or withdraws it from publication.

2. PURPOSE

2.1. Document Purpose

This document’s purpose is to assist cardiovascularpractitioners to provide optimal cardiovascular carewhen employing ionizing radiation in diagnostic andtherapeutic procedures. It is written to serve as anaccessible resource that compiles the current radiationbiology and safety knowledge base applicable to cardio-vascular imaging. The document covers both patient andmedical personnel safety issues for the 3 cardiovascularprocedure classes that employ ionizing radiation: x-rayfluoroscopy, x-ray CT, and radionuclide scintigraphy.It includes discussions of radiation dosimetry andits determinants, radiation harm, basics of equipmentoperation, strategies to minimize dose, and issues ofradiation monitoring and tracking. The document’s goalis to enable cardiovascular practitioners to select theoptimal imaging technique for a given clinical circum-stance while balancing a technique’s risk and benefits,and to apply that technique optimally to generate high-quality diagnostic images that deliver the greatestclinical value with minimal radiation exposure.

2.2. The Radiation Safety Issue

Cardiovascular procedures that employ ionizing radia-tion have transformed the practice of cardiovascularmedicine. These procedures have great value for diag-nosis and treatment of appropriately selected patientswith known or suspected cardiovascular disease.In addition, they enable more refined recognitionand characterization of cardiovascular disease. Theseprocedures are also integral to either planning orexecuting numerous treatment modalities, which canhave profound impacts on the outcomes of cardiovas-cular disorders.

However, ionizing radiation has molecular-level detri-mental effects on exposed human tissue, with potentialfor injury both to patients and to exposed medicalpersonnel. Consequently, it is desirable to minimize ra-diation exposure both to patients and to medicalpersonnel in a manner consistent with achieving optimalhealth benefits. This principle requires that cliniciansemploy judicious selection of and conduct of radiation-employing procedures to achieve an optimal balance ofa procedure’s therapeutic benefit against the incrementalrisk conferred by the radiation exposure.

Currently, cardiovascular diagnostic and therapeuticprocedures are a major source of patient exposure to

medical ionizing radiation, accounting for approximately40% of total medical radiation exposure (exclusive of ra-diation oncology) (1,2). Among occupationally exposedhealthcare workers, interventional cardiologists andclinical electrophysiologists are among the most highlyexposed, and there is potential for exposure to supportpersonnel as well (in particular, nonphysician staff whowork in x-ray fluoroscopy and nuclear cardiology envi-ronments) (3,4).

2.3. The Need for Physician Radiation Safety Education

Cardiovascular specialists have a responsibility to un-derstand the radiation safety knowledge base, in partic-ular, to:

1. Apply knowledge of the radiation safety knowledgebase to make appropriate case selection choices.

2. Conduct radiation-assisted procedures optimally,minimizing exposure to patients and personnel.

There is evidence that many cardiovascular specialistswho order and conduct radiation-employing proceduresare not fully informed about the radiation doses thataccompany the procedure or the associated health impli-cations for their patients and for themselves (5,6).Consequently, there is a need to augment and standardizethe level of knowledge and competence that cardiovas-cular specialists should hold in radiation safety andmanagement. This knowledge base should be incorpo-rated into training curricula and in physician board cer-tification procedures.

Cardiovascular specialists fall into 2 categoriesrequiring different levels of knowledge: those who ordercardiac imaging procedures and those who perform them.Training curricula should furnish the level of knowledgeappropriate for a particular physician’s practice activity.Achieving this goal requires collaboration betweenvarious stakeholders in graduate and postgraduate edu-cation. The blueprints of certification and recertificationexaminations should include specifications of radiationsafety subject matter. Training programs should configuretheir teaching curricula to prepare their traineesappropriately.

2.4. Appropriateness of Medical Radiation

The balance between a procedure’s risk and benefitdetermines its appropriateness. Although the technicalhazards that accompany a procedure are well known,the hazard associated with attendant exposure to ionizingradiation should also be considered a potentiallyimportant determinant of a procedure’s risk-benefitrelationship.

TABLE 1 Typical Effective Doses for Cardiac Procedures

Modality ProtocolTypical Effective

Does (mSv)

MDCT Coronary CT angiography:helical, no tube current modulation

8–30

MDCT Coronary CT angiography:helical, tube current modulation

6–20

MDCT Coronary CT angiography:prospectively triggered axial

0.5–7

MDCT Coronary CT angiography:high-pitch helical

<0.5–3

MDCT CT angiography, pre-TAVR:coronary (multiphase) andchest/abdomen/pelvis

5–50

MDCT Calcium score 1–5

MDCT Attenuation correction <0.5–2.0

EBCT Calcium Score 1

SPECT 10 mCi 99mTc sestamibi rest/30 mCi 99mTc sestamibi stress

11


17


18

SPECT 10 mCi 99mTc sestamibi stress only 2.7

SPECT 30 mCi 99mTc sestamibi stress only 8

SPECT 10 mCi 99mTc tetrofosmin rest/30 mCi 99mTc tetrofosmin stress

9


14


14

SPECT 10 mCi 99mTc tetrofosmin stress only 2.3

SPECT 30 mCi 99mTc tetrofosmin stress only 7

SPECT 3.5mCi 201Tl 15

SPECT Dual isotope: 3.5 mCi 201Tl rest/30 mCi 99mTc sestamibi stress

23

SPECT Dual isotope: 3.5 mCi 201Tl rest/30 mCi 99mTc tetrofosmin stress

22

PET 50 mCi 82Rb rest/50 mCi 82Rb stress

4

PET 15 mCi 13N ammonia rest/15 mCi 13N ammonia stress

2

PET 10 mCi 18F FDG 7

Planar 30 mCi 99mTc-labeled erythrocytes 8

Fluoroscopy Diagnostic invasive coronary angiography 2–20

Fluoroscopy Percutaneous coronary intervention 5–57

Continued in the next column

TABLE 1 Continued

Modality ProtocolTypical Effective

Does (mSv)

Fluoroscopy TAVR, transapical approach 12–23

Fluoroscopy TAVR, transfemoral approach 33–100

Fluoroscopy Diagnostic electrophysiological study 0.1–3.2

Fluoroscopy Radiofrequency ablation of arrhythmia 1–25

Fluoroscopy Permanent pacemaker implantation 0.2–8

Note: Current and ongoing engineering physical design and image processing softwarerefinements enable dose reductions for all 3 modalities since the data in Table 1 werecompiled. These lower doses can be achieved only if radiological equipment is currentgeneration and if operators consciously take advantage of their improved capabilities.As the majority of the currently installed base of equipment is earlier generation, thedata in Table 1 reflect most current exposure levels. Reproduced with permission fromEinstein et al. (7).

CT ¼ computed tomography; EBCT ¼ electron-beam computed tomography; FDG ¼fluorodeoxyglucose; MDCT ¼ multidetector-row computed tomography; PET ¼ posi-tron emission tomography; Rb ¼ rubidium; SPECT ¼ single-photon emission computedtomography; TAVR ¼ transcatheter aortic valve replacement; Tc ¼ technetium; Tl ¼thallium.


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Physicians who either order or conduct such proced-ures need to:

1. Know the magnitude of a patient’s risk associated witha procedure’s radiation exposure.

2. Apply that understanding to determining the appro-priate procedure and selecting the approach that pro-vides the best balance of benefit and risk.

To assess the risk-benefit relationship for a givenpatient, the cardiovascular specialist who orders or

performs the procedure should understand, in the contextof that patient’s clinical characteristics, how the radiationdose that accompanies the procedure may be detrimentalto that patient’s health and how the outcome of the pro-cedure may be beneficial.

3. CURRENT TRENDS IN PATIENT AND MEDICAL

PERSONNEL RADIATION EXPOSURE FROM

CARDIOVASCULAR PROCEDURES

3.1. Trends in Patient and Medical PersonnelRadiation Exposure

The past 2 decades have seen substantial developmentand refinement of the 3 cardiovascular imaging tech-niques that employ ionizing radiation: x-ray fluoroscopy,x-ray CT, and radionuclide scintigraphy. Engineering ad-vances have improved image quality while in many casesreducing the radiation doses employed for image acqui-sition. These advances have greatly enhanced cardiovas-cular diagnostic and therapeutic capabilities, therebyimproving both diagnosis and therapy.

Despite these engineering refinements, the patient ra-diation doses that accompany these procedures remainsubstantial and, for the most part, are at the upper rangeof radiation-based diagnostic studies. Medical pro-fessionals should be aware of the radiation dose thatthese studies deliver to patients. In addition, within aparticular type of study, the radiation dose can varysubstantially depending on image acquisition protocoland patient characteristics. For reference, the commonlyperformed cardiovascular diagnostic studies and theirradiation dose ranges are listed in Table 1. Note that thedoses delivered by x-ray CT and nuclear cardiology canvary substantially depending on particulars of imageacquisition protocols.

TABLE 2Potential Consequences of Patient and MedicalPersonnel Radiation Exposure

IndividualPatient

Although many individual patients receive little or no medicalradiation exposure, some receive lifetime doses in excess of100 mSv. Doses in excess of 100 mSv are associated with adetectable increased cancer risk

Population Increased total exposure incurred by total population ofpatients has the potential to increase the populationincidence of cancer and other radiation-related disorders

OccupationallyExposedWorkers

Occupationally exposed physicians and support staff mayreceive doses as large as 10 mSv per year over a career thatmay span 30–40 years. The implications of this level ofexposure at the level of the individual practitioner areuncertain.



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Patient radiation dose ranges (in millisieverts) for the 3principal radiation-based cardiovascular imaging studies:x-ray fluoroscopy, x-ray CT, and nuclear cardiology. In-dividual procedure categories are further subdivided ac-cording to types of image acquisition protocols. Note thatfor a particular procedure category, the dose can varyconsiderably depending on image acquisition protocoland, within a given image acquisition protocol, procedureconduct and patient characteristics.

However, augmented capabilities have led to increasedutilization levels, resulting in greater radiation exposureboth at the individual and at the population levels. Inaddition, refinement of x-ray fluoroscopic systems,yielding greatly improved image quality, has facilitatedthe development of increasingly complex cardiovascularinterventional procedures. These procedures oftenrequire longer fluoroscopic times, resulting in larger ra-diation exposures than more basic procedures.

Increasing radiation exposure has the potential to in-crease the risk of adverse effects such as radiation-induced cancer. It is uncertain, however, whethermedical radiation is in actuality increasing cancer inci-dence in the population, because a small increase wouldbe difficult to detect against the large background inci-dence of cancer.

During the 2014 calendar year, the U.S. healthcaresystem performed, on Medicare beneficiaries, an esti-mated 925,848 diagnostic cardiac catheterization pro-cedures, 342,675 percutaneous coronary interventions,248,234 clinical electrophysiologic procedures, 61,207cardiovascular x-ray CT scans, and 2,111,558 nuclear car-diology examinations, for a total of 3,689,522 cardiovas-cular procedures that use ionizing radiation in Medicarebeneficiaries (8). Medicare beneficiaries are estimated toconsume 30% to 40% of all cardiovascular procedures.

Natural background radiation averages 3.0 millisieverts(mSv) (see Section 4 for a discussion of the Sievert unit ofradiation exposure) per person/year in the United States—equivalent to 150 posteroanterior chest radiographs (aposteroanterior chest-x-ray dose is 0.02 mSv; combinedposteroanterior and lateral is 0.06 mSv) (9). At the pop-ulation level, between 1987 and 2006, estimated perperson total medical radiation exposure grew from 0.6mSv/year (0.2 � background) to 3.2 mSv/year (1.07 �background) (10). Consequently, patients are currentlyreceiving, on average, more radiation from medicalsources than from natural background sources. 2006 isthe latest year for which compiled data are available. (TheNational Council on Radiation Protection is currentlycompiling contemporary data—expected availability2019—and it is likely that current average medical expo-sure will be found to have increased further). The 2006medical exposure is equivalent to 160 posteroanteriorchest x-rays per person/year. Risks associated with this

exposure must be weighed in relation to the health statusbenefits achieved by these procedures.

Physicians who are invasive cardiovascular procedureoperators are among the most highly exposed of theoccupationally exposed healthcare workers. Measure-ments of interventional cardiologist operator exposureusing current equipment and protection practicesdemonstrate an exposure range of 0.2 to >100 micro-sieverts (mSv) per procedure with a per-procedure averageof 8 to 10 mSv (11). Thus, an active interventional cardi-ologist performing 500 procedures/year employing cur-rent technology may be expected to receive, in addition tobackground exposure, a dose of as much as 10 mSv/yearor, in a most extreme scenario, 300 mSv over a 30-yearactive professional career.

Nonphysician clinical personnel working in an x-rayenvironment should receive substantially smaller dosesthan tableside operators, although nuclear cardiologytechnologists who handle radioactive materials tend to bemore highly exposed. Determinants of nonphysicianexposure include time spent in an active procedure room,location in the procedure room during active procedures,and exposure handling radioactive materials. There areno data that characterize the total number of exposedworkers or their exposure values.

3.2. Potential Consequences of Patient and Medical PersonnelRadiation Exposure

There are 3 important potential consequences of thegrowing use of ionizing radiation in cardiovascular med-icine (see Table 2).

1. At the individual patient level, although many in-dividuals receive little or no medical radiation expo-sure, some receive lifetime doses in excess of 100 mSv.At the population level, such doses are associated witha detectably increased cancer risk (12). Consequently,such exposures may place individuals at increasedpersonal risk of developing cancer or tissue reactions,including skin injury and cataracts. The actual magni-tude of this risk varies substantially with patientcharacteristics (see Section 5.4).


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2. At the population level, the increased total exposureincurred by patients has the potential to increase thepopulation incidence of cancer.

3. With respect to medical personnel, occupationallyexposed physicians and support staff may receivedoses as large as 10 mSv/year over a career that mayspan 30 to 40 years. The implications of this level ofexposure at the level of the individual practitioner areuncertain.

The ongoing magnitude of exposure to the generalpopulation and to occupationally exposed healthcareworkers has health implications at the population leveland for individual patients and healthcare workers. It isimportant that physicians and healthcare workers un-derstand the ionizing radiation knowledge base and applyit to protect patients, themselves, and their colleaguesthrough judicious case selection and appropriate conductof radiation-assisted procedures.

4. THE MANY MEASURES OF RADIATION

4.1. Radiation Exposure and Dose Metrics

Ionizing radiation exposure and dosimetry are not easilycharacterized by simple metrics. Radiation exposure anddose may be considered from the perspective of 5 distinctbut inter-related frames of reference. For this document’spurpose, these metrics have specific meanings as definedin the following text:

n Exposure: the quantity of radiation that impinges on atissue.

n Absorbed Dose: the concentration of energy depositedby radiation in a specific exposed tissue.

n Equivalent Dose: the absorbed dose adjusted by aradiation weighting factor to reflect the different de-grees of biological damage caused by various types ofradiation.

n Effective Dose: a metric that reflects the overall bio-logical effect from radiation on an average subjectfrom a particular radiation exposure scenario.

n Injected Dose: A metric that describes the quantity ofradioactivity of a radioactive substance injected into apatient for a nuclear scintigraphy study (expressed inmillicuries [mCi]). Injected dose is a determinant of the4 dose parameters listed previously. However, theexact relationships between injected dose and absor-bed dose and equivalent dose are complex, and arediscussed in depth in Section 6.4.

A comprehensive assessment of radiation effects re-quires consideration of all 5 parameters. The relationshipsbetween these metrics are complex and are determinedby the properties of both the radiation and the exposedtissue. For clarity in this document, the interaction of

radiation with tissue will be characterized from theperspective of 4 of the previously mentioned inter-relatedframes of reference: exposure, absorbed dose, equivalentdose, and effective dose. It should be noted that in theliterature, the terms “exposure” and “dose” are oftenused with less specific meanings than those used in thisdocument. For this document’s purpose, these metricshave specific meanings as defined in the following text.Exposure

Radiation exposure refers to the presence of ionizing ra-diation at the location of the exposed tissue. This isquantified by standardized measures of a physical quan-tity that represent the amount of radiation present at thatlocation. The typically used measure of radiation quantityis air kerma (Section 4.4.1), which is the amount of energyreleased by the interaction of the radiation with a unitmass of air. Its unit of measure is the gray (Gy). Its unitsare joules (J)/kg. One Gy is the quantity of radiation thatwhen interacting with 1 kg of air releases 1 joule of energy.It should be noted that this is a measure of cumulativeenergy intensity as the energy deposition is normalized toa quantity of the absorbing material.Absorbed Dose

Absorbed radiation dose is a measure of the energy thatradiation deposits in an exposed tissue through in-teractions with its molecular constituents. It differs fromexposure in that the radiation present at a given locationdoes not deposit all of its energy there. The fraction of itsenergy that a given radiation exposure will deposit in theexposed tissue varies with the type and energy of theradiation and the tissue composition.

Absorbed dose is also a measure of the intensity of cu-mulative energy deposition (energy deposited per unitmass of tissue) and is expressed in Gy—joules of energydeposited per kilogram of tissue. In exposure by externalradiation beams, dose is not uniform throughout theexposed volume, but varies, typically as a function ofdepth from the beam entrance port.Equivalent Dose

Different types of ionizing radiation cause varying de-grees of tissue injury for a given absorbed dose. Equiva-lent dose is a construct used to account for differences intissue injury caused by different radiation types. X-raysand gamma rays are the benchmarks against which par-ticle radiation types such as protons, neutrons, and betaparticles are compared. Some particles, in particular,protons, neutrons and alpha particles, cause greater tis-sue injury at a given dose than do x-rays, gamma rays,and electron particles. To adjust for this variability, eachradiation type is assigned a radiation weighting factor bywhich the absorbed dose (in Gy) is multiplied to yield ameasure of the expected tissue injury caused by thatdose. The unit of measure is the sievert (Sv) which is theabsorbed dose in Gy multiplied by the radiation weighting



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factor. Of the different radiation types, x-rays, gammarays, and electron particles (electrons and positrons) areassigned a radiation weighting factor of 1. Other particleradiation types have weighting factors ranging between 2and 20. For medical imaging, which employs x-rays andgamma rays, absorbed dose and equivalent dose take thesame value, that is, an exposure with an absorbed dose of20 mGy has an equivalent dose of 20 mSv.Effective Dose

Effective dose is a measure of the estimated potential fora biological effect on the complete organism caused by aparticular absorbed radiation dose. The effective doseconstruct has been developed as a measure of the esti-mated potential for a stochastic effect (such as cancerinduction) that would be caused by a particular (nonuni-form) absorbed radiation dose. It is the sum of theequivalent doses received by each organ with each organequivalent dose multiplied by a coefficient that reflectsthat organ’s sensitivity to a stochastic effect. The unit ofeffective dose is also the Sv, as discussed in greater depthin Section 4.5. The Sv, like the Gy of the absorbed dose’sunit, is specific to its particular context and is equal to 1joule/kg. The connection between effective dose andabsorbed dose is that an effective dose of 1 Sv is associ-ated with the same estimated stochastic risk that accom-panies a uniform total body exposure with an absorbeddose of 1 Gy of radiation that has a radiation weightingfactor of 1.

In medical radiation exposures, absorbed dose is typi-cally not uniform throughout all tissues. For x-ray imag-ing, dose is concentrated in the body region beingexamined and varies with depth from the beam entranceport. For nuclear imaging, dose is concentrated in thetissues that most avidly take up the tracer or are involvedin its elimination.

Different tissues have different sensitivities toradiation-induced effects. In the effective dose construct,each tissue is assigned a tissue-weighting factor thatspecifies its sensitivity to radiation effects. To calculatethe effective dose in Sv, each exposed tissue’s equivalentdose is multiplied by its tissue-weighting factor yieldingthat tissue’s contribution to the overall risk. The contri-butions to risk from all exposed tissues are summed,yielding total risk, expressed as the effective dose in Sv.(How the effective dose is calculated is discussed ingreater depth in Section 4.5).

It is important to note with regard to childhood andteenage radiation exposure that tissue weighting factorsdo not take into account the increased sensitivity of thetissue of the pediatric population. Thus, for children andadolescents, a given radiation exposure confers a greaterrisk than the same exposure would confer to an adultpopulation. In addition, children who do not have lifethreatening disorders have a long life expectancy, which

provides a longer period for radiation-induced illness topresent (13).

4.2. Challenges in Relating Radiation Exposure and Dose toRisk of Detrimental Effects

Detrimental effects of radiation exposure typically pre-sent weeks to years following exposure. In addition,many detrimental effects, principally cancer, have a largebackground frequency that complicates the attribution ofan effect in a particular subject to prior radiationexposure.

4.3. Types of Ionizing Radiation Used in Medical Imaging

Radiation in cardiovascular imaging consists of photonswith energy >10 kiloelectron volts (keV) (x-rays andgamma rays) and positrons. The physical effect of suchradiation is to eject electrons from atoms that comprisetissue molecules forming ions and free radicals. Thiscauses molecular damage, potentially destroying a mole-cule or altering its function. This is the basis for the term“ionizing radiation” (discussed in detail in Section 5).

4.3.1. X-Rays and Gamma Rays

X-rays and gamma rays are in a class of ionizing radia-tions, which is transmitted by photons. Photons travel atthe speed of light, and have no mass and no charge. Theirelectromagnetic energy ranges from a few electron volts(eV) to millions of electron volts (MeV). The energiescommonly employed in cardiovascular imaging are tensto hundreds of keV.

X-ray or gamma photons cause ionization by collidingwith and ejecting electrons from atoms of constituenttissue molecules. Energy is exchanged in the process,with the ejected electron gaining energy of motion andthe photon losing energy. The incident photon may ormay not be extinguished by the interaction. After aninitial interaction with an atom, photons that were notextinguished continue to travel through the exposedmedium at a degraded energy. The weakened (scattered)photon can collide with additional atoms (furtherexposing the subject), potentially ionizing them as well,until either all of its energy is dissipated and the photonceases to exist, or it escapes from the subject (exposingthe environment).

X-rays used in x-ray fluoroscopy and x-ray CT have a ofphoton energy spectrum between 30 and 140 keV (theenergy spectrum of x-rays generated in typical diagnosticx-ray tubes includes photon energies <30 keV, but themajority of these lower-energy photons are filtered out inthe x-ray tube and do not expose the subject). Thallium-201 and Technetium-99m (Tc-99m) are the principal ra-dionuclides used in cardiovascular nuclear scintigraphystudies. Thallium-201 releases photons primarily in the


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68 to 80 keV range, similar to diagnostic x-rays. Tc-99mreleases photons primarily in the 140 keV range.

4.3.2. Positrons

Positrons are positively charged electrons. They havemass and charge. When positrons travel through a me-dium, their electrostatic charge causes them to interactreadily with electrons in the medium, leaving a trail ofionization. Consequently, they have a very short meanfree path in tissue of 6 to 7 mm with a maximum of 15.2mm. Positrons continue to cause ionization until theirenergy decreases to a critical level, at which point they areannihilated by colliding with an electron of a constituentatom. This annihilation process releases 2 511-keV gammaray photons that travel in opposite directions. Becausethe emitted photons have such high energy, they areminimally attenuated in tissue, and the majority reach theimaging detector. Rubidium-82 is the most commonlyused positron emitter for myocardial perfusion imaging;nitrogen-13 ammonia is used less frequently for thispurpose. Fluorine-18 deoxyglucose is used in cardiologyfor metabolic imaging and to detect myocardial sarcoidand other inflammatory conditions.

4.4. Relationships Between Exposure and Absorbed Dose

Medical radiation exposures occur in 2 ways:

1. Exposure from an external radiation beam (x-ray fluo-roscopy and x-ray CT)

2. Exposure from radioactive decay within the subject(nuclear scintigraphy).

4.4.1. Exposure From External Beams

For external radiation beams, the absorbed dose isdetermined by the total incident exposure, the propertiesof the incident radiation, and the volume of tissueexposed. Exposure from an external beam is measuredwith the parameter air kerma.

Air kerma is the standard unit of measure for x-raybeam exposure. Kerma is an acronym for “kinetic energyreleased in material.” Kerma is an energy intensitymeasured in units of joules of energy released per kilo-gram of absorbing material (J/kg). The kerma unit ofmeasure is the gray (Gy), which represents 1 joule of en-ergy released per kilogram of absorbing material. Themetric “air kerma” is used in medical x-ray fluoroscopicapplications because the measurement is made using airas the absorbing material that is ionized by the incidentradiation beam.

4.4.1.1. Absorbed Dose in Tissue From an External Beam

As described in Section 4.1, radiation absorbed dose, asdistinguished from exposure, is an energy intensity, theconcentration of radiation energy actually deposited in

the exposed tissue. Not all radiation energy that impingeson a tissue is absorbed. Some radiation (a variable quan-tity depending on both radiation and tissue characteris-tics) passes through the tissue without interacting with it,depositing no energy. (This fraction of the radiation iswhat generates the radiological image). Absorbed dose isalso an intensity measured in gray (Gy), which representsdeposition of 1 joule of energy per kilogram of irradiatedtissue.

External beam energy deposition in tissue is not uni-form. X-ray radiation is attenuated as it passes throughtissue. For diagnostic x-rays, in most tissues, x-ray in-tensity decreases by approximately a factor of 2 for each 5cm of tissue that it traverses. Thus, tissue exposed to anexternal x-ray beam, as occurs in x-ray fluoroscopy andx-ray CT, is not exposed uniformly—the dose decreasesexponentially with depth from the beam entrance port.The incident beam air kerma is a good measure of dose atthe body surface, but structures deeper than the bodysurface receive smaller doses. Thus, to estimate the doseto a particular body structure within the path of an x-raybeam but remote from the beam entrance site, adjust-ments have to be made to account for beam absorbance.

4.4.1.2. Kerma-Area Product: Incorporating the Volume ofExposed Tissue in X-Ray Fluoroscopy

Kerma (measured in Gy) is a measure of dose intensity(joules of energy deposited per kg of tissue). The risk ofradiation harm is related both to the intensity of the ra-diation dose and to the quantity of tissue that receives thedose. (The greater the quantity of tissue that receives agiven dose, the greater the risk.) Kerma-area product(KAP) is the product of the beam’s kerma and its cross-sectional area. Thus, this parameter also incorporatesthe volume of tissue irradiated. This concept is particu-larly important in x-ray fluoroscopy, as imaging field sizescan vary considerably leading to very different KAPs fromone examination to another.

4.4.1.3. Kerma-Length Product: Incorporating the Volume ofExposed Tissue in X-Ray CT

CT delivers radiation to a patient in a manner quitedifferent from that of projectional imaging or fluoroscopy.Typically, a narrow x-ray beam with a rectangular crosssection is used to collect images from multiple angles as itrotates around the patient. This distributes the dose muchmore uniformly around the patient compared with pro-jectional imaging. Instead of measuring an entrance airkerma to the patient, “dose” is measured by conventionas an air kerma inside of an acrylic cylinder used tosimulate a patient. Two sizes of cylinders are used: 32-and 16-cm diameters, often referred to as body and headphantoms, respectively. Air kerma is measured inside thephantom using an ionization chamber in the shape of a



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pencil that is placed inside a hole that is appropriatelydrilled in the plastic phantom. This yields the dose“intensity” analogous to the air kerma measurement forx-ray fluoroscopy.

The phantom air kerma is multiplied by the axial scanlength to incorporate the volume of tissue irradiated. Thismethod generates a variety of dose metrics for x-ray CT;these are discussed in detail in Section 6. For example,the computed tomography dose index100 (CTDI100) is ameasure of the dose delivered along a 100-mm scanlength. Computed tomography dose indexw (CTDIWweighted) accounts for the fact that more peripherallylocated structures, which are closer to the beam entrance,receive larger doses than deeper structures.

4.4.2. Exposure From Radionuclides

Unlike external beam exposures, radionuclide exposurescome from radioactive decay within the subject. In nu-clear cardiology applications, a radiopharmaceutical isadministered systemically and distributes throughout thebody. Distribution may be preferential to particular tis-sues depending on the pharmacological properties of theradiopharmaceutical. The dose delivered by a radiophar-maceutical is determined by the activity administered,the tracer distribution, the tracer elimination rate, and thetracer’s time-activity relationships. These data in combi-nation with the tracer radionuclide’s radiation charac-teristics permit estimation of radiation dose delivered toeach organ or tissue. This model is discussed in greaterdetail in Section 6.

4.5. Estimating Effective Dose

The concept of effective dose was formulated to create ametric that estimates a given radiation dose’s contribu-tion to stochastic health risk—namely the risk of cancerinduction and of genetic changes (see Section 5.2.2) (14).The effective dose concept is derived from 2 facts of ra-diation dosimetry:

1. Medical and occupational radiation exposures aregenerally not uniform, with some organs and tissuesreceiving greater exposures than others.

2. Sensitivity to radiation-induced detrimental effectsvaries among different organs and tissues.

The effective dose is expressed in units termed Sv(Section 4.1). The units are a special term for J/kg (thesame as for the Gray). The Sv represents the hypotheticaluniform whole-body dose that confers the same stochas-tic risk as the nonuniform dose actually delivered. Auniform total body absorbed dose of 1 J/kg of radiationthat has a radiation weighting factor of 1 would yield aneffective dose of 1 Sv.

The effective dose construct assigns each organ/tissuea weighting factor that reflects the tissue’s sensitivity to

radiation-induced stochastic risk. The calculation ofeffective dose involves estimating each organ’s actualequivalent dose (in Gy). That dose is adjusted by multi-plying it by the organ’s tissue-weighting factor. The organsensitivity-adjusted individual organ doses are summedto yield a total effective dose (in Sv).

For a chest exposure, absorbed dose is concentrated inthe skin, mediastinal structures, lungs, breast, andthoracic bone marrow. Doses to these organs wouldcontribute the largest components to the effective dosecalculation. Smaller quantities of scattered radiationwould expose the abdominal viscera and upper neck. Asthese organs would receive smaller exposures, theircontribution to the effective dose calculation would besmaller. Other types of radiological examinations, such asx-ray CT and cardiac scintigraphy, would have differentorgan exposure distributions yielding different effectivedose calculations.

Deriving a quantitative measure of a subject’s esti-mated increased cancer risk due to a specified effectivedose is complex because the risk magnitude is determinedby numerous other variables, including subject age (chil-dren and young adults are more susceptible), gender(women are more susceptible), and natural life expec-tancy (longer natural life expectancy confers a longer timeavailable for cancer to present [13]). Statistical modelsthat attempt to quantify the dose-risk relationship havebeen developed based on large population exposures.These models are discussed in Section 5.4.

The individual tissue weighting factors have beenrevised over time as accumulating epidemiological evi-dence permits more precise estimates of organ sensitivity.The International Commission on Radiation Protectionpublished the most recent organ sensitivity estimates in2007 in ICRP Publication 103 (15). The estimates are listedin Table 3.

The tissue weighting factors are measures of the indi-vidual tissue’s intrinsic sensitivities to radiation-inducedcancer. Three applications of the effective dose concepthave relevance for medical exposure to patients under-going cardiovascular procedures and occupationallyexposed medical personnel. They are:

1. For cardiac x-ray fluoroscopy and cardiac x-ray CT, theradiation is concentrated in the subject’s chest. Thus,the exposures that contribute the most to stochasticrisk are the thoracic red bone marrow; the lung; and, infemales, the breast.

2. Measurements of exposure in phantoms providemodels that enable the rough estimation of equivalentdose delivered to particular internal structures by theexposure as measured by the subject exposure param-eters, including KAP product for x-ray fluoroscopy,kerma-length product for x-ray CT, and radionuclide

TABLE 3Tissue Weighting Factors Used to CalculateEffective Dose in Sieverts

OrgansTissue Weighting factors

(ICRP103–2007)

Red bone marrow 0.12

Colon 0.12

Lung 0.12

Stomach 0.12

Breasts 0.12

Gonads 0.08

Bladder 0.04

Liver 0.04

Esophagus 0.04

Thyroid 0.04

Skin 0.01

Bone surface 0.01

Salivary glands 0.01

Brain 0.01

Remainder of body 0.12

Total 1.00

Adapted from the International Commission on Radiological Protection (ICRP) (15).


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doses for cardiovascular nuclear cardiology. These in-dividual organ equivalent doses may then be convertedto effective doses and summed to calculate an estimateof the subject’s effective dose. As noted in the previoustext, the absolute effective dose-risk relationship ismodulated by subject characteristics.

3. Measurements of occupational exposure for healthcareworkers may be used to estimate the worker’s sto-chastic risk.

4.6. Synopsis of Measures of Radiation Exposure and Dose

The existence of the many different measures of radiationexposure and dose has the potential to cause confusionleading to misapplication of units of measure. Table 4contains a synopsis of the principal metrics described inthis section. In reporting radiation from an individualprocedure to a specific patient, modality-specific param-eters should be used. For x-ray fluoroscopic procedures,air kerma at the interventional reference point and KAPshould both be reported. For CT procedures, the CTDIvoland dose-length product (DLP) (discussed in detail inSection 6.3.3) should be reported, along with the size ofthe CTDI phantom (32- or 16-cm). Effective dose has lim-itations for calculating individual patient dosimetrybecause, for example, the tissue weighting factors aregender and age averaged (not accounting for the fact thatchildren and females are more sensitive). However, it maybe useful to make general comparisons between modal-ities, protocols, and imaging strategies. Given that

effective dose is an estimate that involves a number ofassumptions, the calculated and reported values shouldbe accompanied by the actual exposure measurementsand a description of the methodology used for estimation,that is, a conversion factor.

5. HOW RADIATION CAN HARM PEOPLE

5.1. Mechanism of Radiation-Induced Biological Effects

Radiation-induced tissue injury is due to molecular al-terations caused by particles or photons that have suffi-cient energy to induce ionization. Atoms ionized byradiation are frequently chemically unstable and trans-form themselves or their constituent molecules into freeradicals. A common example is ionization of water, whichupon interacting with an x-ray photon, decomposes into afree electron, a proton, and a hydroxyl radical. The hy-droxyl radical, because of its unpaired electron, is highlyreactive and interacts avidly with biomolecules (proteinsor nucleic acids). Similarly, an x-ray photon can ionize anatom that is a constituent of a biomolecule. Thus, abiomolecule can be altered by either reacting with aradiation-generated free radical or by being directlyionized by radiation. The resulting structural change canalter a molecule’s function.

Radiation-induced tissue damage from ionizing radia-tion takes many forms that have variable intervals be-tween exposure and clinical presentation. Some harmfulradiation effects appear within days to months followingthe exposure. Other harmful radiation-induced effectshave long latent periods, becoming evident only manyyears following the inciting exposure, or may not presentin the individual’s remaining lifetime.

Damage to a molecule that is an important tissue con-stituent, such as a protein, can alter the cell’s function. Ifa cell incurs sufficient damage to its constituent mole-cules it may not be able to maintain basic cellular opera-tions and undergo necrosis. If a cell incurs strategicdamage to its deoxyribonucleic acid (DNA), a previouslynormal gene may be transformed into an oncogene or theionization process may lead to other changes in cellularenvironment that promote carcinogenesis.

5.2. Types of Radiation-Induced Health Effects

Radiation-induced health effects are divided into 2 broadgroups that differ in their mechanism, the nature of theireffects, their relationship to absorbed dose, and thetemporal relationships between exposure andmanifestation.

5.2.1. Tissue Reactions (Formerly Called Deterministic Effects)

Tissue reactions are caused by radiation-induced injury tostructural and functional molecules in cells. Cell necrosiswill occur if the amount of molecular alteration incurred

TABLE 4 Synopsis of Radiation Exposure and Dose Metrics

Metric Unit Utility

Absorbed Dose-Related Parameters:Characterize Dose to Organ/Tissue or Whole Body

Absorbed dose Gy Amount of ionizing radiation energy deposited per unit mass of tissue. 1 Gy ¼ 1 Joule of energy depositedper kg of tissue. This metric is a concentration of energy deposition—not the total quantity of energydeposited.

Equivalent dose Sv Absorbed dose adjusted by a radiation weighting factor that adjusts for the specific tissue-injuringpotential of the particular radiation type. Photons (x-rays and gamma rays) have a weighting factor of1. Electrons also have a weighting factor of 1. Neutrons have larger weighting factors that vary withtheir energy level. For medical imaging, because only photons and positrons are used, absorbed doseand equivalent dose take the same value.

Effective dose mSv Calculated whole-body quantity used to roughly compare potential stochastic risks from different partial-body exposures. It is expressed as the uniform whole-body dose that would confer the stochastic riskequivalent to that caused by a regional exposure.

Modality-Specific Parameters

X-ray fluoroscopic air kerma (free-in-air) Gy Used to assess level of radiation present at a location. In x-ray fluoroscopy, cumulative air kerma at theinterventional reference point can be used to approximate beam entrance port skin dose. (Forisocentric C-arms, the reference point is located 15 cm from isocenter in the direction toward the x-raysource. This point in space approximates the location of beam entry into the patient, but due tovariation in table height and tube angulation, is only an estimate of beam entrance port skin dose).

X-ray fluoroscopic Air-KAP, also referredto as dose-area product (DAP)

Gy$cm2 Used to assess the total quantity of radiation delivered by an external beam. It is the product of thecumulated amount of air kerma and the area of a radiographic or fluoroscopic field. KAP is often usedas the basis for estimating effective dose from a fluoroscopic procedure.

Computed tomographic dose index(CTDIFDA, CTDI100, CTDIw, and CTDIvol)

mGy Used to assess relative level of radiation applied during a CT imaging sequence. This metric is aconcentration of energy deposition in the exposed volume. It is not a total deposited energy quantity,as it does not incorporate the actual exposed volume (See DLP below). Different versions are used forvaried purposes.

Computed tomographic dose-lengthproduct (DLP)

mGy$cm Used to assess integrated amount of radiation applied along an axial length of a patient during a CTexamination. Can be used to estimate effective dose from the procedure.

Radionuclide injected dose mCi A measure of the quantity of radioactivity injected for a nuclear scintigraphy study. The relationship ofinjected dose to other dose parameters is complex and includes the nature of the nuclide’s radiation,the nuclide’s half-life, the distribution in the body, and the elimination kinetics.

CT ¼ computed tomography; CTDI ¼ computed tomographic dose index; KAP ¼ Kerma-Area Product.



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exceeds the cell’s ability to repair itself and maintainfunction. If a sufficient fraction of an exposed tissue’scells malfunction, fail to heal properly, or necrose,macroscopically evident tissue injury will occur. Conse-quently, tissue reactions typically exhibit dose-relatedseverity. Above the threshold dose, a greater dose willcause more extensive cellular injury to a greater numberof cells, increasing the severity of the macroscopic effectin a dose-related fashion.

Skin injury is the most common tissue reactionobserved in cardiovascular imaging. It occurs almostexclusively from x-ray fluoroscopic exposures and can besufficiently severe to cause tissue necrosis. Other tissuereactions include cataract formation; bone necrosis; and,in the heart, damage to myocardium, cardiac valves, andcoronary arteries. In addition, if a fetus incurs sufficientcellular injury at critical stages of organogenesis, devel-opment will be impaired (16).

Tissue reactions only become macroscopically evidentif a threshold radiation dose is exceeded. A dose belowthe threshold dose, although it may cause unapparentcellular injury, will not cause a detectable reaction. This isbecause at subthreshold doses, even though some cellsincur radiation-induced molecular change, the tissue is

able to maintain function and viability because the dam-age does not kill a sufficient fraction of the cells or affectcellular function sufficiently to cause macroscopicallyevident tissue injury or functional alteration (below thethreshold dose, a small number of cells in a tissue mayundergo necrosis but not in sufficient numbers to producea macroscopically detectable effect) (17).

Tissue reactions occur with a time delay betweenexposure and the appearance of tissue injury. This is dueto the time required for molecular damage to evolve andto cause sufficient cellular dysfunction to lead to macro-scopically evident injury.

Thus, a subject exposed to a dose that is subthresholdfor a tissue reaction will appear normal initially with nomacroscopic evidence of an effect on the exposed tissue.A subject who receives a greater than threshold dose willinitially appear to be unaffected but the exposed tissuewill, at a later time, show signs of injury. At doses abovethe threshold, the severity of visible tissue injury in-creases as absorbed dose increases.

5.2.2. Stochastic Effects: Cancer

Stochastic effects are caused by radiation-induced dam-age to a cell’s genetic material that causes the damaged


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cell’s DNA to be reprogramed into dysfunctional opera-tion. The principal stochastic event of clinical importanceis radiation-induced cancer.

Stochastic effects differ from tissue reactions in theirdose relationship. Whereas tissue reactions have a definitedose threshold below which they do not occur and exhibitdose-related severity, stochastic events, in contrast, arenot known to have a dose threshold and do not have aquantitative dose-related severity. Rather, their relation-ship to dose is probabilistic, and the severity of a sto-chastic event, should it occur, is not related to the dosethat triggered it. The probability of a stochastic event isthought to increase approximately linearly with dose (18).

The difference between radiation-induced damage toconstitutive structural cell proteins, which causes tissuereactions, and damage to DNA, which causes stochasticevents, is that a single critically located DNA damageevent can create an oncogene. Radiation-induced cancereither does or does not occur (or may not present withinthe subject’s lifetime). The probability that it may occur isrelated to dose. The likelihood that a stochastic event willoccur does not reach 100% even at very large doses.

Damage to a DNA molecule can have variable conse-quences depending on how the particular chemicalchange to a DNA molecule affects gene function. Damageto a noncoding region would most likely be inconse-quential, whereas damage to a coding region could affectits gene product, and damage to a regulatory region couldaffect its regulation. Either of the latter 2 phenomena hasthe potential to transform a normally functioning geneinto an oncogene.

This phenomenon explains how, although radiationexposure likely causes many DNA damage events, most ofsuch events are inconsequential and may be repairedsuccessfully. A clinically evident radiation-induced can-cer event will occur only if radiation-induced injury af-fects a strategic segment of DNA and there is sufficienttime following the radiation event for the cancer todevelop, evolve, and present. Many other biological fac-tors including age, gender, and genetics modulate thelikelihood of inducing a stochastic event.

There is uncertainty about the quantitative relation-ship between dose and risk for stochastic events. At smalldoses, stochastic risk is small and difficult to distinguishstatistically from zero. Theoretically, a single x-rayphoton ionizing a strategic atom within a portion of DNAthat encodes a critical gene could create an oncogene.This is the theoretical basis for the concept that there is nothreshold dose below which stochastic risk is zero (seeSection 5.4) (12).

5.2.3. Stochastic Effects: Heritable Effects in Offspring

Theoretically, radiation injury to DNA in germ cells couldcause a clinically important mutation that would not

affect the exposed individual, but would be transmittedto that individual’s offspring. Such effects have only beendemonstrated in animal models but have not beenobserved in humans with statistical significance (19). Theabsence of any significant finding following exposures inhumans indicates they have a very low probability thatcould be detected only in exposures to very large pop-ulations (20).

5.3. Tissue Reactions: Dose-Effect Relationships

5.3.1. Skin Injury

The most common radiation-induced tissue reaction isskin injury at the beam entrance port following an x-rayfluoroscopically guided invasive procedure (17). Skin in-juries from medical diagnostic radiation virtually onlyoccur following fluoroscopic exposures because fluoros-copy is the only modality that has the potential to delivera skin dose that exceeds the injury threshold. Althoughrare cases of skin depilation and erythema have occurredfollowing x-ray CT examinations, these were caused byexcessive radiation output due to improper programmingof the x-ray parameters or a large number of scans beingrepeated over the same scan area.

Skin entrance port injuries have shapes that reflect theshape of the x-ray beam, which is typically rectangular.These injuries vary in severity from erythema, todesquamation, to ulceration and necrosis. They are al-ways located at the site of beam entrance, which is typi-cally on the subject’s back.

In some cases, transient skin erythema may occurwithin hours to a few days following the exposure andthen resolve. A more significant erythema occurs after adelay of a week to a few weeks. More severe skin injurytypically appears 4 to 8 weeks following the exposure. Inextreme cases, the ulceration can become confluent andfull thickness necrosis of skin may develop exposing un-derlying fat, muscle, and even bone (Figure 1).

The threshold dose that will cause a skin injury isvariable, as is the relationship between dose injuryseverity. The skin dose for a particular procedure may beapproximated by the cumulative air kerma (Section 4.4.1)at the interventional reference point. The x-ray systemcalculates this value. Thus, it is known during and at theconclusion of a procedure and can be used to estimate apatient’s skin injury risk.

General guideline values for the ranges of thresholdvalues for absorbed doses associated with degrees of skininjury severity are tabulated in Table 5. These values areapproximations that will vary among patients. It is note-worthy that these values are for a single first-time expo-sure. The thresholds for injury due to a subsequentexposure are lower and are related to the magnitude ofprior exposure(s) and the length of the interval betweenexposures.

FIGURE 1 Full Thickness Skin Necrosis Caused By a Large-Dose

X-Ray Fluoroscopic Procedure

An example of full thickness skin necrosis (underlying muscle and fat

are exposed) caused by a large-dose x-ray fluoroscopic procedure (90

minutes of fluoroscopy time). Note the rectangular area of skin

discoloration surrounding the area of skin necrosis. The injury is on the

left side of the subject’s back indicating that the exposure was con-

ducted in the right anterior oblique projection (17). (This image is

available on the U.S. Food and Drug Administration Web site and is in

the public domain.)



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Fluoroscopic entrance skin doses vary greatly becauseof variations in procedure complexity, duration, andvariations in patient radiological characteristics (seeSection 6.2). Skin dose accumulates proportionally toprocedure total fluoroscopic and cine acquisition time(the dose rate per frame during cine is typically 6 to 10times greater than during fluoroscopy). In addition, andlikely more importantly, skin dose is strongly affected bythe patient’s characteristics and the procedural

TABLE 5 Radiation-Induced Skin Injuries—Relationship of Sever

Single ExposureDose Range (Gy) 0–2 Weeks 2–8 Weeks

0–2 No

2–5 Transient erythema Possible epilation Re

5–10 Transient erythema Erythema epilation Re

10–15 Transient erythema Epilation, possibledesquamation

Pr

>15 Transient erythema, aftervery high dosesulceration

Epilation, moistdesquamation

De

Adapted from Balter et al. (17).

techniques. Body habitus is the most important patientcharacteristic. Larger patients require a greater skinentrance port dose to achieve adequate x-ray penetrationto the image detector. X-ray input dose is also affected bythe radiological projection. Extreme degrees of rotationalobliquity and cranial or caudal skew require a greater skindose. Dose is also determined by equipment calibrationand imaging protocol settings (see Section 7).

Thus, the prototypical patient at risk for a skin injury isan obese patient with diabetes who has undergone 1 ormore long-duration procedures within the past severalmonths and is under consideration for another procedureto be performed predominantly in a similar radiologicalprojection.

5.3.2. Bone Injury

In addition to skin injury, on occasion, incident radiationcan cause necrosis of superficial bones such as ribs.Although the dose to bone needed to cause osteonecrosisis greater than the dose to cause skin necrosis, the highcalcium content of bone imparts a greater capacity toabsorb x-ray photons. Consequently, the absorbed dose tosuperficial bone underlying the x-ray beam entrance portcan exceed the absorbed dose to the overlying skin,yielding the potential for radiation-induced bone injurywhen doses are sufficiently high to cause skin necrosis.

5.3.3. Eye Injury: Cataracts

The single dose threshold that will cause vision-impairingcataracts in humans is not well characterized but isbelieved to be on the order of 500 mGy with a minimumlatency of approximately 1 year (21). Cataract progressioncontinues for more than a decade after exposure. It isnoteworthy that early cataract lenticular changes are alsoincreasingly being observed in physician operators whohave a long career performing fluoroscopically-guidedprocedures (22). In this circumstance, the relationship ofcataract development to years of continual accumulationof small doses to the eye is currently not well character-ized. This area is currently a subject of ongoing study.

ity to Dose

Skin Reaction

8–40 Weeks Long-Term (>40 weeks)

observable effects

covery of hair loss Complete healing

covery or permanent hair loss At higher doses dermal atrophy or induration

olonged erythema, permanenthair loss

Dermal atrophy or induration

rmal atrophy, secondaryulceration, necrosis

Dermal atrophy, possible late skin breakdown,ulceration, and necrosis of subcutaneoustissues


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Whether or not prolonged low-level radiation exposure(such as occurs in occupational exposures) has thepotential to affect the retina is currently not known.

5.3.4. Tissue Reactions: Managing Skin Injuries

Less severe degrees of skin injury have the potential toheal. This constitutes the basis of management strategy.X-ray–induced skin injuries caused by absorbed doses lessthan that necessary to induce complete tissue necrosiscan be managed successfully with good supportivedermatological care.

X-ray injured skin is fragile. Although a subnecroticinjury has the potential to heal if properly protected,mechanical trauma to the skin can aggravate the injury,causing skin to slough. Thus, the cornerstone of opti-mizing the outcome of a skin injury is mechanical pro-tection while the skin attempts to repair the damage.Dressings and other treatment strategies that help thepatient avoid applying pressure or friction to the affectedarea are important during this period. Skin biopsy shouldnot be conducted in this circumstance, as the healingprocess is impaired.

Early recognition of a radiation-induced skin injury isessential to activate early treatment, which, in turn, pre-vents further injury to the damaged skin. The time delaybetween exposure and manifest injury can impede earlyrecognition. Due to the inherent delay of weeks betweenexposure and the initial signs of skin injury, the patientand his/her physicians may initially fail to recognize thecausal relationship. This recognition delay may causeinappropriate treatments to be applied initially.

The best strategy to facilitate prompt recognition is towarn the patient, family, and primary care physician ofthe potential for skin injury. The ACC/AHA/SCAI 2011 PCIguidelines state that it is a Class I recommendation that allpatients who receive an air kerma at the interventionalreference point >5 Gy should be counseled about thepossibility of a skin injury and instructed how to respondto the earliest signs, should they occur (23).

5.4. Stochastic Effects: Radiation-Induced Cancer

Radiation-induced cancer is potentially the most impor-tant consequence of medical radiation exposure. The po-tential that a patient or a healthcare worker might developa serious or fatal illness as a consequence of diagnosticmedical radiation requires that medical personnel under-stand that risk, and the variables that determine itsmagnitude and strategies to mitigate it. Knowledge of thestochastic risk posed by the patient’s radiation exposure isa variably important component of a procedure’s risk-benefit relationship. Knowledge of the occupational haz-ard posed to healthcare workers who work in a radiationenvironment is an important factor that determinesoccupational radiation protective practices.

5.4.1. Stochastic Effects: Attribution Challenges

One of the complex challenges in this field is to develop aquantitative measure of the incremental cancer riskconferred by medical radiation exposures. Attempts toconstruct evidence-based assessments are complicated bycancer’s large background prevalence in unexposed pop-ulations and the latent period between exposure and theclinical presentation of a malignancy. These phenomenacomplicate efforts to attribute a particular cancer to aparticular medical radiation exposure or group of expo-sures. Current concepts are derived from a combination oflaboratory animal exposures and observational studies oflarge populations who received exposures substantiallyabove background rates. Naturally, there is considerableuncertainty in these estimates.

Cancer is a prevalent family of disorders thatcommonly occurs spontaneously in the absence of radia-tion exposure greater than background. A subject’s life-time risk of developing cancer is roughly 46% (12), andlifetime risk of developing fatal cancer is about 23% (24).Because a radiation-induced cancer cannot be uniquelydistinguished from a cancer due to other causes, the highbackground frequency renders it difficult to attribute aparticular case to radiation exposure.

The latent period between exposure and clinical pre-sentation also adds uncertainty to attribution of aparticular cancer to a particular exposure. The latentperiod between exposure and emergence of most cancersmay be as brief as 2 years or as long as decades. For leu-kemia, the minimum latent period is 2 years. Population-based studies have demonstrated a statistical associationbetween leukemia and other childhood cancers in chil-dren exposed to large medical radiation doses (25,26).Pearce et al. (26) found a 3.18-fold increase in incidence ofleukemia in a large cohort of children exposed to a meandose of 51 mGy from CT scanning. Modan et al. (25), in acohort of 674 children who underwent cardiac catheteri-zation with a mean follow-up of 28.6 years (12,978patient-years), found a 4.75 times increased risk of ma-lignancies with a 6.3 times increase in lymphomas and a4.9 times increased risk of melanoma (25).

5.4.2. Stochastic Effects: Risk Metrics

At the population level, stochastic risk can be quantifiedas an increased cancer incidence in an exposed popula-tion compared with the background incidence in a com-parable unexposed population. The magnitude of this riskis measured using 2 related but different metrics:

1. Excess relative risk. The rate of disease in an exposedpopulation divided by the rate of disease in an unex-posed population minus 1.0.

Excess relative risk is a ratio derived from the diseaseincidence in exposed and unexposed populations. For



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example, if the lifetime rate of developing leukemia in anunexposed population is 500 cases per 100,000 (0.5%)and the rate in an exposed population is 1,250 cases per100,000 (1.25%), the excess relative risk for the exposedpopulation is (1,250/500) � 1.0 ¼ 1.5.

2. Excess absolute risk. The rate of disease in an exposedpopulation minus the rate of disease in an unexposedpopulation.

Excess absolute risk is an incidence. Using the leuke-mia data cited in the first point, the excess absolute riskfor the exposed population is 0.75%. This can also beexpressed as the number of exposures needed to harmone individual, in this case, 1/0.0075 ¼ 133.

Excess relative risk and excess absolute risk are com-plementary measures of the quantitative relationship ofexposure to risk that collectively convey the overallimportance of a risk factor. For example, if a disease has asmall background incidence rate but has a large excessrelative risk for exposure, the overall contribution ofexposure to absolute risk (the number of cases caused bythe exposure) is small because of the small backgroundincidence. On the other hand, if a disease has a largebackground incidence rate and a small excess relative riskfor exposure, the absolute risk impact of exposure may belarge in terms of the number of cases attributable to theexposure because of the large background incidence.

5.4.3. Stochastic Risk: Dose-Risk Relationships

The stochastic dose-risk relationship is an importantdeterminant of medical radiation exposure’s contributionto the future cancer risk of both patients and occupa-tionally exposed healthcare workers. The majority of ourunderstanding of this relationship in humans is derivedfrom epidemiological studies of exposed human pop-ulations. The LSS (Life Span Study), conducted by theRadiation Effects Research Foundation in residents ofHiroshima and Nagasaki over a 50-year follow-up period,relates exposure data to cancer incidence, and providessome of the best quantitative data relating dose to futurecancer risk (27).

This experience has limitations when applied to med-ical and occupational radiation exposure. The exposuresincurred by the Hiroshima and Nagasaki populationsconsisted largely of whole body radiation delivered in alarge, brief exposure that included substantial neutronexposure accompanied by residual exposure from envi-ronmental radioactivity and internalized radionuclides.

5.4.3.1. Stochastic Risk: Qualitative Dose-Risk Relationships

Epidemiological studies such as the LSS have clearlyidentified a dose-related risk for cancers, including

both leukemias and solid tumors. For example, in alarge population, an increased risk of leukemia is sta-tistically detectable at a total bone marrow dose of 1Gy, and risk increases proportionately with larger doses(28). It is noteworthy that this dose is 50 to 100 timesgreater than the estimated total bone marrow dose fora Tc-99 nuclear scintigraphy stress test study. Howev-er, x-ray fluoroscopy and x-ray CT can deliver doses ofthat magnitude to portions of a subject’s bonemarrow (29).

Most statistical models derived from epidemiologicaldata find a linear relationship between dose andincreased future cancer risk, with no dose thresholdbelow which radiation exposure makes no contribution torisk. This direct relationship has led to formulation of the“linear-no threshold” theory (15). Given the low incidencerates associated with small doses, the no thresholdconcept could be validated epidemiologically only bystudies that would employ much larger populationsample sizes than studies to date have achieved. Thelinear-no threshold model is the basis for the concept thatradiation exposure should always be minimized (12). Thisis the foundation of the ALARA principle that radiationexposure should always be maintained “As Low AsReasonably Achievable.”

Children and young adults are more sensitive to radi-ation and, accordingly, for a given exposure, have agreater risk of a radiation-induced stochastic eventcompared with the elderly. The young are more sensitiveto a given radiation exposure because they, particularlygrowing children, have greater overall mitotic activity. Inaddition, because radiation-induced cancer has a latentperiod for induction with risk potentially persistingthroughout a subject’s lifetime, young people, who have along natural life expectancy, are more likely to live longenough for a stochastic event to present. Children bornwith congenital heart disease are at greater risk than otherchildren to increased radiation exposure given theirongoing need for cardiac catheterization and otherradiation-based procedures. On the other hand, theelderly, because of a shorter natural life expectancy, maynot survive long enough for an induced stochastic eventto emerge.

The Committee to Assess Health Risks from Exposureto Low Levels of Ionizing Radiation of the NationalResearch Council has examined a number of statisticalmodels that relate incremental cancer risk to absorbedradiation dose for individual solid organ cancers andleukemia. These models also incorporate important pa-tient characteristics including age and gender. Themodels were published in the 2006 report: Biological Ef-fects of Ionizing Radiation (BEIR) VII (12).

TABLE 6Baseline Lifetime Risk Estimates of CancerIncidence and Mortality

Cancer site

Incidence Mortality

Males Females Males Females

Solid cancer* 45,500 36,900 22,100 (11) 17,500 (11)

Stomach 1,200 720 670 (11) 430 (12)

Colon 4,200 4,200 2,200 (11) 2,100 (11)

Liver 640 280 490 (13) 260 (12)

Lung 7,700 5,400 7,700 (12) 4,600 (14)

Breast — 12,000 — 3,000 (15)

Prostate 15,900 — 3,500 (8) —

Uterus — 3,000 — 750 (15)

Ovary — 1,500 — 980 (14)

Bladder 3,400 1,100 770 (9) 330 (10)

Other solid cancer 12,500 8,800 6,800 (13) 5,100 (13)

Thyroid 230 550 40 (12) 60 (12)

Leukemia 830 590 710 (12) 530 (13)

Note: Number of estimated cancer cases or deaths in population of 100,000 (No. ofyears of life lost per death). The numbers are the estimated number of cases or deathsper 100,000 people. The numbers in parentheses are the estimated number of years oflife lost per cancer death. *Solid cancer incidence estimates exclude thyroid and non-melanoma skin cancers. Reproduced with permission from BEIR VII (12).


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Although there are a variety of models with variablegoodness of fit to available datasets, they all have severalcommon features that are of pragmatic significance.

1. Risk has a graded relationship to total dose. Modelsdiffer with respect to the exact mathematical rela-tionship, but clearly show that risk increases withaccumulated dose.

2. Excess cancer incidence is statistically detectable inpopulation studies at a dose of 100 mSv in adults and insmaller doses in children (30–32). These observationssupport the concept that there is no threshold dosebelow which there is no stochastic risk.

3. Dose-response risk for solid organ cancer correlatesloosely with the organ’s intrinsic mitotic activity. Thus,the solid organs that are most radiation-sensitive are:lung, female breast, colon, bladder, and thyroid.

4. Hematopoietic tissues have high dose sensitivity and ashorter latent period for leukemia induction than solidorgans have for primary cancer induction.

5. Females have greater risk and a steeper dose-riskrelationship than males. Some, but not all of this dif-ference is attributable to breast sensitivity.

6. Risk and dose-risk relationships have a strong rela-tionship to age, with subjects <30 years of age havinggreater dose sensitivity. Children have a particularlyenhanced susceptibility to leukemia and melanoma(25). Beyond age 30, dose sensitivity is less stronglyage-related (13). However, for older subjects, amongwhom background incidence rates are greater, excessabsolute risk may still be important, even thoughexcess relative risk may be smaller

7. The length of the latent period for clinical presentationof an induced cancer may decrease the importance ofconsidering radiation-related risk for elderly patientswho have limited natural life expectancies.

5.4.3.2. Stochastic Risk: Quantitative Dose-Risk Relationships

The quantitative relationship between radiation exposureand increased cancer risk has implications both for a pa-tient undergoing a medical procedure and for an occu-pationally exposed healthcare worker. The quantitativestochastic dose-risk relationship is a component of aparticular procedure’s risk-benefit relationship. Riskmodels attempt to quantify the risk that accompaniesexposure to provide a context for understanding a givenexposed subject’s risk and the implications for occupa-tionally exposed healthcare workers.

The LSS population study has provided the largest andmost rigorously studied dataset on which to build statis-tical models that quantify risk. The BEIR VII Committeeevaluated multiple mathematical models for both cancerincidence and cancer-related mortality to achieve the best

goodness of fit to the LSS data. The models, developed fortotal solid organ cancer, individual solid organ cancers,and leukemia, incorporate gender, age at exposure, andtotal dose to predict the future cancer risk for a particularexposed subject. The models provide our best currentassessment of the quantitative relationship of radiationdose to future cancer risk.

The BEIR VII models’ general structure includes a co-efficient that relates dose linearly to risk and modulatingcoefficients that account for subject gender and age atexposure. These models can be applied to calculate ex-pected excess relative and absolute risks for a variety ofcommon scenarios. These ratios and rates provide acontext with which to judge the contribution of anexposure to a subject’s overall risk.Background Cancer Risk in the Overall Population

Table 6 displays lifetime incidence and mortality risk datafor all cancers. This provides a measure of backgroundincidence and mortality rates in unexposed populationsupon which the excess risks that accompany radiationexposure are superimposed.

Thus, an unexposed subject’s lifetime risk of devel-oping solid cancer or leukemia is approximately 46% andlifetime risk of cancer mortality is approximately 23%.Incremental Cancer Risk Attributable to Patient Medical

Radiation Exposure

The BEIR VII models calculate coefficients that estimatethe excess relative risk and excess absolute risk per Sv ofexposure. Because subject age is an important risk

TABLE 7Estimated Age-Related Gender-Averaged Incremental Risk for Solid Cancer Incidence and Mortality per Sievert ofRadiation Exposure (BEIR VII Model)

Age at Exposure, yrs <15 15-30 30-45 45-60 >60*

Incidence Risk

Excess Relative Risk/Sv 0.78 (0.58, 1.06) 0.63 (0.49, 0.81) 0.42 (0.28, 0.62) 0.43 (0.23, 0.79) 1.7 (0.76, 3.8)

Excess Absolute Risk (cases/10,000 yrs$Sv) 57 (43, 76) 40 (30, 48) 23 (16, 33) 20 (11, 36) 67 (35, 131)

Mortality Risk

Excess Relative Risk/Sv 1.12 (0.80, 1.58) 0.63 (0.46, 0.84) 0.35 (0.22, 0.55) 0.25 (0.10, 0.55) 0.55 (0.19, 1.7)

Excess Absolute Risk (cases/10,000 yrs$Sv) 29 (21, 39) 18 (14, 25) 12 (8, 19) 8 (4, 19) 17 (6, 45)

*Note: Large confidence intervals for age >60 years reflect smaller study cohort size. Adapted from BEIR VII (12).



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determinant, different age ranges have different co-efficients (the coefficients being larger for younger sub-jects). Using these coefficients, a given subject’s excessrelative and absolute risks for a given exposure can becalculated by extrapolation applying the linear-nothreshold concept.

Table 7 tabulates the gender-averaged excess relativerisk ratios and the excess absolute risk rates derived fromthe BEIR VII model for incidence and mortality for allsolid cancers (excluding thyroid and nonmelanoma skin)associated with a dose of 1 Sv calculated as described inSection 5.4.1.1. Calculations are displayed for multiple agecohorts. For example, for the population age 15 to 30years, the cancer incidence in an exposed populationwould be 63% greater than in an unexposed population.An effective dose of 1 Sv is substantially greater thantypical for even extreme patient doses that would occur inmedical practice; typical accumulated doses in clinicalmedicine would be 100 mSv or less. The calculatedprobabilities displayed here are substantially greater thanwould be expected in clinical medicine. The 1 Sv dose wasselected to illustrate the age and gender sensitivityrelationships.

Excess absolute risk is expressed as the estimatednumber of cases or deaths that would occur over afollow-up of 10,000 patient years in a patient cohortexposed to an effective dose of 1 Sv. For example, usingthis metric, an excess absolute risk of 40 cases per10,000 person-years means that a population of 1,000exposed patients would be expected to develop 40additional cancer cases (4% of the population) over a 10-year follow-up period. These cases, which would be inaddition to the background cancer incidence rate, wouldbe attributable to the exposure. It is noteworthy that theprecision of the risk estimates for subjects with ages >60years is limited by small sample sizes in the studiedpopulation. The data in Table 7 demonstrate the age

relationship to sensitivity to induced cancer. It is alsonoteworthy that, although sensitivity is substantiallygreater in children, the excess relative risk of exposure isrelatively constant for ages >30 years. The data aregender averaged, and thus, do not reflect differencesbetween males and females.

An alternative strategy to express the impact of radia-tion exposure on future cancer risk that is more clinicallymeaningful is to calculate the lifetime attributable risk forcancer incidence and mortality. This calculation yieldsthe percent of exposed patients who in the future areprojected to develop a cancer attributable to the expo-sure. Calculations can be made for both cancer incidenceand cancer mortality. Figures 2 and 3 display the results ofestimate calculations for the effect of a whole-body 100-mGy exposure (a moderately large, but more plausiblemedical exposure dose than 1 Sv) on model-predictedincidence and mortality for both genders stratified byage at exposure. The impact of the model’s gender andage at exposure variables is highly evident. Children age15 years and under are projected to have incrementalincidence rates in the range of 2% for males and 4% forfemales (it should be noted that exposures of thismagnitude should occur less frequently in children thanin adults because their smaller body size requires asmaller dose for a given examination; however, this doesnot generally occur in practice). Children who requiresurgery for congenital heart disease will undergo higherdoses than those who do not because of their ongoingneeds for evaluation (33). Model-projected incrementalmortality rates are approximately one-half of the inci-dence risks. In older patient groups, the predicted incre-mental rates are substantially smaller, but are certainlynot negligible—on the order of 0.3% to 0.5% for incidence,with smaller gender differences than in the pediatric agerange.

These data are displayed graphically in Figures 2 and 3.

FIGURE 3 Estimated Cancer Incidence and Mortality for Males Attributable to a 100-mGy Radiation Exposure as a Function of Age

Stacked bar graph depicts the lifetime attributable risk for cancer incidence and mortality for males attributable to a total body 100-mGy (100 mSv) exposure

as a function of age at exposure. Note the strong relationship between age at exposure and risk. Note also the smaller incidence and mortality rates in men

compared with women at each age range. Adapted from BEIR VII (12).

FIGURE 2 Estimated Cancer Incidence and Mortality for Females Attributable to a 100-mGy Radiation Exposure as a Function of Age

Stacked bar graph depicts the lifetime attributable risk for cancer incidence and mortality for women attributable to a 100-mGy total body (100 mSv)

exposure as a function of age at exposure. Note the strong relationship between age at exposure and risk. Adapted from BEIR VII (12).


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5.4.4. Incremental Cancer Risk Attributable to Radiation Exposure

for Occupationally Exposed Healthcare Workers

Occupationally exposed healthcare workers typicallyincur very small doses on a daily basis that can accumu-late over time to a significant exposure. Occupationalexposures are typically smaller and differently distrib-uted than patients’ medical exposures. Healthcareworkers in x-ray environments employ protective gar-ments. Consequently, their exposures are heterogeneousin terms of the exposure magnitude received by different

body parts. Healthcare workers in nuclear cardiologyincur exposure when handling radioactive materials andare at risk of exposure from radiopharmaceutical spills oraccidents.

The BEIR VII models may overstate the stochastic riskto occupationally exposed workers. Most human sto-chastic risk data (such as the BEIR VII models) are derivedfrom comparatively large exposures delivered over rela-tively short time periods. There are few observationalhuman data that assess cancer risk from long-term daily

TABLE 8Lifetime Attributable Risk for All Cancers for Two Radiation Exposure Scenarios Relevant to Occupationally ExposedHealthcare Workers (BEIR VII Model)

Exposure scenario

Incidence (%) Mortality (%)

1 mGy/yr throughoutlife (80 mGy)

10 mGy/yr ages18–65 (640 mGy)

1 mGy/yr throughoutlife (80 mGy)

10 mGy/yr ages18–65 (640 mGy)

Males 0.62 3.06 0.33 1.70

Females 1.02 4.29 0.50 2.39

Adapted from BEIR VII (12).



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small exposures. Most of the available data come fromstudies of nuclear plant operators (34). These observa-tional data have not identified an increased cancer inci-dence in this cohort of occupationally exposed workerschronically exposed at low-dose rates.

Applying the BEIR VII models to dose levels andoccupational exposure durations that are typical forhealthcare workers working in a medical radiation envi-ronment finds a small but measurable increase in futurecancer risk for occupationally exposed workers. The re-sults of these calculations are displayed in Table 8, whichdisplays estimated lifetime attributable risk for cancerinduction and cancer mortality for 2 scenarios:

1. Medical worker receives a very low dose (1 mGy/year)throughout life such as one might experience living athigh altitude. This would result in a lifetime incre-mental exposure of 80 mGy that would confer anadditional cancer mortality of 0.33% in men and 0.50%in women.

2. Medical worker receives the largest anticipatableoccupational dose of a person working in an x-rayfluoroscopic environment for his/her entire adultworking life receiving an upper range of exposure. Thiswould confer an additional cancer mortality of 1.70% inmen and 2.39% in women.

TABLE 9Recommended Exposure Limits forOccupationally Exposed Workers

Total body 20 mSv/yr averaged over defined periods of 5 yrs withno individual annual exposure to exceed 50 mSv.

Lens of the eye 100 mSv/5 yrs (20 mSv/yr)

Skin 500 mSv/yr

Hands and feet 500 mSv/yr

Adapted from the International Commission on Radiological Protection (15).

5.4.4.1. Implications of Occupational Exposure inHealthcare Workers

Although a healthcare worker’s incremental risk of anoccupational exposure-related cancer is small comparedto the background risk, the risk is dose-related and likelynot negligible at higher doses. This phenomenon un-derscores the importance of applying the ALARA principleboth to patients undergoing radiation-employing pro-cedures and healthcare workers who conduct them.

Based on the risk estimates cited in the previous text,the current exposure limits for occupationally exposedworkers recommended by the International Commissionon Radiation Protection (ICRP) are in Table 9 (15).

It should be noted that the ICRP standards (Europe) aremore stringent than the National Council on RadiationProtection (NCRP) standards (United States). Historically,

standards have become more stringent over time.Consequently, the most stringent standards arepresented.

As demonstrated in the calculations based on the BEIRVI models, a full career-length exposure at the upperrange of the ICRP limits would be associated with adetectable increased cancer risk. These calculations andestimates emphasize the importance of rigorouslyfollowing the ALARA principle.

5.4.4.2. Implications of Fetal Radiation Exposure

The human embryo and fetus undergo multiple complexprocesses (cell division, differentiation, and migration)that are sensitive to radiation effects. Consequently, thehuman embryo and fetus are more sensitive to radiationeffects than adults. This phenomenon has implicationsfor the impact of radiation exposure to both patients andto occupationally exposed workers who are known to beor who may be pregnant.

Knowledge of the effects of ionizing radiation on thehuman embryo and developing fetus is derived frommultiple sources including the Hiroshima, Nagasaki, andChernobyl experiences, as well as radiation of pregnantexperimental animals (35). Detrimental radiation effectsinclude embryonic death, fetal malformations, impairedfetal development (particularly neurological), andincreased risk of future cancer (12,36,37). The type ofevent and its dose-risk relationship is variable throughoutthe stages of pregnancy and is summarized in Tables 10and 11 (38–40).

The principal risk of radiation exposure to the earlyembryo during the blastogenesis phase of development isintrauterine death, which would be experienced as failureto establish a pregnancy, as at this stage, critical injury to

TABLE 10 Estimates of Adverse Embryonic and Fetal Events as a Function of Fetal Radiation Dose

Acute Radiation Dose*tothe Embryo/Fetus

Time Post Conception

Blastogenesis (up to 2 wks)Organogenesis

(2–7 wks)

Fetogenesis

(8–15 wks) (16–25 wks) (26–38 wks)

< 0.05 Gy (5 rads)† Noncancer health effects NOT detectable

0.05-0.50 Gy (5-50 rads) Incidence of failure to implant mayincrease slightly, but survivingembryos will probably have nosignificant (noncancer) healtheffects

n Incidence ofmajor malfor-mations may in-crease slightly

n Growth retarda-tion possible

n Growth retardationpossible

n Reduction in IQpossible (up to 15points, dependingon dose)

n Incidence of severemental retardationup to 20%.depending on dose

Noncancer health effects unlikely

> 0.50 Gy (50 rads)The expectant mother may

be experiencing acuteradiation syndrome inthis range, depending onher whole-body dose.

Incidence of failure to implant willlikely be large.‡ depending ondose, but surviving embryos willprobably have no significant(noncancer) health effects

n Incidence ofmiscarriage mayincrease,depending ondose

n Substantial riskof major malfor-mations such asneurological andmotordeficiencies

n Growth retarda-tion likely

n Incidence ofmiscarriage prob-ably will increase,depending on dose

n Growth retardationlikely

n Reduction in IQpossible (>15points, dependingon dose)

n Incidence of severemental retardation>20%, dependingon dose

n Incidence of majormalformations willprobably increase

n Incidence ofmiscarriage may in-crease, dependingon dose

n Growth retardationpossible, dependingon dose

n Reduction in IQpossible, dependingon dose

n Severe mentalretardationpossible, dependingon dose

n Incidence of majormalformations mayincrease

n Incidence ofmiscarriage andneonatal deathwill probably in-crease depend-ing on dose§

Note: This table is intended only as a guide. The indicated doses and times post conception are approximations. *Acute dose: dose delivered in a short time (usually minutes).Fractionated or chronic doses: doses delivered over time. For fractionated or chronic doses the health effects to the fetus may differ from what is depicted here. †Both the gray (Gy)and the rad are units of absorbed dose and reflect the amount of energy deposited into a mass of tissue (1 Gy ¼ 100 rads). In this document, the absorbed dose is that dose received bythe entire fetus (whole-body fetal dose). The referenced absorbed dose levels in this document are assumed to be from beta, gamma, or x-radiation. Neutron or proton radiationproduces many of the health effects described herein at lower absorbed dose levels. ‡A fetal dose of 1 Gy (100 rads) will likely kill 50% of the embryos. The dose necessary to kill100% of human embryos or fetuses before 18 weeks’ gestation is about 5 Gy (500 rads). §For adults, the LD50/60 (the dose necessary to kill 50% of the exposed population in 60days) is about 3-5 Gy (300-500 rads) and the LD100 (the dose necessary to kill 100% of the exposed population) is around 10 Gy (1000 rads). Reproduced with permission from theCenters for Disease Control and Prevention (41).


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a small number of cells is likely to be lethal. Exposureduring the organogenesis phase has the potential to causefetal malformations. Later exposure during the feto-genesis phase can cause growth retardation and impairedneurological development, and can potentially increasethe fetus’ future cancer risk.

TABLE 11Estimated Risk for Cancer from PrenatalRadiation Exposure

Radiation DoseEstimated ChildhoodCancer Incidence*†

No radiation exposure above background 0.3% 38%

0.00–0.05 Gy (0–5 rads) 0.3%–1% 38%–40%

0.05–0.50 Gy (5–50 rads) 1%–6% 40%–55%

> 0.50 Gy (50 rads) >6% >55%

Estimated lifetime‡ cancer incidence§ (exposure at age 10 years). The right columntabulates the estimated lifetime incidence of cancer for the same exposure incurred atage 10 for comparison to the estimated childhood incidence from fetal exposure. *Datapublished by the International Commission on Radiation Protection. †Childhood cancermortality is roughly half of childhood cancer incidence. ‡The lifetime cancer risks fromprenatal radiation exposure are not yet known. The lifetime risk estimates given are forJapanese males exposed at age 10 years from models published by the United NationsScientific Committee on the Effects of Atomic Radiation. §Lifetime cancer mortality isroughly one third of lifetime cancer incidence. Reproduced with permission from theCenters for Disease Control (41).

In considering these risks, it is important to link therisk to threshold radiation doses. This knowledge base hasbeen summarized by the Centers for Disease Control andPrevention in Table 10 (41). In this table, dose ranges areexpressed in Gy rather than in Sv, as the Sv construct isnot applicable to embryos and fetuses.

The increased childhood cancer risk caused by fetalradiation exposure is less well characterized, andwhether or not fetal radiation exposure might confer alifelong increased cancer risk is not known. Estimatesof childhood cancer risk are summarized in Table 11. Theavailable data indicate minimal detectable childhood riskat fetal doses <50 mGy but increased risk at doses >50mGy.

A general synthesis of the fetal radiation dose data in-dicates that fetal doses <50 mGy (as distinguished frommaternal exposures to other body regions) are not asso-ciated with a detectable increase in frequency of anyadverse fetal outcomes. For external beam maternal ex-posures (x-ray fluoroscopy and x-ray CT), fetal exposuresare substantially less than the exposure to the imaged orunshielded body region unless the uterus is directly in the



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imaged field or, in the case of healthcare worker occupa-tional exposure, if the mother’s abdomen is not shielded(discussed in Section 6). Fetal doses between 50 and 500mGy are associated with detectable increased frequenciesof all of the adverse fetal effects. Fetal doses >500 mGyare likely catastrophic (and may well be catastrophic tothe mother as well) and are unlikely to occur in medicalradiation circumstances.

The American College of Radiology (ACR) providesguidance about the need to screen for pregnancy prior toperforming a diagnostic test involving ionizing radiation.Prior to an examination, all patients of menstrual age(typically ages 12 through 50 years) should be questionedabout pregnancy status using a standardized questionnaireand/or direct questioning by the technologist with docu-mentation added to the medical record. Most diagnosticstudies deliver far less than 20mGy to the uterus, includingsingle-phase CT studies of the abdomen (42). However, afluoroscopic interventional procedure may deliver dosesabove 100 mGy and demands planning and caution. Forsuch studies, the ACR recommends a pregnancy test beobtained within 72 hours of the procedure, unless medicalurgency prevents it. Chest radiography during the first andsecond trimesters, and extremity or head and neck radi-ography,may not be altered by pregnancy status andwouldnot require pregnancy testing (42).

The most effective way to limit radiation exposure tothe pregnant patient is to consider the indications andnecessity for a particular examination, carefully weighingthe risks and benefits. Cardiovascular imaging teamsshould follow the screening and counseling recommen-dations established for other ionizing radiation imaging.Diagnostic x-rays pose no risk to lactation (43). In addi-tion, lactating women do not need to discontinuebreastfeeding after receipt of intravascular iodinatedcontrast because <1% will be excreted into breast milkand <1% of that will be absorbed by the infant (43).

For nuclear imaging, there are concerns about theadministration of radioactive iodine as it readily crossesthe placenta, preferentially accumulates in the thyroidgland, and has a half-life of 8 days, and thus may injurethe fetal thyroid gland. Tc-99m, however, which has ashorter half-life, will cause less fetal exposure. For nu-clear cardiology perfusion imaging, Rb82 positron emis-sion tomography (PET) would cause the least fetalexposure.

6. MODALITY-SPECIFIC RADIATION EXPOSURE

DELIVERY

6.1. General Principles

6.1.1. Characteristics of Medical Diagnostic Radiation

The radiation employed in all cardiovascular diagnosticmodalities (x-ray fluoroscopy, x-ray, CT, and nuclear

cardiology imaging techniques) is low linear energytransfer radiation. Ninety-five to ninety-nine percent ofx-ray energy that enters the subject is either absorbed orscattered within the subject. The remaining 1% to 5% ofthe incident x-ray penetrates the subject, reaching theimage detector to form the image. Absorption is requiredto form diagnostic x-ray images, but degrades nuclearscintigraphy images. For both diagnostic x-ray and nu-clear scintigraphy, the majority of radiation energyreleased by the x-ray tube or radioactivity administered tothe subject either exposes the subject or is scattered outof the subject with the potential to expose nearby medicalpersonnel.

6.1.2. Tools Used to Estimate Absorbed Dose

Estimates of absorbed dose for x-ray fluoroscopy andx-ray CT are based on models developed by exposinginstrumented phantoms to incident x-ray beams thatreplicate the beams used in diagnostic imaging, andmeasuring absorbed dose at different points within thephantom. These models are applied to calculate an esti-mated absorbed dose based on the incident beam in-tensity, photon energy characteristics, exposure time,and the size of the exposed body region. Model-derivedestimates are just that—estimates. Since models do notinclude all of the important dose-determining variables,actual absorbed doses may vary considerably fromestimates.

Estimating absorbed dose from radionuclides is anentirely different discipline that is discussed in Section 6.4.

6.2. X-Ray Fluoroscopy

6.2.1. X-Ray Fluoroscopy Subject and Operator Dose Issues

X-ray fluoroscopy differs from other ionizing radiationimaging techniques in that the beam entrance port isrelatively small. Consequently, the skin at the beamentrance port is the most intensely exposed tissue.Subject skin doses can reach levels that cause skin tissuereactions (see Section 5). Typically, x-ray fluoroscopy isthe only imaging technique with the potential to causesuch a reaction. X-ray fluoroscopy also has the potentialto deliver a dose to internal structures in the imagingfield remote from the beam entrance port that is largeenough to cause stochastic effects. X-ray photons arealso scattered within the subject and deliver dose tosubject tissues that are outside the imaging field.Consequently, assessment of the implications of subjectexposure from x-ray fluoroscopy must consider entranceport skin dose, which is the dose received by internalstructures within the imaging field and by other internalstructures outside the imaging field. Other scatteredphotons exit the subject and can expose nearby medicalpersonnel.

FIGURE 4 Diagrammatic Representation of an X-Ray Fluoroscopy System to Illustrate X-Ray Exposure Modality

The primary beam, collimated to a rectangular cross section, enters the patient, typically through the patient’s back. It is attenuated and scattered within the

imaging field. The primary beam exposes the subject within the imaging field. The scattered primary beam radiation can expose structures within the subject

that are remote from the imaging field.


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6.2.2. Basics of Operation of an X-Ray Cinefluorographic Unit

An x-ray cinefluorographic unit generates controlledx-rays in an x-ray tube that are collimated to regulate thesize and shape of the x-ray beam. The beam passesthrough the subject forming images that are detected by aflat panel detector (Figure 4). The x-ray tube output (andaccordingly the exposure to the subject) is modulated byfeedback circuitry from the unit’s imaging chain to ach-ieve an optimally exposed image.

There are 2 different x-ray fluoroscopic systemparameters that are used to characterize x-ray exposureand dose:

1. Cumulative dose at the interventional reference point.This parameter is a measure of the radiation dose(expressed as air kerma) that enters the subject. It ismeasured and displayed in real time by current state-of-the-art x-ray fluoroscopic systems.

2. Dose at the image detector. This is a measure of theattenuated radiation dose that penetrates the subjectand reaches the detector to form the image. It istypically <5% of the dose at the interventional refer-ence point.

6.2.2.1. X-Ray Cinefluorographic Unit Operating Parameters

There are multiple imaging parameters that influence thex-ray exposure associated with an x-ray cinefluorographicexamination. These are:

1. X-ray image detector dose per pulse. This is the dosefor each x-ray pulse (typically measured in nanogray

(nGy) that reaches the x-ray system detector. Thisparameter is set by the x-ray unit calibration and is adeterminant of image clarity and detail (discussed ingreater depth in Section 7.2). It is important to pointout that the detector dose is considerably smaller thanthe subject dose, as generally #5% of the incident ra-diation penetrates through the subject and reaches thedetector.

2. X-ray unit framing (pulsing) rate. This is the number ofpulses that the x-ray system generates per unit of time.This is an operator-selectable parameter that generallyranges between 4 and 30 pulses/second and is adeterminant of image temporal resolution.

3. Imaging field size. This is the cross-sectional area ofthe x-ray beam that impinges on the subject. This isdiscussed in greater depth in the section “Kerma-AreaProduct.”

4. X-ray beam filtration. An x-ray tube produces aspectrum of x-ray photon energies. The lower-energyphotons (photon energies <30 keV) do not havesufficient penetrating power to reach the detector,and thus expose the subject without contributing toimage formation. These “undesirable” photons aretypically “filtered” out of the x-ray beam by inter-posing layers of aluminum and copper in the x-raytube exit port.

The x-ray dose delivered to a subject during a fluo-roscopic examination is determined by the combinationof the previously mentioned parameters and also by

FIGURE 5 Diagram Showing the Estimated Decreasing Intensity of

X-Ray Exposure With Depth Within the Subject



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subject characteristics. Larger doses per pulse, largerfield sizes, and faster framing rates all increase subjectexposure.

Because x-ray systems are calibrated to image with aparticular detector dose, the intrinsic radiographic den-sity of the subject influences the dose that must bedelivered to the subject to achieve a particular dose thatreaches the detector. Thus, all other parameters being thesame, a larger, heavier subject will receive a larger dosethan will a smaller, lighter subject.

In this example, (right anterior oblique projection), the beam enters

the left side of the subject’s back. Beam intensity decreases with depth

within the subject due to a combination of beam divergence with

distance (inverse square law) and absorption within the subject. The

overall effect of these processes is to attenuate the beam intensity

that exits the subject to 5% or less of the incident intensity.

6.2.3. Measures and Determinants of Subject and

Operator Exposure

6.2.3.1. Cumulative Air Kerma at the InterventionalReference Point

The cumulative air kerma of the incident x-ray beam atthe interventional reference point (described in Section4), is the basic measurement used to calculate estimatesof subject entrance port skin dose and is the metric thatmost accurately estimates tissue reaction risk. It is ameasure of the energy density (measured in Gy) of theradiation that enters the subject. X-ray fluoroscopic sys-tems sold in the United States since 2006 are required tomeasure or calculate and display an estimate of thisparameter both instantaneously and cumulatively for acomplete examination. Operators can potentially applythis information to make procedure conduct decisions,enabling them to weigh the risk of a skin injury againstthe importance of continuing a procedure. This informa-tion can also be used to inform a patient of his/herpotential skin injury risk.

Cumulative air kerma at the interventional referencepoint is a much more meaningful subject exposureparameter than the total fluoroscopic time, which doesnot account for subject density, cine acquisition time, orchanges in frame rate or angulation. The x-ray system’sautomatic exposure controller determines the x-rayexposure intensity. Exposure intensity is determined bythe system’s set detector dose and the subject’s intrinsicradiological density. Subject density is determined bybody habitus (larger patients are more dense) and pro-jection angle (extremes of obliquity and skew requiregreater dose). Consequently, the exposure rate deliveredduring an x-ray fluoroscopic examination can vary over alarge range.

Subject skin exposure within the beam entrance port issomewhat greater than the air kerma at that location. Thisis because some of the radiation that enters the subject isscattered in the opposite direction (backscatter). Thebackscattered radiation sums with the incident radiation

from the primary beam, increasing entrance port skinexposure. Depending on x-ray field size, entrance skinexposure is typically around 10% to 40% greater thanincident air kerma.

X-ray exposure to the subject is not uniform. As an x-ray beam passes through a subject, tissue absorption at-tenuates its intensity (see Figure 5). Consequently, tissuecloser to the beam entrance port receives a larger dosethan deeper-lying tissue and tissue at the beam exit port(closest to the imaging detector) receives the smallestdose. The dose delivered to a location deeper within thesubject’s body is determined by the beam intensity andthe specific x-ray absorbance of the tissue at that location.For soft tissue, the dose is about 6% greater than the airkerma at that location. Bone absorbs x-ray more avidlythan soft tissue. Consequently, dose to bone is 3 to 4 timesgreater than air kerma at that location. Model-based esti-mates of the relationship between incident air kermaexposure and absorbed dose at deeper subject structurescan be applied to calculate an estimate of the dose deliv-ered at more deeply located structures within the subject.

Because of scattering within the subject, some scat-tered x-ray is also directed to locations outside the area ofthe x-ray beam and exposes tissue outside the imagingarea (see Figure 4). The magnitude of this exposure is

FIGURE 6 Diagrammatic Representation of the Pattern of X-Ray Scatter From a Subject Undergoing X-Ray Fluoroscopy

Note that scattered x-ray emanates from the subject in all directions.


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considerably less than that of tissue within the imagingarea. Scattered radiation that leaves the subject’s body isthe principal source of exposure to nearby medicalpersonnel (Figure 6).

6.2.3.2. Kerma-Area Product

KAP, sometimes referred to as dose-area product(described in Section 4), is commonly used as a metric toestimate a subject’s total absorbed dose. It incorporatesboth dose and exposed tissue volume into a single mea-surement. Air kerma is a measure of dose intensity(measured in J/kg) reflecting the number of ionizationevents per unit mass of exposed tissue. However, it doesnot measure the quantity of tissue that receives thatdensity of radiation energy, and thus, does not measurethe total quantity of ionizing radiation that impinges on asubject. For a given incident radiation exposure, thelarger the quantity of tissue that receives that exposure,the more tissue ionization events will occur conferring agreater potential for a stochastic event.

KAP is expressed in units of Gy $ cm2. It is calculated bymultiplying the beam air kerma by its cross-sectionalarea. It should be noted that some x-ray system manu-facturers report KAP in units of mGy$m2 (1 Gy$cm2 ¼ 100mGy$m2). It should also be noted that air kerma and KAPrepresent cumulative doses from an exposure, not expo-sure rates.

An important characteristic of KAP is that its value ina diverging x-ray beam is independent of the distance

from the source. This is because the beam intensity(air kerma) decreases proportionally to the square of thedistance while the beam area increases with square ofthe distance.

KAP is affected both by air kerma output and by colli-mation or field size. Thus, if exposure intensity is con-stant, reduction in exposure field size decreases KAP. At aconstant exposure field size, increasing radiation outputincreases KAP. This phenomenon underscores theimportance of minimizing the exposed field size in x-rayfluoroscopic examinations.

6.2.3.3. Application of KAP in Cardiovascular X-RayFluoroscopy to Estimates of Effective Doseto Patients

The total cumulative KAP is the best readily availablemeasurement of the total radiation dose delivered to asubject by an x-ray fluoroscopic procedure. A number ofassumptions are involved in estimating an effective dosevalue from a KAP measurement. General estimatesof the relationship between KAP exposure to thethorax in Gy$cm2 and effective dose in Sv made usingmeasurements in phantoms derive a coefficient of0.20 mSv/Gy$cm2 (44). By this estimate, a combinationcoronary arteriography and percutaneous coronaryintervention that delivers a KAP exposure of 50 Gy$cm2

would impart an effective dose to the subject of 10 mSv.The implications of that dose magnitude are discussedin Section 5.4.



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6.2.4. Measures and Determinants of Physician Operator and

Healthcare Worker Occupational Exposure

6.2.4.1. Application of KAP in Cardiovascular X-RayFluoroscopy to Estimates of Effective Dose toMedical Personnel

Medical personnel who conduct x-ray fluoroscopic pro-cedures are rarely exposed directly to the primary x-raybeam. They are exposed by scattered radiation that em-anates from the patient (Figure 6). The scattered radiationintensity is directly related to the KAP rate, and the totalscattered radiation for a procedure is related to the pro-cedure’s cumulative KAP. The amount of scattered radi-ation that reaches and delivers absorbed dose to medicalpersonnel is determined by:

1. The distance of the exposed medical personnel from thex-ray source—scattered x-ray intensity decreases pro-portionately to the square of thedistance from the source.

2. The effectiveness of shielding employed by theexposed medical personnel.

6.2.4.2. Physician andMedical Personnel ExposureMonitoring

For exposed medical personnel, estimates of actual dosesdelivered to different structures and organs cannot bemeasured directly and must be calculated from modelsderived from instrumented phantoms. Most estimates arebased on measurements made by personal radiation moni-tors (formerly known as “film badges”), which can be wornboth outside protective garments (at the collar level on theleft side) andunder protective garments (atwaist level). Theoutside badge measures the dose that reaches unshieldedstructures of the head while the badge underneath theapron measures the radiation level that penetrates theprotective apron reaching the subject. Effective dose can beroughly estimated from the reading of a single badge wornoutside the apron at the collar (neck) or it can be moreaccurately estimated by combiningmeasurements from the2 badges. Typically, the under-apron badge reading, rep-resented as H(u) and measured in units of mSv, shouldbe <5% of the collar badge reading for an 0.5-mm lead-equivalent protective apron. The collar badge reading isrepresented by H(col), also measured in units of mSv.

Several different, but similar, models for both single-and dual-badge applications have been developed. TheNCRP (Reports 122 and 168) recommends the followingformula for estimating effective dose—E(estimate)—fromsingle- or dual-badge readings (38,45):

For a single-badged worker, E in units of mSv may beestimated as:

EðestimateÞ ¼ HðcolÞ.21

The divisor of “21” in this formula is consistent with thefact that the protective garments intercept approximately

95% of the incident radiation. This formula assumes that athyroid collar is worn shielding cervical bone marrow andthe thyroid. Other models developed for no thyroid collarshielding have divisors of 14 (46,47). The difference be-tween these 2 divisors emphasizes the importance of thethyroid collar.

When 2 badges are worn, one under the apron and theother over the apron around neck level, E in units of mSvis estimated as:

EðestimateÞ ¼ 0:5 HðuÞ þ 0:025 HðcolÞ

These calculations assume the presence of a thyroidshield.

Caution is advised regarding regulatory requirementsin the assessment of E from personal radiation monitors.Regulatory agencies may adopt rules for calculation ofeffective dose (or the closely related, older quantity calledeffective dose equivalent) that differ greatly fromthose described here. This text reflects the NCRP’srecommendations for effective dose (as opposed to thesomewhat different effective dose equivalent). Regula-tory agencies do not have to adopt the recommendationsof the NCRP in their rules and may choose more restrictivemethods of estimating regulated personnel exposures.

6.2.4.3. Exposure Levels for Operating Physicians

Multiple studies have endeavored to identify a quanti-tative relationship between a procedure’s cumulativeKAP and the effective dose to operating personnel. Thereis necessarily considerable variation among estimates.Sources of variation include operators’ distance from thex-ray source and the degree and effectiveness with whichoperators employ shielding to protect themselves. Thedose to operators per Gy$cm2 to the patient hasdecreased moderately over time with equipmentimprovement, increased operator awareness, and betterutilization of shielding. The most current studies find arange of 0.02 to 0.12, with typical values clustering about0.1 uSv/Gy$cm2 cumulative dose (48). (Note that theestimated patient exposure is 200 uSv/Gy$cm2 suggest-ing that patient exposure is roughly 2000 times operatorexposure.) Applying these values, a “typical” combinedcoronary arteriogram and straightforward coronaryinterventional procedure utilizing a cumulative KAP of50 Gy$cm2 would deliver a 5 uSv effective dose to thephysician operator standing roughly 1 meter from thecenter of the primary beam while delivering 10 mSv tothe patient.

Observational data indicate a per-procedure operatordose range from 0.2 to 38 uSv with typical median valuesbeing 5 uSv (11,48). An operator performing 500 proced-ures/year at a typical effective dose of 5 uSv per proced-ure, would be expected to receive a total effective dose of

allocgr


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2.5 mSv/year—well below the ICRP recommended doselimit of 20 mSv/year. It is important to point out thatthese estimates are based on effective use of all protectivemeasures.

These estimates of operating physician exposure arerepresentative for physician operators who are in theclosest proximity to the radiation source. The exposure toother medical personnel who are in the procedure roombut not in immediate proximity to the radiation sourcewould be expected to be less—decreasing as the square ofthe distance from the source. Thus, a scrubbed assistant,who would be approximately twice as far from the source,would be expected to receive roughly one-fourth theexposure of the primary operator. Personnel who arecirculating in the room at a distance at least 4 times thedistance from the source as the primary operator wouldbe expected to receive roughly 1/16 or less of the dosereceived by the primary operator.

6.2.4.4. Radiation Protection Considerations for Pregnant orPotentially Pregnant Occupationally Exposed Workers

As discussed in Section 5.4.4, although radiation expo-sure to the human embryo and fetus should always beminimized (as is the case with all human exposure), nomeasurable increase in adverse fetal outcomes has beendetected at fetal or embryonic exposures <50 mGy. Foroccupationally exposed workers in an x-ray fluoroscopyenvironment, proper shielding and practices should keeputerine exposures well below this level for the duration ofa pregnancy. Because the uterus is a deep structure and isinside protective garments, the dose to the uterus deliv-ered by scattered x-ray is greatly attenuated comparedwith the dose to unshielded areas. Measurements made inphantoms indicate that the uterine dose in a subjectwearing a 0.25-mm lead equivalent apron is <2% of thecollar dose (outside protective garments). Thus, for anoccupationally exposed worker to receive a uterine doseof 50 mGy would require an accumulated collar badgedose of 2.5 Gy.

A pregnant female occupationally exposed workershould wear, in addition to the customary collar filmbadge, an abdominal badge worn under the apron toestimate the uterine dose. This will verify that the uterinedose is within the range that is considered to be safe forthe fetus. Detailed recommendations for protection of thepregnant or potentially pregnant worker in interventionalradiology have been published, and these are directlyrelevant to cardiology operators and staff (49). Unfortu-nately, a recent survey indicates low adherence to suchrecommendations. Only 20% of women reported the useof fetal radiation badges while participating in radio-graphic imaging procedures while pregnant, and 24% re-ported using additional lead protection. Over 60%reported either having no workplace policy on pregnancy

or being unaware if their department had a policyregarding radiation exposure during pregnancy (50).Thus, although it is clear that with adequate precautionsand protection a pregnant healthcare worker can work inan x-ray fluoroscopy environment without detectablyjeopardizing her fetus (51,52), all workplaces shoulddevelop and enforce training and policies regardingmonitoring and radiation reduction procedures for preg-nant operators and staff (50). Thus, it is clear that withadequate precautions and protection a pregnant health-care worker can work in an x-ray fluoroscopy environ-ment without detectably jeopardizing her fetus (51,52).

6.3. X-Ray CT

6.3.1. X-Ray CT Subject and Operator Dose Issues

Although x-ray CT, like x-ray fluoroscopy, is an externalbeam exposure technique, unlike x-ray fluoroscopy, theincident beam is distributed circumferentially around thesubject. Consequently, x-ray CT subject skin doses shouldnever approach levels that could cause skin injury.(The potential for skin injury exists if scanners areimproperly calibrated or if multiple scans of the same bodyregion are performed close together in time.) Thus, for thex-ray CT subject, harm issues should be confined to sto-chastic risk. Similarly, x-ray CT clinical operating personnelare not routinely in the room with the subject duringexposure so personnel exposure should be negligible.

6.3.2. Basics of Operation of an X-Ray CT Unit

The dose delivered by an x-ray CT examination can varysubstantially depending on patient characteristics and thesettings of multiple scanner operating parameters. Con-figurable CT technique parameters that can affect doseinclude x-ray tube potential (measured in kV); x-ray tubecurrent (measured in milliamperes [mA]); and scan pro-tocol, for example, axial or helical, pitch, gating protocol,scan rotation time, beam width, scan length, and beamfiltration.

Image quality is affected by imaging parameter selec-tion. This selection involves a conscious tradeoff betweenimage quality and dose. Other parameter selections, suchas gating protocol, do not necessarily affect image qualitybut do affect the amount of radiation used to acquire animage set.

Electrocardiographic (ECG) gating is important in car-diovascular imaging to minimize motion artifact. Gatingprotocol selection can have a major influence on subjectdose in cardiovascular imaging.

There are 2 types of gating (Figure 7):

Retrospective gating involves x-ray exposure continu-
y over the cardiac cycle. Because exposurecurs continuously, retrospective gating deliverseater exposure than prospective triggering.

Reuoasstuprcoamancade

toprwhnoprne

FIGURE 7 Comparison of Retrospective ECG Gating With Prospective ECG Gating

With retrospective gating, the intensity-modulated x-ray beam is on for the entirety of the R-R intervals during imaging. With prospective gating, the x-ray

beam is on for about 26% of every other R-R interval. Reproduced with permission from Shuman et al. (53).



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trospective gating can be valuable when contin-us 4-dimensional data are needed for functionalsessments, analyses of intracardiac shunts, anddies directed toward cardiac valves or valveostheses. When retrospective gating is used forronary CT angiography, images with the smallestount of motion-related artifacts are used toalyze coronary anatomy; however, all image setsn be available to assess myocardial motion, ifsired.

Prospective triggering involves synchronizing exposure
a selected portion of the cardiac cycle. The goal ofospective triggering is for exposure to occur onlyen cardiac motion is minimal. For studies that aret intended to assess motion-related functionality,ospective triggering, if successful, can avoid un-cessary exposure.
As with x-ray fluoroscopy, x-ray CT images can beacquired at different dose levels with concomitant impacton image noise. Operators can select among dose levelsdepending upon the study’s purpose. Coronary computedtomography angiography (CTA) requires greater spatialresolution than do examinations for basic cardiovascularstructure. Imaging protocols and their impact on patientdose are discussed in greater detail in Section 7.

6.3.3. X-Ray CT Measures of Subject Exposure

As x-ray CT imaging technology has evolved, it hasbecome apparent that multiple dose parameters are

needed to precisely specify the dose delivered by an ex-amination. The dose delivered by an x-ray CT examina-tion should be considered from 2 perspectives:

n Dose intensity: Dose per unit mass of tissue. This is ameasure of the intensity of the dose used to generatethe images and is determined by the combination ofscanner parameter settings and subject characteristics.

n Volume of Tissue Exposed During the Examination.

This incorporates, in addition to the dose intensity, thesubject volume scanned. The total dose delivered to asubject is the product of the dose intensity and thevolume of tissue exposed.

There are a number of different metrics in use asmeasures of subject dose in x-ray CT. It is noteworthy thatmany of these metrics are derived from the measurementof x-ray tube air kerma. However, in the CT lexicon, theterm “dose” is very widely used. Accordingly, thisdiscussion will use the term “dose” rather than “kerma.”

6.3.3.1. CTDI: A Measure of Dose Intensity

Several related CT dose index terms have been formu-lated. CTDI was first defined in 21 CFR 1020.33(c) as theaverage dose detected over a 100-mm length in a phan-tom from an imaging acquisition of 14 slices (notnecessarily a 100-mm imaging length). Thus, CTDI is anindex of dose imparted by a unit scan length. All CTDIterms are measures of dose intensity, not measures ofthe total dose delivered to a subject. Given that scanlengths vary depending on the purpose of the


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examination, the actual subject dose will also be pro-portional to the scan length.

6.3.3.2. CTDI100

CTDI100 is a refinement of CTDI that standardizes all doseindex measurements to a standardized scan length of 100mm. However, this definition also has some utility prob-lems. The dose delivered by an x-ray CT examination isnot uniform across the exposed subject volume. Doses atthe more superficial locations within the exposed volumeare substantially greater than doses at deeper locationscloser to the exposed volume center.

6.3.3.3. CTDIw

The CTDIw, or weighted CTDI100, is an index developed toapproximate the average radiation dose delivered to across section of a subject’s body. It allows for nonunifor-mity of dose with depth. CTDIw is a modification toCTDI100 in which the peripheral dose and center dose areadded together in a weighted fashion (2/3 peripheraldose þ 1/3 central dose).

6.3.3.4. Volume CTDI

Pitch is a fundamental characteristic of helical scanningand is defined as the ratio of the scanning beam width (inmm) to the length of table movement during 1 gantryrotation. If the tablemovement length is equal to the widthof the scanning beam, there is no beam overlap betweenslices and the pitch is 1.0. If the beam width is greater thanthe table movement, there will be beam overlap betweenrotations and the pitch will be <1.0. If the beam width isless than the table movement, the pitch will be >1.0.

Pitch determines how the scanning mode distributesradiation along the scan length. A value of pitch <1.0delivers a higher overall dose and a value >1.0 deliverslower overall dose.

To account for the impact of pitch on dose, a newrefinement to CTDIw, CDTIvol, was introduced. It is simplycalculated as CTDIw/pitch. This is the dose index mostcurrently used.

CTDIvol is the weighted absorbed dose to air of a 1-cmaxial length of the examined subject located in the mid-dle section of a 100-mm length scan of an acrylic cylinderfor a specific CT technique. Its unit of measure is the mGy.It accounts for both the exposure directly delivered to the1-cm thick slice and also for exposure delivered to thatslice by scatter from adjacent imaged tissue. The cylindermust be specified as either 32- or 16-cm diameter andmust be positioned in the center of scanning bore of theCT unit during measurement.

CTDIvol Special Considerations for Exposure

in Children

It is noteworthy that for identical techniques, smallersubjects receive a higher dose than larger subjects.

Estimates of CTDIvol for body imaging made utilizing a 32-cm thick phantom substantially under-represent the dosereceived by small individuals, especially children.Although using a 16-cm phantom might provide a moreaccurate assessment of the actual amount of radiationdelivered to small subjects, the 32-cm phantom iscurrently specified by the International ElectrotechnicalCommission and is most commonly used. As an approxi-mation for small subjects, the dose index measured usingthe 16-cm phantom will be about a factor of 2 greater thanthe dose index measured using a 32-cm diameter phan-tom. For similar reasons, the use of a 32-cm phantom mayresult in overestimation of dose for subjects with thoracicdimensions >32 cm.

6.3.3.5. Size-Specific Dose Estimate

Initial efforts to develop a dose index that better reflectsdifferences in subject habitus have led to the introductionof a quantity called the size-specific dose estimate. Thesize-specific dose estimate is a normalization of CTDIvolthat takes into account subject size. This may become amore important measure for radiation management in thefuture. However, current scanners do not yet automati-cally determine or report the size-specific dose estimate.Its incorporation into practice is still to be determined.

6.3.3.6. DLP: A Measure of the Total Dose Absorbedby the Subject

CTDIvol is a measure of dose intensity that, when multi-plied by the longitudinal length of the scan, provides ameasure of dose received, the dose-length-product (DLP).Thus, if the scan length for a procedure is 30 cm andCTDIvol is 20 mGy, then the DLP is 600 mGy$cm. If thescan length were only 15 cm, using the same protocol theDLP would be 300 mGy$cm. DLP is a better predictor ofstochastic risk when compared with CTDI.

6.3.4. X-Ray CT Measures of Effective Dose

The benefits and shortcomings of effective dose(expressed in units of mSv) as an indicator of stochasticrisk are addressed in Section 4.5. For CT imaging, Euro-pean Commission–sponsored guidelines from 2000 (54)and 2004 (55) have suggested a simple approximation ofthe effective dose that can be obtained by multiplying theDLP by a conversion factor k (unit: mSv $ mGy�1 $ cm�1)that varies dependent on the radiation sensitivity ofdifferent body regions and patient ages. The Europeanguideline documents offer conversion factors for CT of thehead, neck, chest, abdomen, pelvis, and legs, all based onMonte Carlo simulations of single-slice CT scanners andthe then-current definition of effective dose, which wassubsequently updated with the introduction of the newtissue weighting factors (Table 3) in ICRP Publication 103(15). Conversion factors for chest CT in these documents



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ranged from 0.014 to 0.019 mSv $ mGy�1 $ cm�1 in adults,and 0.013 to 0.039 mSv $ mGy�1 $ cm�1 in children. The0.014 mSv $ mGy�1 $ cm�1 factor, which was subsequentlyincluded in American Association of Physicists in Medi-cine guidelines (56), is most commonly used for chest CTin adults. However for CT examinations confined to thecardiac region, such as most coronary CT angiography andcalcium scoring scans, since the cardiac region is moreradiosensitive than the rest of the chest, several studies(57–61) have demonstrated that estimates of cardiac con-version factors are considerably greater, ranging from0.017 to 0.043 mSv $mGy�1 $ cm�1 in adults, depending onthe scanner, protocol, and methodology used, with anaverage figure of 0.026 mSv $ mGy�1 $ cm�1 (62).

6.3.4.1. X-Ray CT Measures of Effective Dose in Children

Pediatric CT dosimetry is complicated by the fact thatscanners and studies have variably used 32- or 16-cmphantoms for the determination of DLP (see Section6.3.3.4). For that reason, when reporting CTDI or DLP inchildren, the phantom size used should always be speci-fied. In children, European guidelines for chest CT con-version factors (63), based on the 32-cm phantom, rangefrom 0.013 mSv $ mGy�1 $ cm�1 (10 years) to 0.039 mSv $

mGy�1 $ cm�1 (0 years), depending on age. Only a fewstudies, and only 2 using contemporary cardiac scanners,have determined cardiac CT–specific conversion factorsfor children. Normalized to the 32-cm phantom, the con-version factors by Podberesky et al. (64) averaged 0.092mSv $mGy�1 $ cm�1 for age 1 year and 0.082 mSv $mGy�1 $

cm�1 for age 5 years, whereas the conversion factors byTrattner et al. (63) were 0.099 mSv $ mGy�1 $ cm�1 for age1 year and 0.049 mSv $ mGy�1 $ cm�1 for age 10 years.Thus, when estimating effective radiation dose fromCTDIvol and DLP in cardiac CT, it is important to under-stand that the conversion factor used is specific to either a32- or 16-cm phantom, may be cardiac- or chest-specific,and may or may not be scanner-, protocol-, and age-specific. For that reason, when reporting effective dose,both the phantom size and the specific conversion (k)factor used should be specified. Dose calculations that areadjusted for age and size should not be used inter-changeably with background radiation estimates that arenot similarly adjusted, or to compare between modalitieswithout similar adjustments in calculated dose (65).

6.3.5. X-Ray CT Dose Alert Monitoring

The ‘‘Protecting Access to Medicare Act of 2014’’ requiresthat imaging providers comply with the National Elec-trical Manufacturers Association XR-29 Standard Attri-butes on CT Equipment Related to Dose Optimization andManagement, also known as Medical Imaging & Tech-nology Alliance Smart Dose (http://www.medicalimaging.org/policy-and-positions/mita-smart-dose/). To be

compliant with the Medical Imaging & Technology Alli-ance Smart Dose, a CT scanner must possess 4 attributes:

1. DICOM-compliant radiation dose structured reporting(https://www.nema.org/Standards/Pages/Standard-Attributes-on-CT-Equipment-Related-to-Dose-Optimization-and-Management.aspx).

2. Dose check features (https://www.nema.org/Standards/Pages/Computed-Tomography-Dose-Check.aspx).

3. Automatic exposure control.4. Reference adult and pediatric protocols (https://www.

nema.org/Standards/Pages/Supplemental-Requirements-for-User-Information-and-System-Function-Related-to-Dose-in-CT.aspx).

Since January 1, 2016, Medicare has reduced by 5% thereimbursement for the technical component of imagingprocedures performed in imaging centers, physician of-fices, and hospital outpatient settings on CT equipmentthat is not in compliance with all 4 attributes of theMedical Imaging & Technology Alliance Smart Dose.

6.4. Patient and Personnel Exposure in Nuclear Cardiology

6.4.1. Patient Exposure in Nuclear Cardiology

In contrast to projectional or tomographic transmissionimaging with x-rays, radiation dose to the subject fromscintigraphy comes from within; from ionizing radiationemitted by a radiopharmaceutical that has been adminis-tered to the subject. Most often, the radiopharmaceuticalis administered intravenously and distributes throughoutthe body. Consequently, unlike x-ray imaging, whichprincipally exposes the imaged structures, the radioactivetracer exposes the entire body, not just the heart andadjacent structures. Organs receiving the highest radiationdose may not be the imaged structures. Furthermore, thepatient’s behavior after study completion can alter the rateof radiopharmaceutical excretion, which can affect theoverall radiation dose from the procedure.

The following information is employed to estimate theeffective dose from a radiopharmaceutical exposure(which is meant to approximate the equivalent uniformwhole-body dose):

1. Quantity of radioactivity administered.2. Radiopharmaceutical distribution within the subject.3. Kinetics of distribution to and elimination from each

organ.4. Radiosensitivity of each exposed organ.5. Physical half-life of the radionuclide and its emitted

photon or particle energy.

Much of the methodology for performing these calcu-lations as well as relevant regulations is governed by theInternational Commission on Radiological Protection(ICRP), whose publications specify methodology and best

http://www.medicalimaging.org/policy-and-positions/mita-smart-dose/

http://www.medicalimaging.org/policy-and-positions/mita-smart-dose/

https://www.nema.org/Standards/Pages/Standard-Attributes-on-CT-Equipment-Related-to-Dose-Optimization-and-Management.aspx



https://www.nema.org/Standards/Pages/Computed-Tomography-Dose-Check.aspx

https://www.nema.org/Standards/Pages/Computed-Tomography-Dose-Check.aspx

https://www.nema.org/Standards/Pages/Supplemental-Requirements-for-User-Information-and-System-Function-Related-to-Dose-in-CT.aspx





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practices for estimating radiation dose fromradiopharmaceuticals.

In addition, there is a second framework for estimatingthe radiation dose from radiopharmaceuticals, called theMIRD formalism (Medical Internal Radiation Dose). Manyof the MIRD methods have been developed by the Societyof Nuclear Medicine and Molecular Imaging’s Committeeon Medical Internal Radiation Dose, which has resulted inseveral publications that review this topic in greater detail(66,67).

The MIRD approach measures the concentration of theradiopharmaceutical in several organs/body compartments/whole body at multiple time points after administration.These concentrations at different times are used toestimate a residence time curve for each organ andthe area under that curve allows the calculation of thecumulative activity from a given radiopharmaceuticaladministration.

Different radiopharmaceuticals have different elimi-nation kinetics. For example, a typical small moleculeused for cardiac imaging like Tc-99m sestamibi has quiterapid initial uptake in tissues followed by a slowerwashout that generally follows first-order (exponential)kinetics. On the other hand, a radiolabeled immunoglob-ulin G antibody can remain in the circulation for days orweeks with much slower uptake into other tissues.

Most radiopharmaceuticals used for diagnostic imagingemit gamma rays. A high-energy photon emitted from theheart will have the potential to deposit ionizing energy inthe heart, but can also deposit energy in any tissue thoughwhich it passes. Therefore, the MIRD method uses organresidence times as input into anthropomorphic phantomsthat model the radiation dose delivered to a given organ,both from radionuclides within the organ and also fromactivity in other organs, to estimate the total dose (in Gy)received by each organ. These values are multiplied by theindividual organ radiation sensitivities to yield the indi-vidual organ doses, which are then summed to calculatethe whole body effective dose for the subject in mSv.

Although the MIRD applies to dose from gamma rays,PET radiopharmaceuticals emit positrons, which areessentially positively charged electrons that are emittedfrom the nucleus and travel only a few millimeters priorto incurring an annihilation event that emits a pair of 511keV photons. These photons have very high penetratingpower and do not significantly expose the subject as theyexit from their site of origin. PET nuclides, thus, deliverall of their energy close to the site where the decay occursand exposure is essentially limited to exposure from ac-tivity within that organ.

Radioactivity within the subject decreases over timedue to a combination of physical decay of the radionu-clide and elimination of the nuclide from the subject. Thecombination of these 2 processes is expressed as the

radiopharmaceutical’s “effective” half-life. The effectivehalf-life is shorter than the shortest of the physical orbiological half-life, although in cases when 1 of the 2components is very long, the effective half-life can bealmost the same as the shorter of physical or biologicalhalf-life. For example, Tl-201 chloride has very slowelimination from the body and so the effective half-life isclose to the 73-hour physical half-life.

A commercially available U.S. Food and Drug Admin-istration (FDA)-approved software package (OLINDA/EXM, VU e-Innovations, Nashville, Tennessee) imple-ments the MIRD method for calculating effective dosefrom a radiopharmaceutical administration. This softwarepermits calculation of an estimated whole body effectivedose (in mSv) per megabecquerel administered. (Onemegabecquerel is the quantity of a radioactive materialthat produces 1 million radioactive decay events persecond.)

It is important to emphasize that these models simulatean “average” person. The exact radiation dose to a givensubject may vary substantially from the expected values.For example, a subject with poor renal function willeliminate a renally-excreted radiopharmaceutical moreslowly than will the “average” normal volunteer used tomodel dosimetry estimates, and consequently, willreceive a larger actual dose than the model estimate. It iscritical to understand this limitation and not to considerthe radiation doses estimated for a given subject to behighly precise calculations for a specific individual,although they are quite accurate on a population level.

Consensus guidelines do not exist regarding the needto screen patients for pregnancy prior to diagnostic nu-clear medicine testing. However, such guidelines areparticularly needed because nuclear cardiology usingphoton-emitting nuclides has the potential to cause sub-stantial uterine exposure. Two groups have advocateduniversal screening by questioning women of child-bearing potential with postponement or pregnancytesting for those whose last menstrual period was over 10days (68), or for an expected radiation dose over 1 mSv(69). Laboratories should develop their own policies todirect their approach to screening, testing, and coun-seling pregnant or potentially pregnant patients. Theadministration of radionuclides to pregnant womenshould be undertaken only after careful deliberationabout the clinical benefit to the mother and potentialharm to the fetus. Physicists should be consulted to pro-vide an accurate assessment of exposure. Most agentsresult in low overall exposure, but some have advocatedavoiding the use of 18F-fluorodeoxyglucose in pregnantwomen (70,71). Adjustments to protocols should beconsidered to reduce exposure if image quality is notaffected. A patient should be encouraged to void herbladder frequently after injection to reduce prolonged

TABLE 12Core Principles for the Use of Medical IonizingRadiation for Diagnostic and TherapeuticProcedures

1. The examination should be conducted such that the dose received by thepatient and attendant medical personnel is the smallest necessary to yieldsatisfactory diagnostic efficacy.

2. Diagnostic and therapeutic efficacy should not be compromised in the interestof sparing radiation dose.

3. If the study’s purpose can be achieved employing a modality that does notemploy ionizing radiation, serious consideration should be given to thealternative modality.



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fetal exposure (72). For some nuclides, including Tc-99mpertechnetate and Tl-201, interruption in breastfeedingis recommended, for hours to days (73).

Finally, there are 2 additional issues to consider:

1. Because most radiopharmaceuticals are excreted, thefrequency of elimination can greatly alter the dose tospecific organs in contact with the excreta. Forexample, for a renally excreted radiopharmaceutical,radiation dose to the bladder wall is increased byexcreted activity in the urine. Thus, the less often thata subject voids, the higher the dose. Dosimetry studiesusually specify a minimum rate of urination (e.g.,asking the subject to void at least every 2 hours). Ifclinical subjects are not given adequate instructions onhydration and voiding, their bladder dose (and thebladder contribution to overall effective dose) could besignificantly higher than published values.

2. Radiopharmaceutical imaging studies, both PET andsingle-photon emission computed tomography(SPECT), which employ attenuation correction (animportant adjunctive technique that improves accuracyof reconstructed images), utilize a hybrid radiation-based technique to estimate attenuation. These tech-niques employ either a rod radiation or an x-ray radia-tion source. This is now commonly done with CT(termed PET/CT or SPECT/CT). Thus, the total radiationdose that the subject receives is the sum of the radio-pharmaceutical effective dose and the CT or rod sourcedose to the area exposed. In most cases, rod source doseis very low and negligible, or nearly negligible. How-ever, depending on the CT imaging protocol used, theCT dose may be greater and, in some cases, can equal orexceed the radiopharmaceutical dose. Therefore, dosereduction strategies need to consider all sources ofexposure from the study as well as to tailor the tech-nique to get the necessary information with the mini-mum radiation dose to the subject.

6.4.2. Personnel Exposure in Nuclear Cardiology

In contrast to the dosimetric calculations needed to esti-mate effective dose to a subject from radiopharmaceuticaladministration, the dose to personnel is generally morestraightforward and closely mirrors the exposures dis-cussed in Section 4 with 2 important caveats:

1. The photons emitted from the subject from radio-pharmaceuticals are generally of higher energy thanthe x-rays emitted from fluoroscopy or CT devices.Only the most energetic x-rays used in CT have anenergy of 140 keV; the vast majority of Tc-99m gammarays have an energy of around 140 keV (some photonsthat exit the subject will have decreased energy as aresult of scatter interactions), and the photons emitted

from positron annihilation have an energy of 511 keV.Photons in this energy range readily penetrate the 0.5-mm lead equivalent of conventional diagnostic x-rayprotective materials. Therefore, personal shieldingdevices, such as lead aprons or leaded glasses, are lesseffective and consequently are rarely used. Instead,nuclear cardiology personnel rely on the principles oftime and distance. Because the total photon flux fromnuclear studies is far lower than from x-ray tubes,limiting the duration spent near a radioactive subject isgenerally sufficient limitation of exposure. Therefore,personnel should limit the duration they spend in closeproximity to either the dose syringe or the injectedsubject as much as reasonably possible.

2. Whereas the X-ray tube used in CT or fluoroscopy iseither generating x-rays or not, the radiopharmaceu-tical is a continuous source of activity that can beexcreted via body fluids or spread during administra-tion. Thus, subject blood and excreted body fluids areradioactive and are a potential source of radiationexposure to personnel, particularly if an accident or anerror causes a healthcare worker to become contami-nated. Therefore, careful and routine monitoring forcontamination is required. If contamination occurs, amedical physicist often needs to be involved to esti-mate the dose received by the worker. The reason forinvolving a medical physicist in cases of contaminationis that monitoring devices (body dosimeters or ringbadges) assume a relatively uniform dose to the personthat can be accurately represented by the dose to thesmall dosimeter. However, a spill of radiopharmaceu-tical on a technologist’s shoe could result in a mean-ingful dose to the foot but barely register on adosimeter worn on a coat lapel.

7. MODALITY-SPECIFIC DOSE REDUCTION

STRATEGIES


Table 12 indicates core principles to follow for the use ofmedical ionizing radiation for diagnostic and therapeuticprocedures.


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7.1.1. Case Selection

The most effective way to reduce patient radiationexposure is not to perform the radiation-based procedurealtogether; a radiation-based procedure should be usedonly when it is the preferred choice among alternativemodalities that do not involve radiation exposure (e.g.,stress echo or stress cardiovascular magnetic resonance).If equivalent diagnostic information can be obtained bythe alternative imaging modality, that study should beselected in preference to a radiation-based procedure, allother considerations being equal. This is particularlyimportant in children and adolescents.

It is critical to utilize appropriate use criteria in selectingpatients to undergo diagnostic and therapeutic proced-ures. One factor among many incorporated in the deter-mination of appropriateness in these documents isprocedural risk, including risk from radiation. In choosingbetween a radiation-based modality and one that does notuse ionizing radiation, one must weigh both the clinicalefficacy, including sensitivity and specificity of eachalternative modality, and importance of the risk to theparticular patient conferred by the radiation exposure.Although it is important to always seek tominimize patientradiation exposure (this is a particular consideration inyounger patients who have long natural life expectancies),it is equally important to not withhold appropriate studiesdue to undue concern of the radiation-related risk.

7.1.2. Dose-Determining Variables

The radiation dose delivered to patients and medicalpersonnel (regardless of modality) is affected by severalvariables that are under the operator’s control. These are:

1. Equipment quality and calibration2. Equipment operating protocols3. Operator conduct

Radiological equipment image quality is stronglyinfluenced by the quantity of radiation that reaches theimage detector and is used to form the image. This“detector dose” is different from the dose that the patientreceives, as it is a small fraction of the total incidentradiation and refers only to the radiation that penetratedthe patient to reach the detector.

As each of these variables influences the dose deliveredto the patient (and also, potentially to operatingmedical personnel), each provides an opportunity toreduce dose.

7.1.3. Image Quality Issues

Image quality is a major determinant of an examination’sdiagnostic accuracy. Inadequate image quality mayeither cause incorrect diagnoses or a need to repeat anexamination—requiring additional patient exposure.

Consequently, it is imperative that radiological equipmentmeets current image quality standards, be maintainedin prime working order, and is used properly to producehigh-quality diagnostic images.

In addition, there are choices that balance imagequality and dose. In the earlier days of x-ray and nuclearimaging, image quality was sufficiently poor to bediagnosis-limiting, and only the best image qualityachievable could be accepted. This led to equipmentcalibration that employed detector doses that were largeby today’s standards.

Current imaging equipment produces much higher-quality images with the potential to employ smaller de-tector doses. There are circumstances in which the “best”or lowest-noise image that the system can deliver is betterthan needed for diagnosis. Consequently, there are cir-cumstances in which operators can choose to accept alower image quality, which is still sufficiently diagnostic,to reduce patient (and operator) radiation dose.

Image quality is determined by both spatial and tem-poral resolution, the signal-to-noise ratio, the contrast-to-noise ratio, and the presence of imaging artifacts. Mosttactics that increase either spatial resolution (byimproving signal-to-noise ratio and contrast-to-noise ra-tio) or temporal resolution (by increasing framing rate) doso at the cost of increased dose. The challenge is to opti-mize these properties by balancing the tradeoffs betweendose and image quality.

7.1.3.1. Spatial Resolution: Detector Input Dose, Pulse Width,and Nuclear Scan Acquisition

In general, image detector signal-to noise ratio isinversely proportional to the square root of the detectordose. Low signal-to-noise ratio images have a “grainy”appearance because a small number of x-ray photonsreach the detector to form an image. This grainy quality,termed “quantum mottle,” becomes smoother as doseincreases, improving the ability to perceive image detail.This principle applies to all ionizing radiation-basedimaging techniques.

Examples of the impact of detector dose on imagenoise for x-ray fluoroscopic imaging are presented inFigure 8. These are images of a line pair phantom that areacquired at different detector doses ranging from 10 to1,200 nGy/frame. As the number of photons reaching thedetector increases, image noise decreases and the imagebecomes smoother. Over a defined range, as image noisedecreases, perceptible image spatial resolution increases.For each imaging modality there is an upper limit of dosebeyond which further dose increase, although it mayproduce a smoother appearing image, does not yieldgreater image detail of diagnostic importance. Thus, doseis an important determinant of noise, and noise affectsthe ability to perceive detail. Consequently, for each

FIGURE 8 Images of a Line Pair Phantom Acquired in an X-Ray Fluoroscopic System at Different Detector Doses (as Labeled on the Individual Images)

Note the progressive decrease in image noise and the ability to perceive image detail as the dose increases: 10 nGy/frame, an unacceptably low dose; 18 nGy/

frame, representative dose for low-dose fluoroscopy; 40 nGy/frame, representative dose for standard-dose fluoroscopy; 200 nGy/frame, representative

dose for cine acquisition; 1,200 nGy/frame, representative dose for digital subtraction imaging.



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imaging purpose, there is a dose level that delivers suf-ficient image quality for diagnosis while minimizingdose.

Similarly, the image noise in x-ray CT images is deter-mined in part by the number of x-ray photons that rea-ches the scanner detectors. Larger doses will yield imageswith less noise and, within limits, greater spatial resolu-tion. For x-ray CT, the spatial resolution required toassess myocardial contours, and, accordingly, the doseneeded to achieve it, is less than that required to char-acterize coronary artery lesions. For nuclear scan images,the number of gamma ray counts that are acquired toconstruct the image determines the image noise and,accordingly, its spatial resolution, which improves as thenumber of counts acquired increases. The number ofcounts acquired is determined by both the amount ofradioactivity administered for the examination, whichdetermines the number of counts per unit time, and theimage acquisition time, with longer acquisition timesacquiring a larger number of counts.

7.1.3.2. Temporal Resolution: Pulse Duration and Frequency

The cardiovascular system moves. This imposes addi-tional requirements on cardiovascular imaging systems,and raises 2 issues:

1. If image acquisition time is too long, object motion willcause the image to be blurred (motion unsharpness)just as a photograph of a moving object will be blurredif camera shutter speed is too slow. X-ray fluoroscopysystems deliver x-ray in a brief (2 to 10 milliseconds[ms]) pulse for each fluoroscopic or cine acquisitionframe. The pulse duration is analogous to a camerashutter speed (5 ms pulse duration is equivalent to1/200 s camera shutter speed).

2. If an image series (such as an x-ray fluoroscopy cineacquisition) is acquired at too slow of a frame rate,events that occur during time periods shorter than theframing rate will not be resolved. In addition, at slowerframe rates, object motion will cause the resulting im-age to have a jerky quality.


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The cumulative dose from an examination is propor-tional to the total number of frames exposed. Thus, forthe same total duration of exposure, a faster frame ratewill deliver a proportionately greater total exposure.

These issues pose different challenges for the differentcardiovascular imaging modalities.


Of the 3 imaging modalities, x-ray fluoroscopy has thegreatest variation in dose per procedure and has the po-tential to deliver the largest dose to patients and also tooperators and nearby medical personnel. In addition,operator choices and behavior and equipment quality andcalibration have substantial influences on dose. Conse-quently, it is important for physicians who perform x-rayfluoroscopically guided procedures to be well-versed inequipment operation and the parameters that affect dose.

7.2.1. General Principles

As discussed in Section 4, there are 2 patient dose pa-rameters that are reported by current fluoroscopy sys-tems. These parameters are determined in part by x-rayequipment calibration and in part by operational conductdecisions that are under the operator’s control. The 2parameters are:

1. Cumulative air kerma, which is a measure of exposureintensity and correlates with the risk of tissuereactions.

2. Cumulative air KAP, which is a measure of the totalenergy delivered to a subject and correlates with sto-chastic risk.

The total dose delivered during an x-ray fluoroscopicimaging examination is the product of the dose per frameand the total number of frames in the examination(determined, in turn, by the framing rate [in frames/s]and the imaging duration). Each of these parameters is adeterminant of the total dose, and each presents an op-portunity to control dose. Each also presents potentialtradeoffs in diagnostic utility. Optimal imaging requiresoperator attention to each of the parameters to achieve anoptimal balance between image quality and dose.

7.2.2. Digital X-Ray System Operating Modes

Digital x-ray imaging systems operate in 3 modes thatemploy different detector doses to achieve different im-age spatial resolution (see Figure 8 for examples).

1. Fluoroscopy: the lowest dose per frame imagingprotocol that yields images with the lowest spatial res-olution. Fluoroscopy is intended to provide visualguidance for catheter manipulation but not to generateimages suitable for anatomic diagnosis. Typical fluoro-scopic dose rates range between 20 and 40 nGy/frame.

2. Cine Acquisition: an intermediate dose per frame im-aging protocol intended to provide diagnostic qualityimages for archiving and diagnostic interpretation.Cine acquisition images have reduced image noisecompared with fluoroscopic images but should stillhave visible noise. Typical cine acquisition dose ratesare in the range of 200 nGy/frame. Interpretation ofthese somewhat noisy images is frequently aided byvisual integration of a moving display of sequentialimages. Current x-ray units are generally configured fora single dose per frame for cine acquisition that hasbeen selected to provide an optimal balance betweendose and image noise.

3. Digital Subtraction Algorithms: the highest dose perframe imaging protocol for digital subtraction algo-rithms. Digital subtraction enhances the visualizationof low concentration of x-ray contrast, enabling smallercontrast doses. However, because digital subtractionalgorithms are highly sensitive to image noise, highdoses per frame are needed to enable the subtractionalgorithms to function effectively. Consequently, digi-tal subtraction algorithm per frame dose rates are muchhigher (typically 1,200 nGy/frame) than for cineacquisition.

7.2.3. X-Ray System Calibration, Operation, and Dose

X-ray fluoroscopy systems are typically designed to adjustthe dose to the detector automatically to achieve a pro-grammed image brightness. The dose to the detector is setby the system calibration. Current radiological equipmenthas selectable operating protocols that can vary the radi-ation dose per frame employed for fluoroscopy and theframing rate for both fluoroscopy and cine acquisition.Equipment operators should be familiar with the uses andcapabilities of the different operating protocols andshould select the protocol that is most appropriate for aparticular patient’s clinical circumstances (38,74).

At a typical framing rate of 15 frames/s with a typical5-ms pulse duration, the x-ray beam is only on for 5 ms ofthe 66.7 ms occupied by each video frame. Currentdigital video systems employ image gap-fill to eliminatethe image flickering that historically accompanied slowframe rates. However, although gap-fill avoids the flickerin the video image presentation, it does not improvetemporal resolution. Slow frame rate fluoroscopic andcine acquisitions, although gap-filled, have an obligatoryjerky quality because of the longer interval betweenframes.

The goals and purposes of a particular examinationdetermine the optimal balance between radiation expo-sure and the image’s spatial and temporal resolution.For example, for x-ray fluoroscopy, the spatial andtemporal resolution required for general catheter



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placement and manipulation is less than that required toperform coronary or structural cardiac interventionalprocedures. Consequently, in this application, slowerframe rates and lower doses per frame can be usedto reduce exposure without compromising clinicaleffectiveness.

In addition to selecting the optimal dose-determiningimaging protocol, operator conduct can also affect pa-tient dose. Dose is also affected by equipment posi-tioning, radiation field size, and exposure time. This isdiscussed in detail in subsequent sections.

7.2.3.1. Temporal Resolution Issues and Dose Tradeoffs

Because the cardiovascular system moves, x-ray fluoro-graphic imaging requires short (typically between 3 and 8ms for adults, as short as 2 ms for children) pulse dura-tions to limit image motion unsharpness.

In addition, to capture the details of a moving object,an x-ray fluorographic system must have a framing ratecommensurate with the degree of motion. Temporal res-olution requirements vary depending on the nature of theprocedure. Faster framing rates deliver a greater dose.The optimal compromise balances the smoothness ofimage presentation against the dose required by theframing rate.

Fluoroscopic temporal resolution requirements varysubstantially depending on the examination’s purpose.General catheter placement can be accomplished withfluoroscopic frame rates as slow as 3 to 4 frames/s. Morecomplex procedures such as coronary and structural in-terventions require greater temporal resolution andemploy frame rates between 10 to 15 frames/s.

Cine acquisition frame rates also vary with the purposeof the examination. For coronary arteriography, a framerate of 10 to 15 frames/s is generally adequate. For adultventriculography, 30 frames/s is preferred to achievemore precise identification of end diastole and end sys-tole. In pediatric applications, framing rates as frequentas 60 frames/s are occasionally needed.

7.2.4. Determinants of Total Dose for an Exposure

7.2.4.1. Dose Per Frame and Framing Rate

The total dose for a particular exposure is the product ofthe dose per pulse and the total number of pulses in theexposure. Thus, 3 parameters—dose per pulse (in nGy perpulse), the pulse rate (in pulses per second), and theexposure duration (in seconds)—combine to determinethe total dose for an exposure.

For fluoroscopy mode, current x-ray units typicallyprovide 3 tableside-selectable fluoroscopy detector dosesper frame levels—these doses produce different degrees of

image noise and tableside fluoroscopy pulse rates rangingfrom 4 to 30 pulses/s.

For cine acquisition mode, the detector dose per pulseis set by the service engineer. The operator can selectamong multiple pulse rates.

The optimal parameter settings for a fluoroscopic ex-amination or a cine acquisition run are determined by thepatient’s particular circumstances, which affect diag-nostic requirements for spatial and temporal resolution.The operator is responsible for determining the operatingparameters (dose per pulse and pulse rate) that balancethe diagnostic requirements of image spatial and tempo-ral resolution and the dose associated with theexamination.

7.2.4.2. X-Ray Imaging Field Size and System Positioning

While the dose per pulse and the number of pulsesdetermine the total dose intensity (in Gy) delivered to thepatient, the product of the total dose and the imagingfield size determines the total amount of radiation energy(expressed as the KAP in Gy$cm2, discussed in Sections 4and 5) that the patient receives. The total KAP determinesthe patient’s stochastic risk associated with the exposure.In addition to the examination’s total number of pulsesand the detector dose per pulse, the KAP is affected by 2additional parameters that are under the operator’s con-trol: the imaging field size selected and systempositioning.

While reducing KAP is beneficial to the patient, it isalso beneficial to the operator and nearby medicalpersonnel, because scattered x-ray dose (to the operatorand nearby personnel) is directly related to KAP. Conse-quently, the operator has a personal interest to minimizeKAP.

X-Ray Imaging Field Size

Current x-ray systems link brightness stabilizationdetection to a collimator position that samples only thedetector area receiving the collimated x-ray beam.Consequently, the dose per pulse to the detector is notaffected by collimator position. However, the KAPis directly related to the size of the imaged area.The consequence of this phenomenon is that, at a givendetector zoom (magnification or input phosphor size)mode, smaller image area sizes deliver proportionatelysmaller KAPs. Thus, at a given detector zoom mode,reducing exposed field size by collimation to the smallestsize necessary minimizes the KAP that the patient re-ceives. This is not true for changing detector zoom modes.Detector dose per pulse increases as the zoom magnifi-cation increases. Thus, for the same actual image size, theKAP delivered by an uncollimated image at a greater zoom

FIGURE 9 Diagrammatic Representation of the Effect of System Positioning on Patient and Operator Radiation Exposure During X-Ray Fluoroscopy

Note that in the “table too low” circumstance, the entrance port dose delivered to the patient is increased compared with optimal positioning. In the “table

too low, detector too high” circumstance, the entrance port dose to the patient is further increased. In addition, in the “table too low” circumstance, the

scattered dose to the operator increases because less of the scattered dose is intercepted by the detector (17).


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mode is greater than an image of the same size acquiredcollimated at a lesser zoom mode.

X-Ray System Positioning

There is an optimal distance between the patient’s skinsurface and the x-ray source (typically approximately 70cm). If the patient is positioned too close to the x-raysource, the full x-ray output is concentrated on a smallerarea of the patient’s skin, increasing the patient’s beamentrance port exposure rate. This can increase the pa-tient’s skin injury risk. If the patient is positioned too farfrom the x-ray source, the image receptor necessarilymust also be positioned further away from the source andthe inverse square law requires a greater x-ray output toachieve the requisite dose to the detector. Although thislatter positioning does not increase patient entrance portskin exposure rate, increased kVp required to achieveadequate detector dose causes decreased image contrast,potentially degrading image quality.

X-ray detector positioning is also an important deter-minant of dose to the patient as well as the exposure tomedical personnel from scattering. If the detector ispositioned substantially above the thorax, the imagemagnification caused by beam divergence will decrease

the size of the beam entrance port, causing the patient toreceive a larger skin dose. In addition, the x-ray imagedetector, when positioned close to the patient’s chest,intercepts a substantial portion of the radiation scatteredwithin the patient that would otherwise reach medicalpersonnel; accordingly, x-ray detector positioning con-tributes to medical personnel protection (Figure 9).

7.2.5. Procedures and Practices to Minimize Patient and

Personnel Exposure

Because so many variables affect patient and medicalpersonnel exposure, there are abundant opportunities tominimize exposure to both constituencies.

7.2.5.1. X-Ray Equipment Quality, Calibration, andMaintenance

Invasive cardiovascular x-ray imaging facilities have aresponsibility to maintain and update x-ray equipment toproduce quality images at the minimum detector inputdose needed to generate such images. Equipment shouldbe well maintained and its calibration should be surveyedperiodically to verify that it is operating within appro-priate specifications. Facility clinical leadership shouldcollaborate with the x-ray system vendor and the



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institution’s radiological physicist to verify that detectordoses are minimized consistent with optimal imagequality.

The x-ray system should provide beam spectralfiltering that is consistent with current standards. Thesestandards have become more stringent over time. Currentsystems provide both aluminum and copper filtration thatis selected automatically by algorithms that use overallsubject characteristics to achieve optimal image qualityand beam filtration.

The x-ray system should provide reduced-dose oper-ating protocols for low-dose and low frame rate imagingprograms that can be employed to reduce exposure incircumstances where this will not compromise procedureconduct or diagnostic image quality. Ideally, the low-dosefluoroscopic calibration should be very low (on the orderof 2 0 nGy per pulse). This will provide an image of suf-ficient quality for general catheter placement but not formore advanced coronary interventional procedures. Atypical standard (or intermediate)-dose fluoroscopic doserate is on the order of 40 nGy/pulse and is typicallysuitable for all but the most demanding coronary inter-ventional procedures.

Typical cine acquisition detector input doses rangefrom 140 to 240 nGy/pulse. The facility’s physician di-rector collaborates with the service engineer to determinethe detector dose that provides satisfactory diagnosticquality images with an appropriate level of image noise. Itis important that the physician director understand thevariables that determine image quality and the tradeoffsbetween image noise and dose to arrive at the optimalcompromise dose rate.

7.2.5.2. Physician Operator Conduct

Dose Awareness and Monitoring

Appropriate physician operator conduct begins withawareness of dose and a commitment to minimize radia-tion exposure to patients and to healthcare personnel.Operators should be cognizant of the variables thatdetermine image quality and dose to achieve the bestbalance of image quality (as clinically necessary toconduct a procedure effectively) and radiation exposure(75,76).

Current x-ray units display real-time values for airkerma dose rates and cumulative air kerma and KAP. Thephysician operator should be aware of these values andtheir interpretation throughout a procedure. Where clin-ically appropriate, the physician operator should considertotal accumulated dose in making procedure conductdecisions.

X-Ray System Operational Issues

Imaging modality, imaging time, and image field sizeare 3 important dose-affecting parameters that are under

the operator’s direct control. Operators should select thelowest-dose imaging modality that is appropriate for aparticular application. This includes using an image fieldsize that confines exposure to the structures of interest,using the lowest-dose fluoroscopy program, and using theslowest fluoroscopy pulse rates that yield appropriatequality images (77).

Operators should use the x-ray system collimator tominimize the exposed field size. Operators should opti-mize system positioning with the procedure table at theoptimal distance from the x-ray tube and the imagedetector as close to the patient as possible.

The operator should pay particular attention to mini-mizing exposure time. This includes limiting fluoroscopyto actual catheter manipulations that require fluoro-scopic visualization and minimizing cine run durations.Maneuvers that do not require fluoroscopic visualiza-tions such as pullbacks can be conducted withoutfluoroscopy.

Current x-ray systems also provide a number of capa-bilities that enable some common tasks to be performedwithout additional fluoroscopy. “Last image hold” main-tains the last acquired frame on the monitor available formore detailed study, avoiding a need for additional x-rayexposure. Current systems provide virtual collimator po-sition indicators that allow the operator to position thecollimator without an x-ray exposure—enabling optimalfield size adjustment while avoiding unnecessary expo-sure due to adjusting collimator position. In addition,current x-ray systems provide a virtual image of the effectof repositioning a patient, reducing the need to useadditional fluoroscopy for the sole purpose of setting up aparticular position for an examination.

7.2.5.3. Physician and Medical Personnel Shielding andProtection

Protective shielding of operators and personnel providessubstantial protection. Standard shielding for diagnosticx-ray ranges between 0.25 and 0.5 mm of lead or anothermaterial, such as titanium, in a thickness that has an x-rayabsorbance equivalent to 0.5 mm of lead. A 0.5 mm leadequivalent apron absorbs 95% of 70 kVp x-ray and 85% of100 kVp (78,79). The 70 kVp value is a closer representa-tion of the spectral distribution of scattered x-ray photonsthat can expose medical personnel.

Medical personnel working in an x-ray procedure roomshould wear 0.25- or 0.5-mm equivalent lead apronsaugmented with neck thyroid shields and humeralshields. The protection provided by thyroid collars isparticularly important. The thyroid collar shields thethyroid and the cervical bone marrow, 2 highly radio-sensitive structures that are located in an area of highradiation scatter. By attenuating the dose to these struc-tures, the thyroid collar decreases the effective dose to

Checklist of Dose-Sparing Practices for X-Ray Fluoroscopy

Case selection , Consider patient age, comorbidities, natural lifeexpectancy

, Consider appropriateness and utility ofnonradiation-based imaging techniques

Equipment calibration , Fluoroscopic and cine doses as low as compatiblewith diagnostic image quality

Procedure conduct , Minimize beam-on time, Use lowest-dose fluoroscopy setting suitable for

a particular task

, Collimate imaging field size to the area ofinterest

, Use the slowest framing rates suitable for aparticular task

, Minimize cine acquisition run durations

, Minimize patient-detector distance

, Maximize employment of operator shielding


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the operator by approximately one-half. In addition tolead aprons, medical personnel who work close to thex-ray source should wear leaded eye protection with sideshields. Lead or lead-equivalent hats may reduce cranialdose, but potential benefits are to date theoretical basedupon anecdotal reports of increased left sided braintumors in interventional cardiologists (80).

The protection afforded by lead garments should beaugmented by portable shielding. Typical in-roomshielding includes a ceiling-mounted lead-impregnatedpoly(methyl methacrylate) shield that can be placed be-tween the patient’s thorax and the operator’s upper body.This can intercept scattered radiation that would other-wise strike tableside personnel. The importance ofceiling-mounted shields cannot be overstated. Proper useof these shields reduces operator eye exposure by a factorof 19 (81). Under-table mounted 0.5-mm lead-equivalentshielding intercepts backscatter off of the patient andthe x-ray table that would otherwise strike the operator’slower body.

In addition to lead-equivalent shielding, the inversesquare law is one of the best sources of protection. X-rayintensity decreases as the square of the distance from thesource. This relationship has implications for physicianoperators, because operator position in relation to the x-ray source can make a large difference in exposuremagnitude. It also has important implications for circu-lating medical personnel.

Circulating personnel should be positioned remotelyfrom the x-ray source and, as a result of that distance,should receive negligible exposure. When circulatingpersonnel need to approach close to the patient, thephysician operator has a responsibility to not operate thex-ray system until the circulating person has finished andis no longer in close proximity to the x-ray source (74,82).

7.2.6. Pregnant Occupationally Exposed Workers

7.2.6.1. Uterine Exposure Considerations for Pregnant orPotentially Pregnant Occupationally ExposedWorkers

As discussed in Section 5.4.4, no measurable increase inadverse fetal outcomes has been detected at fetal or em-bryonic exposures below 50 mGy. For occupationallyexposed workers in an x-ray fluoroscopy environment,proper shielding and practices should keep uterine expo-sures well below this level for the duration of a pregnancy.Because the uterus is a deep structure and is inside pro-tective garments, the dose to the uterus delivered by scat-tered x-ray is greatly attenuated compared with the dose tounshielded areas. Measurements made in phantoms indi-cate that the uterine dose in a subject wearing a 0.25-mmlead apron is <2% of the collar dose (outside protectivegarments). Thus, for an occupationally exposed worker toreceive a uterine dose of 50 mGy would require an

accumulated collar badge dose of 2.5 Gy. The shieldingprovided by a standardwell-fitted lead apron is sufficient toprotect the fetus for typical exposures during procedures.Aprons specifically designed for pregnant workers provideadditional lead inserts over the pelvis, although the addi-tional weight of such aprons may limit their use (49).

7.2.6.2. Radiation Protection and Monitoring Practices forPregnant or Potentially Pregnant OccupationallyExposed Workers

A pregnant female occupationally exposed worker shouldwear, in addition to the customary collar film badge, anabdominal badge worn under the apron to estimate theuterine dose. This will verify that the uterine dose iswithin the range that is considered to be safe for the fetus.Thus, it is clear that with adequate precautions andprotection, a pregnant healthcare worker can work inan x-ray fluoroscopy environment without detectablyjeopardizing her fetus (51,52). As for patients, breast-feeding need not be interrupted for occupational ionizingradiation exposures.

7.2.7. Alternative Imaging Techniques

There are a number of alternative imaging techniques thatprovide structural and guidance information that cansupplement or replace x-ray fluoroscopic imaging. Theseinclude intravascular and intracardiac ultrasound, otherforms of ultrasound, cardiac magnetic resonance, andelectromagnetic mapping. In some circumstances, theseimaging techniques are superior to x-ray fluoroscopy andhave the additional advantage that they do not requireionizing radiation. They are being increasingly widelyadopted in clinical electrophysiological procedures and instructural heart interventional procedures as adjuncts toand partial replacements for conventional x-ray fluoro-scopic imaging.

7.2.8. Summary Checklist for Dose Sparing in X-Ray Fluoroscopy



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7.3. X-Ray CT

7.3.1. X-Ray CT General Principles

The past 2 decades have seen an explosive growth in thesophistication and capabilities of x-ray CT scanners.Current-design scanners provide enhanced-qualitynoninvasive images of the cardiovascular system. Theeasy availability of high-quality images provides bothimportant adjuncts to cardiovascular diagnosis and astrong incentive for utilization.

Achieving optimal images at minimal dose requires anexpert team to coordinate patient management and pro-tocol selection including image acquisition, reconstruc-tion, and interpretation. The team needs to select theimaging protocol most likely to acquire diagnostic-qualityimages that achieve the examination’s goals whileexposing the patient to the smallest necessary radiationdose (83–85).

The keys to minimizing radiation exposure in cardiacCT are:

1. Appropriate case selection.2. Scanner capability and protocol selection.3. Proper patient preparation.4. Appropriate examination conduct.

7.3.1.1. Case Selection Appropriateness

The first principle to reduce patient radiation exposuredue to CT examinations is to avoid performing examina-tions that will prove to be nondiagnostic. Appropriatecase selection should consider whether a CT examinationwill answer the clinical question(s) posed. This choiceincludes weighing relative contraindications due to con-ditions (such as marked obesity, atrial fibrillation, exten-sive coronary calcification, or a patient’s inability tocooperate) that degrade image quality. Case selectionshould incorporate the appropriate use criteria formu-lated collaboratively by the ACC and other organizations(86–88).

7.3.1.2. Procedure Planning and Patient Preparation

In planning the examination, it is important to select theacquisition protocol that is optimal for the patient’s con-dition and physical characteristics, and the examination’sgoals. Newer x-ray CT scanners provide improved acqui-sition modes that have the potential to decrease patientdose substantially.

Depending upon the clinical indication for the exami-nation, patient preparation for cardiac CT may be vitallyimportant both to achieve an optimal quality study and tominimize radiation dose. It is important to minimizemotion artifacts caused by rapid heart rates and patientmotion. This requires effective patient education tooptimize cooperation to eliminate body motion and

breathing during the examination. When targeting thecoronary arteries, heart rate control with appropriate useof beta blocker premedication is particularly importantbecause the lowest-dose scan protocols require a low andconsistent heart rate to maximize the effectiveness ofECG gating and image quality. Target heart rates willdepend on scanner hardware, but in general a regularrhythm with rates in the range of 50 to 70 beats/min ispreferred.

The dose delivered by the examination can alsobe influenced substantially by examination conductdecisions. The most obvious choice is to take care toconfine the examination to the part of the body that isrelevant to the examination’s goal and to take carenot to expose irrelevant anatomical regions. Equallyimportant is to tailor the acquisition protocol tothe spatial and temporal resolution needed for theparticular examination’s purpose. For example, imagingof structure for congenital heart disease requires lessspatial resolution (and accordingly, lower doses) thancoronary imaging.

7.3.2. Equipment Quality and Calibration

Equipment calibration and preventive maintenance aspart of quality assurance and control programs play animportant role in reducing radiation dose by facilitatingdose optimization. This is discussed in greater detail inSection 9. CT scanner design has improved substan-tially in recent years with improved detectors and morerefined acquisition protocols enabling current scannersto produce higher-quality images at lower patientdoses.

7.3.3. Variables That Affect Patient Dose for X-Ray CT

The radiation dose to a patient is determined by a com-bination of the patient’s physical characteristics andscanner protocol selection. Patient exposure will neces-sarily increase with patient size and body mass index.Depending upon the specific acquisition parameters, theincreased exposure need not increase dose to radiosen-sitive tissues. Patient size is not a variable that de-termines exam appropriateness as long as the patient’ssize does not preclude obtaining diagnostic qualityimages.

X-ray CT systems may either use a constant x-ray tubeoutput or, in some acquisition protocols, use ECG-gatedvariable output. The operator selects the acquisitionprotocol based on patient characteristics and the studypurpose with the intent to deliver a sufficient exposure topermit an acceptable degree of noise in the reconstructedimages.

The x-ray CT system operator is responsible to selectthe scanning protocol that optimizes the examination’s


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diagnostic yield while minimizing dose. The following areessential considerations in this process:

1. Scan length. Scan length, defined as the distanceimaged along the cranio-caudal axis, should be kept toa minimum to encompass only the anatomy of interestand not expose structures that are not relevant to theexamination’s purpose. Care needs to be taken toensure that the diaphragm position seen on the topo-gram is the same as during the scanning. This requiressimilar breath hold instructions.

2. X-ray beam intensity. X-ray beam intensity is deter-mined by both the x-ray tube potential (in units of kV)and the x-ray tube current (in units of mA). Modern CTscanners modulate the tube current dynamicallythroughout the CT acquisition to minimize radiationexposure.Tube potential: Studies of radiation dose reduction havedemonstrated that the most important single factor incontrolling radiation dose is adjustment of x-ray tubevoltage (89–91). Increasing tube voltage increases thex-ray beam’s mean photon energy level, and increasesradiation dose roughly proportionally to the square of thevoltage. Thus, at a constant tube current, a decrease oftube voltage from 120 to 100 kV reduces the radiation doseby almost 40%. In most scanners, the x-ray tube voltagemay be adjusted between 70 to 140 kilovolts (kV). Thevoltage is selected by the operator based on subject weightor body mass index. A commonly used adjustment scalethat provides diagnostic quality in most scanners is: 120kV for patients with body mass index $30 kg/m2, 100 kVfor body mass index 21 to 29 kg/m2, and 80 kV for bodymass index <21 kg/m2 (83). Image noise decreases as po-tential increases, so that in extreme cases (body massindex $40 kg/m2) the maximum tube potential of 150 kVmay be necessary to produce diagnostic quality images.Consequently, selecting the tube potential involves atrade-off between image noise and dose.Tube current: The x-ray tube current (in mA) is defined asthe number of electrons accelerated across the tube perunit of time and is proportional to the number of x-rayphotons produced per unit time. The radiation dose islinearly proportional to the tube current. Image noise isinversely proportional to the square root of the tubecurrent. Thus, decreasing tube current at a giventube potential decreases the radiation dose at the expenseof increased image noise. The tube current may be modi-fied based upon patient size assessed by visual inspection,measurement of body weight or body mass index, thoraciccircumference or diameter, or noise measurement from across-sectional prescan or topogram. Most modern scan-ners offer tube current modulation based upon the thick-ness of the body estimated from the topogram.Modulation may be applied longitudinally as well as

circumferentially. This approach can reduce radiationexposure of thoracic CT examinations by 20% withoutincreasing image noise (92). A specific form of tube currentmodulation that is applicable to retrospective ECG gating(see the following text) modulates the tube currentduring each heart beat relative to position within the R-Rinterval. This method is discussed in specific detail withinthe subsequent section on retrospective ECG.

3. Rotation time. The time required for the gantry toperform 1 rotation is a selectable parameter. Exposureincreases linearly with rotation time.

4. X-ray beam filtration. Filters placed beneath the x-raytube are used to selectively attenuate low-energyx-rays that do not significantly contribute to theimage but do contribute to radiation dose (85). The neteffect is to increase the mean energy of the x-rayswhile not altering the maximum energy. Filters may besmall, medium, or large and either flat or bowtie.The choice of filter depends on the size of the patientand the acquisition field of view.

5. Scan acquisition mode. This is a major determinant ofradiation dose. Different acquisition modes can deliversubstantially different doses while producing similarimages. There are 3 principal CT scan modes: axial or“conventional” scanning, helical scanning, and fixedtable or single-station scanning.Axial scanning may or may not be ECG triggered. Itimages a portion of the anatomy during a single gantryrotation while the table is stationary. The table advancesto the next contiguous position, which is based upon thewidth of the detector array, and another scan ensues. Theprocess is repeated until the full anatomy of interest hasbeen imaged.Helical scanning combines continuous gantry rotationwith continuous table advancement to trace a contiguoushelical or “spiral” path from the origin to the terminus ofthe scan. The ratio of the width of the detector array to thedistance that the table advances per complete gantryrotation is the scan pitch. Radiation exposure for helicalscanning at a pitch of 1 is comparable to axial scanning.When the pitch is <1, the radiation exposure is greater;when the pitch is >1, then radiation exposure isdecreased. A specialized form of helical scanning, called“high pitch” scanning, has been developed for use in dual-source CT scanners. Dual-source scanners with 2 x-raytube/detector systems can interleave 2 sets of projectiondata acquired simultaneously; however, the 2 beams areseparated in-plane by approximately 90%, allowing thepitch to increase to >3. When applied to imaging theheart, the reduced overlap between gantry rotations inthis scanning mode reduces radiation dose more than anyof the other scan modes, to values of <1 mSv (93,94).However, high-pitch scanning at its current stage ofdevelopment is vulnerable to image artifacts, and suitable



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for coronary artery imaging only in patients with slow,very regular heart rates. It does have value in circum-stances where minor imaging artifacts can be accepted(such as pulmonary vein mapping).Fixed table scanning is a specialized form of axial scan-ning performed when using a CT scanner with a detectorarray with a width that equals or exceeds the length of theanatomy of interest. In this instance, the table remainsstationary during a single or multiple gantry rotations.Radiation exposure approximates axial scanning when asingle gantry rotation is applied and increases linearlywith increased gantry rotation time.

6. Cardiac motion compensation. Compensation for car-diac motion is rarely applied outside of direct cardiacand aortic root imaging. Thus, the majority of cardio-vascular medical imaging does not employ ECG gatingor triggering. In contrast, when imaging the heart oraortic root, cardiac motion compensation is critical toavoiding motion-related artifacts that substantiallydegrade image quality. Depending upon the scanmode, 1 of 2 cardiac compensation methods is used.Prospective ECG triggering: Prior to the scan, the operator“prospectively” selects an imaging window within thecardiac cycle, which may be defined as a percentage fromone R-wave to the next or an absolute time delay aftereach R-wave (6). Scans are then triggered to coincide withthe selected scan window. Prospective triggering may beapplied to each of the 3 scan modes. In the case of axialscanning, ECG triggering is used to trigger the acquisitionat each table position. Scanning occurs during every otherheartbeat, and the table is incremented during inter-vening heart beats. The principle is the same with fixedtable scanning; however, here a single scan is acquired,initiated based upon the ECG trigger. Finally, in the caseof high-pitch scanning, a single sub-second helical acqui-sition may be triggered based upon a prospectivelyacquired ECG. For the specific application of coronaryCTA, prospective triggering has been associated with thelowest-dose scans; however, effective prospective trig-gering requires a regular, slow heart rate (typically 50 to 65beats/min in most scanners). The data acquisition windowmay be widened (padding) to allow for retrospective ad-justments of the acquisition window at the expense ofincreased radiation. A disadvantage of prospective gatingis the potential that, if the image quality proves to beunsatisfactory, the entire scan must be repeated becauseno projection data are available from other portions of thecardiac cycle.Retrospective gating: Applicable to both helical and fixedtable scan modes. With helical scanning, acquisition isperformed using a low pitch of approximately 0.2. Theslow acquisition images the entire cardiac anatomy acrossthe entirety of a cardiac cycle, providing a 4-dimensional

dataset that allows each spatial location within the heartto be reconstructed at any time-point across the cardiacperiod. Data are continuously acquired along with the ECGsignal while covering the anatomy of interest. The dataare subsequently rebinned at each slice location for imagereconstruction, according to the time of the cardiac cyclefrom the ECG signal. The selection of a specific time-pointfor reconstruction is determined after the scan iscompleted or “retrospectively.” The principle is the samefor fixed-table scanning; however, here, there is no tablemovement. Multiple gantry rotations with a wide detectorprovide the 4-dimensional, temporally resolved data.Retrospective gating is the only method that allows theassessment of dynamic cardiac structures such as nativeand prosthetic valves, myocardium, and chamberdimensions. It can also reveal intracardiac shunts anddehiscent graft anastomoses, owing to variations in iodineenhancement across the cardiac cycle. When comparedwith prospective triggering, coronary CTA performed withretrospective gating offers diagnostic image quality of thecoronary arteries in patients with higher basal heart ratesand a greater degree of beat-to-beat variability and allowsassessment of regional myocardial function; however, it isassociated with a higher radiation dose. Typically, imagereconstruction for “static” coronary CTA uses only thetime period during the cardiac cycle when cardiac motionis minimal (diastole), and the helically arranged projectiondata from other periods of the cardiac cycle are ignored.This scanning mode, while radiation-inefficient, has anumber of advantages. In particular, the entire imagedataset is available for image reconstruction, enablingpost-processing selection of the best quality images. Inaddition, because image data are available from the entirecardiac cycle, images from different portions of the cardiaccycle can be combined to construct cine loops that can beused to examine global and regional left ventricularfunction.ECG-triggered tube current modulation: As discussed inthe preceding section, “Tube Current,” ECG-triggeredtube current modulation is used to reduce radiationdose during systole when there is the greatest cardiacmotion and can reduce the radiation exposure signifi-cantly. In this circumstance, tube current is at nominalvalue only during the portion of the cardiac cycle likely tobe used for reconstruction (typically end diastole). Duringthe remainder of the cardiac cycle, the tube current isreduced to reduce radiation output. Recent refinementsof this technique have allowed reduction of the length oftime (“window”) during which tube current is nominaland reduction of tube current during the undesired por-tions of the cardiac cycle by 20% and to as little as 3% to5% of the nominal value (95–97). A potential disadvan-tage of this technique is that images reconstructed from


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projection data acquired with low tube current may betoo noisy to be diagnostic for coronary anatomy.Retrospectively ECG-triggered tube current modulationworks best in patients with stable sinus rhythm and lowheart rates (specific thresholds depend on scannercharacteristics).

7. Image reconstruction. Filtered back-projection hashistorically been used to reconstruct CT images fromprojection data. The advent of greater computingpower has made an alternative statistical method—iterative reconstruction—practical for CT. This methodpredicts projection data based upon an initialassumption about the attenuation in each voxel andcompares that data to measured projection data. Thevoxel attenuation values are modified iteratively untilan acceptable level of error between the predicted andmeasured data is obtained. The resulting reconstructedimages have lower noise values compared with thoseobtained with filtered back projection. This permitsreducing tube voltage and/or current to obtain imageswith comparable noise and lower radiation dose(98,99). One important characteristic of iterativereconstruction is that excessively low-dose images donot appear grainy, as is the case with filtered backprojection. Instead, structures become blurred and candevelop a blotchy appearance, undermining theirdiagnostic effectiveness.

8. Image postprocessing filters. These may also be appliedto acquired images to reduce image noise whilepreserving image contrast and edges. The feasibility ofusing these filters for radiation dose reduction hasbeen recently demonstrated (100).

7.3.4. Summary Checklist of Dose-Sparing Practices for X-Ray CT

Checklist of Dose-Sparing Practices for X-Ray Computed Tomography


, Consider appropriateness and utility of nonradiation-based imaging techniques

Equipmentcalibration

, Acquisition detector doses as low as compatible withdiagnostic image quality

Procedureplanning

, Select lowest-dose acquisition protocol compatiblewith study goals. Retrospective gating should beselected when feasible

, Use ECG-gated variable tube output if retrospectivegating is used

, Use the lowest x-ray tube voltage compatible withadequate diagnostic quality image acquisition

, Use the lowest x-ray tube current compatible withdiagnostic quality image acquisition. Use topogram-based tube current modulation

, Use the largest scan pitch compatible with adequatediagnostic quality image acquisition

Study conduct , Minimize patient heart rate, Confine scanned body area to the area relevant to the

study’s diagnostic purpose

7.4. Nuclear Cardiology Techniques

7.4.1. Nuclear Cardiology General Principles

There are 2 categories of nuclear cardiac imaging:

1. Single photon imaging, which is intrinsically a planarformat technique in which images can be acquired ineither planar or tomographic (SPECT) formats.

2. Positron imaging, which is an obligatory tomographicformat (PET).

PET generally administers a smaller radiation dose tothe patient and is less affected by patient attenuation. It iscurrently more expensive to perform than SPECT becausethe scanners, being more complex, are more expensive toacquire, and because some of the radiopharmaceuticalsused in PET are currently more expensive than those usedin SPECT.

Achieving optimal image quality in nuclear cardiologyinvolves considering more than just spatial resolution.Nuclear cardiology images compare tracer activity atdifferent locations in moving structures. Consequently,image noise, contrast, and temporal resolution are oftenmore important imaging attributes as long as imagespatial resolution meets a necessary minimum value.

Like x-ray imaging, nuclear image quality is in partdetermined by the quantity of radiation that reaches thedetector to form the image. This presents dose-image qualitytradeoffs that are similar to the tradeoffs in x-ray imaging.Quality is also influenced by the quality of the equipment,with more recent scanners being more sensitive and able togenerate a quality image from a smaller number of counts.

7.4.2. Nuclear Cardiology Equipment Quality, Calibration, and

Maintenance

In nuclear cardiology imaging, achieving an optimal bal-ance of image quality and patient dose requires that theimaging equipment be in good operating order. Nuclearcardiology imaging equipment has benefited from sub-stantial engineering progress. These improvementsinclude new detector designs and better electronics, bothfor PET and SPECT, ultimately resulting in greater sensi-tivity. For example, current state-of-the-art solid state(cadmium zinc telluride) detectors achieve a direct elec-trical signal output from an incident photon, eliminatingthe need for photomultiplier tube scintillation detectionand improving energy resolution. Novel cardiac-specificdesigns allow for a much higher effective detector sizefocused on the heart.

All of these improvements enable acquisition of asatisfactory image from a smaller radiopharmaceuticaldose, reducing patient exposure. Nuclear cardiologyfacilities should endeavor to have recent-generationequipment and an organized program of equipmentperformance surveillance. This is discussed in greaterdetail in Section 9.



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7.4.3. Nuclear Cardiology Spatial Resolution and Image

Detector Dose

Overall, achieving an optimal balance of patient exposureand image quality requires a judicious balancing of imageacquisition parameters, hardware, imaging time, andradiopharmaceutical dose.

To have a diagnostic image, the “good” signal mustoutweigh the noise (the signal to noise ratio). Typically,noise is approximately the square root of number ofcounts (101). Thus, signal to noise ratio is exponentiallyproportional to the overall number of counts that form theimage. For example, if 100 counts are collected, noisewould account for about 10 counts (square root of 100 is10), and signal to noise ratio would be 10. On the otherhand, if 10,000 counts are collected, the signal to noiseratio would be 100.

The impact of signal to noise and system sensitivityaffects choices of subject radiopharmaceutical dose,comfort, and convenience. An image, to be of diagnosticquality, should be properly collimated and be formedfrom a requisite number of counts.

Three variables affect the number of registered counts:

1. The imaging system’s sensitivity.2. The amount of radioactivity administered to the sub-

ject. This affects the dose the subject will receive fromthe procedure.

3. The length of time that counts are acquired to form theimage. This affects the scan acquisition time, with im-plications for the subject’s experience, and the amountof scanning time needed to complete the study withimplications for facility throughput.

Typically, a given scan acquisition is conducted until arequisite number of counts has been collected. If theradiopharmaceutical dose administered is small, althoughthe radiation dose to the subject will be smaller, a longerscan acquisition time will be required. Patients cannot liestill indefinitely. Consequently, there are limitations topractical acquisition duration without introducing motionartifacts.

7.4.4. Procedures and Practices to Minimize Patient Exposure

The variables that affect patient dose that can be adjustedin nuclear myocardial perfusion imaging include: radio-pharmaceutical selection and dose, image acquisitiontime, whether to routinely acquire rest images, and modeof stress.

7.4.4.1. Radiopharmaceutical Choice

SPECT Imaging Agents: Tc-99m and Tl-201

The most commonly used SPECT radiopharmaceuticalsuse Tc-99m bound either to sestamibi or tetrofosmin.These radiopharmaceuticals have largely supplantedTl-201 chloride because of Tc-99m’s superior imaging

characteristics and lower radiation dose to the subject. Atypical effective dose range for a 1-day Tc-99m rest-stressimaging protocol is 9.8 to 16.3 mSv (102). However, Tl-201has a pharmacological advantage in that it redistributesover time, providing a viability assessment with noadditional radiation exposure. Thus, Tl-201 has a role thatmay be considered in cases when viability data is needed.

PET Imaging Agents: Rb-82 Chloride, N-13 Ammonia,

F-18 Fluorodeoxyglucose

PET myocardial perfusion imaging can be done with anumber of radiopharmaceuticals. All positron emittersyield photons with energies of 511 keV, which is idealfor imaging.

The most commonly used agent is Rb-82 chloride. Rb-82 has a very short physical half-life, which confers anadvantage, an operational challenge, and a disadvantage.The advantage is that the rapid physical decay can ach-ieve high count rates with a substantial decrease in sub-ject dose. The operational challenge is that theradionuclide dose must be administered promptly after itis generated, and the patient must be imaged immediatelyafter nuclide administration. The disadvantage is that theshort half-life requires that only pharmacological stresscan be used. This means that the independent prognosticinformation available from exercise stress is lost unless aseparate nonimaging exercise stress is performed.

PET’s radiation dose advantage is slightly offset by thefact that PET imaging requires attenuation correction,which requires an additional x-ray CT exposure. Theamount of additional radiation from the x-ray CTcomponent is variable depending on the technique used,but should be very small compared with the radionuclidedose. Many centers currently perform CT attenuationcorrection for SPECT as well, so if a high-dose CT protocolis used, the total amount of radiation employed may notbe that different between SPECT and PET.

N-13 ammonia is an excellent imaging agent for quan-tification of myocardial blood flow. It is cyclotron-generated and has a 10-minute half-life. The latter re-sults in a subject radiation dose for a rest-stress study ofonly 2.2 mSv, which is even lower than Rb-82 rest-stressmyocardial perfusion imaging. Because of its very shorthalf-life, N-13 imaging can only be conducted in a facilitywith an on-site (or very nearby) cyclotron, and productionmust be very tightly paired to administration. This pre-sents logistical and financial challenges that limit theavailability of N-13 imaging.

Rb-82 chloride, which has a 75-second half-life, isgenerator-produced. This makes it more readily availablethan N-13. It provides a viable alternative to Tc-99 SPECTimaging in centers that have the infrastructure of PETscanners and sufficient clinical volume to amortize thegenerator cost. Rb-82’s short half-life results in a subject


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radiation dose for a rest-stress study of 3.3 to 3.8 mSv thatis a fraction of the dose from a Tc-99m sestamibi 1-dayrest-stress imaging study (103,104).

F-18 is cyclotron-generated and has a 110-minute half-life. The 18-fluoride anion is then incorporated intoglucose to make 18-fluorodeoxyglucose. Because of therelatively short half-life, it must be administeredpromptly. It is widely used to image tumors and inflam-mation. In cardiovascular medicine, it is of value to detectviable and hibernating myocardium and to imagemyocardial sarcoid and inflammation.

7.4.4.2. Imaging Protocol Choice

Imaging Agent

There are a variety of imaging protocol strategies thatprovide different opportunities to reduce subject dose.Each involves compromises and tradeoffs.

As Tc-99m imaging agents and Rb-82 do not redis-tribute, separate radiopharmaceutical injections areneeded for the stress and the resting scans. Because of Rb-82’s short physical half-life, both injections can utilize thesame dose, as the first dose’s will have decayed by thetime the second dose is administered (typically 20 to 30mCi). However, for a Tc-99m same-day stress-rest proto-col, the first administration is with a lower dose (typically10 mCi) and the second employs a higher dose (typically30 mCi). The basis of this strategy is that the higher doseof the second administration overwhelms the residualcounts from the first administration. This necessarilymeans that the lower-dose administration may be count-poor and may be nondiagnostic, particularly in largerpatients. In such cases, a 2-day protocol using themaximal dose each day is preferred.

Stress-Rest Versus Rest-Stress Protocol

Stress-first and rest-first acquisition protocols havedifferent merits, and choosing between them involvesbalancing a number of considerations. The choice balancesthe important goal of reducing patient dose against therequirement to ensure acquisition of a fully diagnosticstudy. A low-dose stress-first protocol offers the potentialto acquire a diagnostic study at reduced total patient dose ifthe stress images are of good quality and are normal. If thestress image is normal, there is no need to do a rest image,and the study can be completed with a single low-doseinjection. Use of stress-first imaging in some patients hasbeen recommended as a best practice protocol by the In-ternational Atomic Energy Agency (101) and is widelypracticed effectively in Europe. However, although stress-first imaging offers the potential of saving dose when thestress images are normal, it also means that the stress im-age, arguably the more important of the 2, is acquired witha lower radiopharmaceutical activity; in some patients—particularly in those with significant obesity—this may

compromise image quality and thereby the study’s overalldiagnostic accuracy. In addition, if the stress image isabnormal, the rest image must be obtained anyway for acomplete study.

The American Society of Nuclear Cardiology imagingguidelines balance these competing priorities in protocolselection (102,105). While noting that “in patients withouta high pre-test probability of a stress perfusion defect orleft ventricular dysfunction or dilatation, a low-dosestress/high-dose rest Tc-99m protocol is advantageousbecause a significant percentage of these patients willhave normal stress imaging, thereby obviating the needfor the rest imaging with its additional radiation expo-sure,” the guideline allows that in “larger patients (e.g.>250 lbs or BMI >35) or in female patients where signifi-cant breast attenuation is anticipated, a low dose of Tc-99m radiotracer may result in suboptimal images and a2-day imaging protocol with higher activities (18 to 30mCi) for each injection may be preferable.”

Thus, stress-first imaging is advisable in subjects whoare good imaging subjects and who do not have a highpretest probability of an abnormal study. Of note, the useof attenuation correction and/or prone imaging mayincrease the proportion of patients who are appropriatesubjects for stress-first imaging. For larger patients orfemales with significant breast attenuation, a 2-dayprotocol is advisable. Depending on the practice setting,rest-first imaging would be appropriate in subjects who arelikely to have abnormal stress imaging, or who will bedifficult to image but forwhoma 2-day study is not feasible.

7.4.4.3. Image Acquisition Practices

The patient dose for a nuclear cardiology study is deter-mined by the radionuclide used and the dose injected.The dose required is determined by the capabilities of theimaging equipment, how it is used, and the practical time-period to acquire images. Increasing the efficiency ofimage acquisition lowers the amount of radioactivityneeded, reducing the dose administered to the patient.Current scanners acquire counts more efficiently thanearlier models and make it possible to use lower injectedradioactivity doses.

The activity required to acquire a diagnostic study isalso influenced by aspects of the acquisition technique.There are a number of practices that, if followed, willenable acquisition of quality diagnostic images at smallerradioactivity doses. It is important to emphasize thatthese are study conduct decisions that should be madeindividually, based on the particulars of the patient andthe study goals by the physician responsible for the study.

1. Camera positioning. Radioactive emission intensitydecreases with the square of distance from the sourceto the detector. Consequently, it is critical to position

Checklist of Dose-Sparing Practices for Nuclear Cardiology



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the camera as close as possible to the patientthroughout the acquisition. By doing this diligently,image quality at a specific dose will be maximized,enabling the use of a smaller radioactivity dose. Manymodern systems automatically contour to the patient’sbody. Accordingly, it is important to position the pa-tient such that no extraneous material will increase thepatient-to-detector distance (e.g., excess blankets, pa-tient’s arms, and so on).

2. Iterative image reconstruction. Iterative reconstructiontechniques can improve image quality from a givendataset. Iterative reconstruction capabilities are incor-porated into current SPECT and PET systems. Thus,virtually all SPECT and PET acquisitions should bereconstructed with an iterative technique because,compared with filtered back projection, iterative tech-nique improves image quality for a given radiationdose. The specific reconstruction parameters cansignificantly alter the image smoothing/noise charac-teristics, and so these parameters can be adjusted tothe preferences of the reader, with input from themanufacturer and medical physicist.

3. Attenuation correction x-ray dose. The x-ray doseemployed for attenuation correction adds to theexposure from the radionuclide tracer. While attenua-tion correction is important to improve diagnostic ac-curacy, it is important to minimize the magnitude ofthe additional radiation exposure. Attenuationcorrection-capable scanners utilize either rod-source orCT-source radiation. Rod-source is intrinsically lowdose (typically <1 mSv and has been reported to be aslow as 0.04 mSv—substantially less than the radionu-clide tracer dose). CT-source, on the other hand, can behighly variable and substantially greater depending onthe CT acquisition protocol employed. Consequently, ifa scanner utilizes a CT scan for attenuation correction,operators should ensure that the acquisition protocolgenerally employs the lowest dose available (106,107).


, Consider appropriateness and utility of nonradiation-based imaging techniques

Modalityselection

, Select the appropriate technique and radionuclide thatprovides diagnostic quality information at the leastpatient radiation dose.

Dose relationships:

N-13 H3 PET< Rb-82 PET< Tc-99m SPECT< Tl-201 SPECT

Equipmentcalibration

, Use scanners with cadmium zinc telluride [CZT]detectors

Procedureplanning

, Use stress-rest protocol in preference to rest-stresswhen the overall clinical situation (clinical scenario andpatient imaging characteristics) is appropriate

, Use the smallest radionuclide dose compatible withadequate count acquisition rates

, Position camera head as close to the patient as possible

, Use iterative reconstruction

, Minimize radiation exposure for attenuation correction

7.4.5. Procedures and Practices to Protect Occupationally

Exposed Healthcare Workers in Nuclear Cardiology

Facilities

Occupationally exposed healthcare workers are at expo-sure risk from 3 sources:

1. Exposure incurred when handling radiopharmaceuti-cals prior to administration.

2. The ambient radiation from the radioactive patient.3. Residual radiation from an inadvertent or unrecog-

nized contamination or spill.

Contained doses of radiopharmaceuticals should beproperly shielded to minimize the escape of radiation intothe environment to expose healthcare workers. Although

some radiation emanating from a patient who hasreceived a radiopharmaceutical dose inevitably escapesinto the environment, the magnitude of exposure tohealthcare workers can be managed by protocols thatminimize the time that workers are in close proximity toradioactive subjects. There is also the potential for radi-ation exposure to members of the public, including thepatient’s family. However, at the doses administeredclinically, exposure to the public is small and well withinregulatory limits. Therefore, in this case, no specific in-structions are warranted.

The consequences of contamination or a radiophar-maceutical spill can be much more severe. This is partic-ularly problematic as human senses cannot detectradiation. Thus, there is a potential for a spill orcontamination to occur without the workers realizing it.Because of this possibility, it is necessary to have in placerigorous spill-protective practices; rigorous safety pro-tocols that govern personal conduct in “hot” areas; andrigorous surveillance of equipment, work surfaces, andpersonnel for radioactive contamination.

In particular, a substantial radiation exposure topersonnel can occur if they ingest or otherwise take inradiopharmaceutical from contamination, volatilization,and so on. Therefore, it is critically important to avoideating, drinking, applying cosmetics, and so on in areaswhere radioactivity may be present. Furthermore, alcohol-based hand sanitizers, although useful in preventing spreadof infection, are not effective at removing radiopharma-ceutical contamination. Consequently, rigorous handwashing is the principal protective strategy to remove/neutralize potential radiopharmaceutical contamination.

7.4.6. Summary Checklist of Dose-Sparing Practices for

Nuclear Cardiology

TABLE 13 Dose Minimization Strategies

X-RayFluoroscopy

Use alternative nonradiation-based imaging techniques(ultrasound, magneticresonance imaging,electromagnetic mapping)when appropriate.

Radiological equipment:Current state-of-the-art equipmentcalibrated for minimaldose exposures.

Operator conduct:Optimal systempositioning, minimalimaging field size,minimal exposuretime.

X-ray system operatingmodes: Slowest frame rateand the smallest dose perframe consistent withdiagnostic quality imagingand appropriate procedureguidance.

Medical personnel protection:Optimal use of protectivegarments and shields,maximize distance from thex-ray source, and minimizepatient exposure (which, inturn, minimizes medicalpersonnel exposure).

X-Ray CT Study appropriateness:Ensure that an x-ray CTexamination is the optimalimaging technique toanswer the clinicalquestion.

Radiological equipment:Equipment should becurrent state of theart, in good workingorder, and calibratedfor minimal doseexposures.

Scan protocol: Selectthe lowest-dose scanprotocol that willprovide images ofdiagnostic quality toanswer the clinicalquestion.

Scan size: Confine the imagedbody region to the smallestarea needed to answer theclinical question.

NuclearCardiology

Study appropriateness:Ensure that nuclearcardiology study is theoptimal imaging techniqueto answer the clinicalquestion. Consider PETrather than SPECT imagingif feasible and appropriate.

Imaging equipment:Equipment should becurrent state of theart, in good workingorder, and calibratedfor minimal doseexposures.

Scan protocol: Select astress-rest protocolwhen appropriate(according to ASNCguidelines).

Radiopharmaceutical choice:Tc-99m is preferred forSPECT. Avoid Tl-201 exceptfor studies focusedparticularly on myocardialviability issues.

Radiopharmaceutical dose:Use the smallest injecteddose that will providesufficient counts forimaging in a practical timeperiod.

ASNC ¼ American Society of Nuclear Cardiology; CT ¼ computed tomographic; PET ¼ positron emission tomography; SPECT ¼ single-photon emission computed tomography;Tc ¼ technetium; Tl ¼ thallium.


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7.5. Summary of Dose Minimization Strategies in X-RayFluoroscopy, X-Ray CT, and Cardiovascular NuclearScintigraphy

Table 13 summarizes dose minimization strategies for theimaging modalities discussed in this document.

8. MODALITY-SPECIFIC OPERATOR EDUCATION

AND CERTIFICATION


Physicians who operate or supervise the operation ofradiological equipment should be able to operate it in amanner that achieves optimal image quality whileminimizing radiation exposure to patients and toattendant medical personnel (including themselves).To achieve this successfully requires the physicianoperator/supervisor to hold the following requisite coreknowledge bases:

1. The basics of radiation physics, radiation biology, andradiation protection.

2. Equipment operation knowledge base specificallyrelevant to the imaging modality:

n How the equipment operates.n How settings, configuration, andother operational

choices affect image quality and radiation dose.

Although overall competency in radiological tech-niques requires understanding image interpretation in

addition to equipment operation, this section focuses onthe knowledge required to achieve optimal equipmentoperation to minimize radiation exposure withoutcompromising image quality.

8.1.1. Regulatory Authority

Organizations that have governance over training forusers of ionizing radiation include the U.S. NuclearRegulatory Commission (NRC), the U.S. Department ofLabor Occupational Safety and Health Administration,The Joint Commission, and state governments. There isconsiderable state-to-state variation in the degree ofregulatory oversight and establishment and enforcementof minimum mandatory training requirements. Thehealthcare institutions have credentialing responsibilityand, accordingly, are responsible to ensure that allhealthcare providers who operate radiological equipmenthold a requisite understanding of the relevantmodalities.

8.1.2. Professional Society Guideline and Position Statements

The Conference of Radiation Control Program Directors isa consortium of radiation safety officers that has pro-duced a series of resolutions including “That the CRCPDencourage healthcare facilities to require appropriateeducation and training of all personnel, to include phy-sicians, before they are permitted to operate fluoroscopicmachines, and that this training include radiation safetyand the biological effects of radiation exposure.” This is



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codified in a statement applicable principally to x-rayfluoroscopy and issued in 2004 (108). The Conference ofRadiation Control Program Directors recommends to stateregulatory agencies that all persons operating fluoro-scopic x-ray systems complete training in the followingbefore using fluoroscopy independently: 1) biological ef-fects of x-ray; 2) principles of radiation protection; 3)factors affecting fluoroscopic outputs; 4) dose reductiontechniques for fluoroscopic x-ray systems; 5) principlesand operation of the specific fluoroscopic x-ray system(s)to be used; 6) fluoroscopic and fluorographic outputs ofeach mode of operation on the system(s) to be used clin-ically; and 7) applicable requirements of these regula-tions. Regulatory authority to implement theserecommendations as regulations resides with individualstates’ departments of health.

The ACC has published expert consensus documentsand training standards (COCATS [Core CardiovascularTraining Statement]) for all aspects of cardiovascularmedicine and specifies training in radiation safety andprotection in those documents (75,109–112). Similardocuments have been published by the ACR and theAmerican Council for Graduate Medical Education(16,113,114).


8.2.1. Physician Responsibilities

Operator choices and conduct are of paramount impor-tance in affecting patient and operator dose during x-rayfluoroscopically guided procedures (discussed in detail inSection 7). There are many equipment calibration andoperational conduct choices under the physician opera-tor’s control that influence the radiation dose received bythe patient and by the attendant healthcare personnel. Itis important that physician operators understand thevariables affecting patient and personnel radiationexposure and that they apply this understanding tominimize exposure to patients and to attendantpersonnel. This requires initial education for traineesand, for experienced operators, periodic refreshertraining and orientation to new equipment features andcontrols.

8.2.2. Operator Training/Education Recommendations and

Requirements

There are no regulatory requirements for specific trainingof physician operators of x-ray fluoroscopic units.

In 2000, the European Commission published recom-mendations that interventional cardiology specialistsachieve a high level of knowledge regarding the generalprinciples of radiation protection, operational radiationprotection, and radiation protection of patients and staff.Achieving a medium level of knowledge is recommendedfor:

1. Radiological quantities and units.2. Physical characteristics of the x-ray or therapy

machines.3. Fundamentals of radiobiology and biological effects of

radiation.4. Quality control and quality assurance.5. Regulations and standards for ionizing radiation.

The European Commission suggests 20 hours ofinstruction to achieve this knowledge base (115).

In 2004, the ACC, in conjunction with the AmericanHeart Association, Heart Rhythm Society, and Society forCardiovascular Angiography and Interventions, crafted aclinical competence statement on physician knowledge tooptimize patient safety and image quality in fluoroscopi-cally guided invasive cardiovascular procedures (75).

The ACC, in its 2008 COCATS training statement, out-lined a detailed blueprint for the knowledge base to beheld by practicing interventional cardiologists. Thisincluded understanding x-ray imaging, including thedesign and operation of fluoroscopy and fluorographicunits, digital imaging and storage, radiation physics,factors that influence image quality, radiation qualityassurance, and the physiology using x-ray contrast media(116). This statement was updated in 2015. This compre-hensive work outlined a detailed curriculum that con-forms to The Joint Commission standards. Specificrecommendations for the appropriate clock hours fortraining were not made, but a range between 2 and 20hours of instruction time has been recommended by otherexperts in the field. The most recent ACC training state-ment on training in cardiac catheterization states thattrainees should understand the principles of radiationsafety (111).

The American Board of Internal Medicine includes thebasic principles of x-ray imaging, radiation protection,and radiation safety in its interventional cardiology ex-amination blueprint (117). The Society for CardiovascularAngiography and Interventions published recommenda-tions for establishing a radiation safety program for thecardiac catheterization laboratory in 2011 (74). The NCRPhas recommended initial training for fluoroscopic cre-dentialing and refresher training for recredentialing (39).

8.3. X-Ray CT


Cardiovascular CT, and in particular coronary CT angiog-raphy, is a complicated procedure that is demanding to dowell. Many protocol decisions are required to optimizedose and image quality. Currently available radiologicalequipment can employ many different image acquisitionprotocols, which can have major impacts on both imagequality and patient dose (discussed in depth in Section7.3). There is good evidence that special dose-reduction


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training results in sustained lower median doses, and thisis especially true in lower-volume centers (118). Conse-quently, physicians who conduct cardiovascular CT ex-aminations are responsible to hold the requisiteknowledge base to employ optimal image acquisition andto be able to make informed equipment selection choices.Physicians who supervise x-ray CT facilities are, in addi-tion, responsible to understand equipment capabilities tomake optimal choices in equipment selection.

8.3.2. Society-Developed Operator Training/Education

Requirements

The SCCT and the ACR have published guidelines on ra-diation dose and dose optimization strategies in cardio-vascular CT (33,119).

Standardized training has been endorsed in the UnitedStates (110,120,121) with appropriate emphasis on radia-tion dose, radiation exposure factors, hazards of radiationexposure to both patients and CT personnel, and tech-nical aspects of scan acquisition. The ACR has publishedcertification criteria for radiologists who perform cardio-vascular CT (114).

Many of the practices that impact radiation safety incardiovascular x-ray CT are related to scanner quality andimaging protocol selection, which are determined princi-pally by the facility clinical leadership. The cognitive andtechnical skills necessary for the competent performanceof cardiovascular CT are addressed in an American Col-lege of Cardiology Foundation/American Heart Associa-tion clinical competence statement, endorsed by multipleprofessional societies dedicated to cardiac imaging, in2005 (120) and updated in 2015 (110). This statement fol-lows the standard 3 clinical competence-level model, withthe 3 levels requiring different training durations andminimum numbers of mentored examinations performedor interpreted. Level 3 training is required to qualify in-dividuals to serve as a director of a cardiac CT facility, andrequires 6 months of training at a minimum of 35 h/week.“Knowledge of the physics of CT and radiation generationand exposure” is among the cognitive skills listed asrequired for competence in cardiovascular CT, but theamount of time or specifics of instruction in radiationbiology, dosimetry, and safety are not specified.

Certification and recertification in cardiovascular CTcan be achieved in various ways. The SCCT endorses thecardiovascular CT experience as required by the AmericanCollege of Cardiology Foundation/American Heart Asso-ciation clinical competence statement (122). The SCCTalso offers designation as fellow to members who show“evidence of ongoing interest and contribution in car-diovascular diseases through cardiovascular CT”following completion of training.

The Certification Board of Cardiovascular ComputedTomography, part of the Alliance for Physician

Certification and Advancement, administers a certifica-tion examination to document “mastery of a defined bodyof knowledge” (123). Certification is for a period of 10years. Criteria for recertification have been publishedonline (124).

8.4. Nuclear Cardiology Techniques


Nuclear cardiology equipment used for cardiovascularimaging has a range of capabilities and complexity span-ning from relatively simple single-headed gamma cam-eras for planar imaging to cardiac-specific solid-statedetector multipinhole collimated devices and to highlysensitive time-of-flight capable PET devices with andwithout hybrid CT scanners. The practitioner must befamiliar with the specific capabilities of the system beingused as well as its calibration and quality control re-quirements (see Section 9.4.2). The facility directorshould oversee equipment selection, maintenance andcalibration, and imaging protocol policies so that the fa-cility will generate consistently high-quality images whileminimizing patient dose. The physician director must alsobe familiar with the workings of the instrumentation be-ing used to be able to recognize and identify signs ofequipment malfunction. The physician director must bean authorized user, is responsible to oversee all aspects ofthe radiopharmaceutical use, and is responsible for theconduct of the technologists who are administering ra-diopharmaceuticals to patients and acquiring images.

8.4.2. Summary of Current Regulatory Requirements

Nuclear cardiology is subject to considerably greatergovernmental regulation than the other radiation-basedimaging modalities. There are multiple regulatory re-quirements that govern nuclear imaging procedures.

8.4.2.1. Authorization to Prescribe Radiopharmaceuticals

Radiopharmaceuticals are under the purview of the FDAand, under Federal law, can only be administered to pa-tients by a prescription of a physician who has satisfiedfederal (NRC) criteria for status as an Authorized User(AU). AU status is obtained by meeting the legal criteria ofthe NRC as legally outlined in 10.CFR.35.200 (125) and10.CFR.35.290 (126) for “Imaging and Localization.” Boardcertification by the American Board of Nuclear Medicine,American Board of Radiology in Diagnostic Radiologyand/or Nuclear Radiology, or the Board of Nuclear Cardi-ology are pathways to AU status, because these certifica-tion examinations meet 10.CFR.35 criteria.

The education and training requirements to be an AUare issued by the NRC and specify didactic and practicaltraining requirements. Education and training include 80hours of classroom instruction and 700 hours of clinicalexperience. Although passing a certifying examination



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developed by 1 of the previously mentioned organizationsprovides evidence of AU eligibility, status can also beapproved on the basis of attestation of training by an AU.

NRC AU status is specific to particular categories ofradiopharmaceutical use, such as dilution studies, imag-ing and localization studies, and various therapeutic uses.In general, those authorized on the basis of CBNC certi-fication will be authorized only for imaging and localiza-tion studies.

The NRC rules are based on federal laws but manystates have become “agreement states” wherein a signif-icant portion of the oversight is transferred to the state (orlocal) regulatory agencies. These states or municipalitiesmust adhere to the minimum regulations set forth by theNRC, but can impose stricter limitations in some aspects(some areas, like training requirements, cannot be modi-fied). Therefore, the AU must understand the specificconstraints imposed by the local regulatory agencies andnot assume that federal guidelines are the only ones thatmust be followed.

8.4.3. Operator Training/Education Requirements

As discussed in the previous section, much of the trainingrequired for handling radiopharmaceuticals and for con-ducting and interpreting nuclear cardiac imaging isspecified by the NRC guidelines for AU status and is alsoaddressed in the ACC COCATS-4 training statement (109).

For hybrid imaging techniques, the reader should havesome knowledge and understanding of x-ray CT cardio-vascular imaging. Although the CT images acquired withthe minimum radiation dose are not of full diagnosticquality and are intended principally for attenuationcorrection and anatomic localization, there is the poten-tial that important and recognizable anatomic data maybe evident on the CT component. Thus, while the CTimages should be reviewed with a primary focus onregistration, there is also a responsibility to examine themfor potentially important incidental findings that wouldwarrant clinical follow up (e.g., pulmonary parenchymalnodules). These interpretations should be made with thecaveat that the CT images are not of full diagnosticquality.

9. QUALITY ASSURANCE

9.1. Introduction and General Principles

A robust quality assurance process is central to the safeand efficient operation of any clinical service and isparticularly important when the service’s activitiesexpose the patient and personnel to potential risk, as isthe case with procedures that utilize ionizing radiation.Because radiation is undetectable by human senses, thereis potential to undervalue its importance as a hazard andfor inadvertent or unrecognized excessive exposure

either to patients or to personnel. Consequently, qualityassurance procedures must be in place to verify that pa-tient and personnel exposures are minimized. Qualityassurance, guided by the ALARA principle, involves bothexamining current practices and monitoring exposures ofpatients and personnel.

Quality assurance has 2 principal goals:

1. To survey a facility’s current operations with respect tothe exposures being delivered to patients andpersonnel and to compare them to benchmark values(where available).

2. To identify improvement opportunities to decreaseexposures.

Action to limit patient ionizing radiation exposure wasproposed in a 2010 FDA white paper (127). In this publi-cation, FDA recommended 3 actions to optimize the safeuse of radiation-based medical imaging:

1. Establish requirements for manufacturers of CT andfluoroscopic devices to incorporate additional safe-guards into equipment design, labeling, and usertraining.

2. Partner with the Centers for Medicare and MedicaidServices to incorporate key quality assurance practicesinto accreditation and participation criteria for imagingfacilities and hospitals.

3. Collaboration between the healthcare professionalcommunity and FDA to develop diagnostic referencelevels for CT, fluoroscopy, and nuclear cardiologyprocedures locally and also through a national radia-tion dose registry.

The ACR has implemented a dose index registry thatpermits participating sites to compare their dose indexesto regional and national values (128). This registrycurrently is principally oriented toward diagnostic refer-ence dose levels for x-ray CT abdominal and head exam-inations with little emphasis on x-ray CT of the chest andx-ray fluoroscopy.

Successful quality assurance involves monitoring andtabulating radiation exposure to patients and clinicalstaff. These data should be monitored over time for trendsas well as to identify individual outlier cases. Clinical staffand operators should receive feedback characterizingtheir individual performances and exposure. Thereshould be ongoing efforts to continue to reduce patientand personnel exposure as new developments or prac-tices emerge.

Modifiable determinants of radiation exposure thatform the basis of radiation safety quality assuranceinclude:

1. Equipment quality and calibration.2. Imaging protocol selection.


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3. Equipment operator and personnel conduct.4. Identification and follow-up of high exposure

procedures.

The interactions of each of these variables in theaggregate determine the magnitude of exposure to bothpatients and personnel. Success of a quality assuranceprogram, and consequently minimizing exposure, re-quires support and collaboration at the level of the insti-tution, the facility administrative leadership, and thefacility clinical personnel.


9.2.1. X-Ray Fluoroscopy Regulatory Issues and

Societal Policy Statements

Currently, dose calibration for x-ray fluoroscopic units islargely at the discretion of the equipment manufacturer.Statutory regulation of permissible dose ranges is sparse.The FDA governs the maximum permissible fluoroscopicdose at the interventional reference point at 88 mGy/minin standard fluoroscopy mode and permits a high-dosefluoroscopy mode of 176 mGy/min for limited use underspecial circumstances (129).

The ACC has previously published recommendationsfor quality assurance in cardiac catheterization labora-tories. These recommendations address radiation safetyin x-ray fluoroscopy (130).

9.2.2. X-Ray Fluoroscopic Radiological Equipment Quality

and Calibration

Quality assurance in x-ray fluoroscopy begins with as-suring that x-ray equipment is in good operating orderand is calibrated to achieve optimal image quality atminimal dose. Interventional catheterization and elec-trophysiology laboratories should have regular evaluationby the institution’s qualified medical physicist. Equip-ment survey should include an assessment of imagequality and radiation dose. The steps required include:

1. Acceptance testing: measurement of the baselineequipment performance when initially installed (toconfirm that the unit is performing to baselinespecifications).

2. Longitudinal surveillance testing: performance mea-surement should be conducted periodically to identifypossible deterioration in equipment performance, andto verify correction of deterioration. If performancedeteriorates, the equipment manufacturer shouldrecalibrate the unit or renovate it if necessary.

Typical x-ray system calibrations are expressed as thedose reaching the detector in nGyper pulse. Current x-rayfluoroscopic systems display this dose value in the met-adata accompanying recorded images. For current state-of-the-art equipment, the following are representative

factory default image detector dose settings with the de-tector in a 22-cm zoom mode (typical zoom for coronaryangiography):

n Low-dose fluoroscopy: 20 nGy/pulsen Standard-dose fluoroscopy: 40 nGy/pulsen Cine acquisition: 200 nGy/pulse

These should be considered benchmark values. X-rayfluoroscopic units that deliver doses substantially greaterthan these values should be recalibrated or renovated toachieve similar doses or, if not remediable, retired.

9.2.3. X-Ray Fluoroscopic Imaging Protocol Selection Practices

A laboratory should develop a culture of selecting theimaging protocol that provides satisfactory image qualitypermitting accomplishment of the task at hand whileemploying minimum dose. Operators should be accul-turated to use low-dose fluoroscopy and slow pulse rateswhen applicable, such as for general catheter placement,while reserving standard dose fluoroscopy and fasterpulse rates for tasks that require greater spatial andtemporal resolution. X-ray system default parametersshould be set to low-dose protocols so that a conscious actis required to select a higher-dose protocol. Pyne et al.(131) evaluated the clinical and radiation dose impact ofreducing fluoroscopy and cine framing rates from thecommonly used 15 to 10 pulses/s. This demonstrated theexpected 38% reduction in dose per procedure, and theyfound that operators rapidly adapted to the slowerframing rates. Thus, they achieved a 38% dose reductionwithout compromising clinical efficacy (131).

9.2.4. X-Ray Fluoroscopic Operator and Personnel Conduct

Physician operators are responsible to operate x-rayfluoroscopic equipment in a manner that optimizes thesafety of patients, laboratory occupationally exposedpersonnel, and themselves. This requires a radiationsafety awareness culture on the part of both the physicianoperators and the laboratory personnel. Physician opera-tors should be acculturated to minimize beam on time,optimize system positioning, use collimators to minimizeimage field size, and use protective shielding. In partic-ular, physician operators should be cognizant of protect-ing the laboratory personnel. This includes maximizinglaboratory personnel distance from the radiation source,interrupting fluoroscopy when laboratory personnel needto approach a patient, providing portable shielding withinthe laboratory for circulating personnel, and providing anout of laboratory control room for the personnel who aremonitoring but not directly attending to the patient.

Similarly, laboratory personnel need to be cognizant ofthe factors that affect their personal radiation exposure tooptimize their personal protection. As the catheterization



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laboratory’s physician personnel and case mix expand toencompass new medical disciplines and procedures, it isessential that all operating physicians—cardiologists andcardiac and vascular surgeons—hold requisite radiationprotection knowledge and adhere to appropriate radiationsafety procedures and protocols. It is noteworthy thatradial access for cardiovascular procedures has the po-tential to increase operator exposure unless the operatoris careful to maximize his/her distance from the radiationsource (132).

Initial training in radiation safety with annual updatesshould be the norm for all personnel who work in an x-rayfluoroscopic environment.

9.2.5. X-Ray Fluoroscopic Patient Radiation Exposure Monitoring

There are 2 types of readily available patient exposuremetrics that are calculated by the x-ray unit:

n Procedure fluoroscopy and cine acquisition timen Patient exposure metrics: total air kerma at the inter-ventional reference point and total procedure KAP

Patient dose and imaging time monitoring should beconducted with 2 purposes:

1. To identify trends for excessive dose and imagingtimes in group data from overall facility experience.This will enable identification of physician operatorswho employ excessive fluoroscopic and cine times.

2. To identify individual outlier procedures that receiveexcessive exposures. This will enable recognition andprompt treatment of patients who are at risk of radia-tion skin injury.

Appropriate quality assurance surveillance shouldinclude both of these parameters. In particular, patientinterventional reference point doses that exceed thethreshold for potential skin injury (5 Gy) should bereviewed and the affected patients should be counseledto be aware of the skin injury potential. A follow-upprotocol should be in place for these patients. The JointCommission has identified a skin entrance dose >15 Gy asa reviewable sentinel event (133).

9.2.6. Effectiveness of Programs to Minimize Patient Radiation

Exposure in X-Ray Fluoroscopy

Approaches to minimizing patient radiation exposure inthe fluoroscopy suite include optimizing equipment per-formance and improving operator behavior. Changingoperator behavior to reduce radiation dose can beaccomplished with physician education and increasingoperator awareness of radiation dosing (134).

A comprehensive program for radiation dose reduc-tion was undertaken at the interventional cardiologylaboratories of the Mayo Clinic, Rochester, Minnesota, in2008. Along with technical optimization of the

equipment and fellow education, radiation exposure wasannounced during the case in air-kerma increments of3,000 mGy. Procedures exceeding 6,000 mGy werereferred to the radiation safety committee, and imme-diate physician feedback was provided. Over the courseof 3 years, a 40% reduction in radiation dose was ach-ieved (76).

The Mayo Clinic experience also provides benchmarkdata for fluoroscopy time and patient dose at the inter-ventional reference point. They report a median fluoros-copy time for left heart catheterization and coronaryarteriography of 5.8 minutes and for percutaneous coro-nary intervention of 15.7 minutes. The corresponding skindoses for coronary arteriography were median 467 mGywith a 75th percentile of 936 mGy. For percutaneouscoronary intervention, the median skin dose was 952 mGywith a 75th percentile of 1,491 mGy. These values reflect acombination of operator proficiency and x-ray equipmentquality and calibration.

9.3. X-Ray CT

9.3.1. X-Ray CT Regulatory Issues and Societal Position

Statements

9.3.1.1. Governmental Regulation

The FDA regulates CT imaging systems under 2 statutes(135):

1. As radiation-emitting electronic products under theRadiation Control for Health and Safety Act

2. As medical devices under the Medical Device Amend-ments to the Food, Drug, and Cosmetic Act.

The regulations implemented under these laws placecontrols or requirements on the manufacturers of CTscanners rather than on the users. Under the RadiationControl for Health and Safety Act, the FDA administers anequipment performance standard for diagnostic x-raysystems (136). This standard:

1. Establishes minimum radiation safety requirements forCT scanners.

2. Requires that manufacturers produce CT scanners thatcomply with the radiation safety requirements of theperformance standard.

3. Requires manufacturers to certify that their productsmeet the standard.

As part of this certification, manufacturers set forth intheir quality assurance manuals lists of standard tests tobe performed, and specify the phantoms to use for thetesting.

9.3.1.2. Societal Standards

The ACR, American Association of Physicists in Medicine,NCRP, International Electrotechnical Commission, and


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the International Commission on Radiation Units andMeasurement are prominent among the organizationsthat are involved in setting x-ray equipment standards,and provide guidance on how to comply with the regu-lations set forth by law, including the implementation ofquality assurance and quality control programs.

The pertinent ACR/American Association of Physicistsin Medicine technical standards require that all CTequipment undergo performance evaluation upon instal-lation, and be monitored by a qualified medical physicistto ensure that it is functioning properly at least annually,or more often if required by state or local regulatoryagencies. A qualified medical physicist is competent topractice independently in 1 or more subfields in medicalphysics, as evidenced by certification (e.g., from theAmerican Board of Radiology, the Canadian College ofPhysics in Medicine, or the American Board of MedicalPhysics), continuing education, and experience. Thequalified medical physicist must be familiar with:

1. Principles of imaging physics and of radiationprotection.

2. The guidelines of the NCRP.3. Laws and regulations pertaining to the performance of

CT scanners.4. The function, clinical uses, and performance specifi-

cations of the CT scanner.5. Calibration processes and limitations of the in-

struments used for testing performance (137).

9.3.2. X-Ray CT Equipment Quality and Calibration

Equipment calibration and preventive maintenance aspart of quality assurance and control programs play animportant role in reducing radiation dose by facilitatingdose optimization. Regularly scheduled performancemonitoring is important to verify that x-ray CT systemsproduce optimal-quality diagnostic images at a radiationdose appropriate for an examination’s imaging purpose(83,137). Periodic surveys of equipment performance areimportant, as x-ray CT units can drift out of calibrationwithout readily noticeable changes in imaging perfor-mance. Performance evaluation and quality control mayreveal deviations in either radiation output or imagingperformance (or both) (83).

Components of regular (at least annual) performancemonitoring as part of a quality assurance program includethe following radiation output characteristics immedi-ately pertinent to radiation dose:

1. Radiation beam width (collimation).2. Reconstructed image thickness.3. Measurement of radiation output (CTDIvol or the

equivalent) for representative examinations.4. Estimates of patient radiation dose for representative

examinations.

Measured scanner output and patient dose should becompared with the values reported by the scanner consoleand with the appropriate guidelines, recommendations,or reference levels (if available) (137).

Each facility, in accordance with national and stateregulatory agencies, can decide to conduct additional,more frequently performed routine testing of additionalimaging performance characteristics. A continuous qual-ity assurance program should evaluate at least:

1. CT number accuracy (average and standard deviationof water).

2. Image noise and field uniformity.3. Image artifacts.4. The acquisition workstation (98).

Quality assurance testing may make use of a phantomsuch as that developed by the ACR (138), which isdesigned to measure CT number accuracy, slice thickness,low contrast resolution, CT number uniformity, and highcontrast resolution.

If performance evaluation or quality assurance showthat the measured values for any of these parametersfall outside the established tolerances, appropriateinvestigative or corrective actions should be under-taken. Such actions may include a service request by afield engineer, typically from the scanner manufacturer.The medical physicist should evaluate in a timelyfashion the need for repeat performance testing after amajor component of a CT scanner has been repaired orreplaced (137).

9.3.3. X-Ray CT Imaging Protocol Selection

Adoption of and adherence to standardized imagingprotocols for standard imaging circumstances is impor-tant for consistency, particularly with CTA imaging.Guidance for imaging protocols has been proposed byexpert consensus to derive the maximum diagnosticyield with the minimum radiation exposure (83).Although the specific imaging protocol should be tailoredto the individual patient and the goals of the imagingstudy, patient exposure can be minimized by employingbest practices and adhering to standardized protocolswhenever possible. A research collaborative of 15 hospi-tal imaging centers in Michigan undertook an initiativeto reduce the radiation dose in cardiac CTA scanning. Abest-practice model with physician and technologist ed-ucation was employed to promote consistent applicationof dosage reduction techniques. As a result of thisinitiative, patients’ median radiation dose decreased by53% without change in image quality. It was concludedthat to achieve and then maintain this positive change,ongoing radiation dose monitoring and review would beneeded (118).



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9.3.4. X-Ray CT Patient Radiation Exposure Monitoring

Currently, at a national level, it is not routine practice tomonitor and tabulate the radiation dose received duringindividual CT scans. However, trends are emerging at thelevel of individual states toward more rigorous assess-ment. Since 2012, California requires that either CTDIvol orDLP as displayed on the scanner console be included inevery radiology report. Beginning in 2016, the JointCommission’s new diagnostic imaging standards alsorequired documentation of CT dose in a retrievableformat for every patient examination (139).

9.4. Nuclear Cardiology

9.4.1. Nuclear Cardiology Regulatory Issues

9.4.1.1. Governmental Regulation

Nuclear imaging facilities, because they handle radioac-tive material, are subject to statutory regulation by a va-riety of governmental bodies. These include the NuclearRegulatory Commission, the FDA, the Department ofTransportation, and the Department of Homeland Secu-rity. Additional regulation also is provided by state andmunicipal departments of health.

These regulations, which are extensive, govern thehandling, storage, and administration of radioactive ma-terials. Additionally, regulations govern the design of fa-cilities to ensure appropriate radioactive material storageand handling as well as shielding appropriate for the ra-dionuclides to be used in a facility. Regulations alsogovern radiation surveillance procedures including re-quirements for radiation detecting and measuringequipment.

9.4.1.2. Societal and Industrial Standards and Guidelines

The National Electrical Manufacturer’s Association pro-vides guidance on acceptance testing including definedmetrics of equipment performance both for SPECT andPET systems. The American Association of Physicists inMedicine adds additional recommended SPECT and PETacceptance testing procedures.

Laboratory accreditation agencies, such as the ACR andthe Intersocietal Accreditation Commission, also provideguidance documents that detail the required specific testsfor specific instrumentation and procedures and the re-cords that must be kept and available for inspection. Inadditional, The Joint Commission may require review ofany documents relating to patient care, including gammacamera and PET quality assurance records (140–145).

9.4.2. Nuclear Scintigraphy Equipment Quality and Calibration

SPECT and PET scanners can drift out of calibrationwithout clearly evident changes in clinical images. Thus,it is essential that these instruments’ performance be

surveyed regularly. Comprehensive quality assuranceprograms cover many different aspects of nuclear imag-ing. Specific measurements are recommended on a daily,weekly, and quarterly basis. In most facilities, calibrationand quality control is conducted with assistance of amedical physicist and/or vendor service. Some accredi-tation criteria, for example ACR accreditation, requireoversight by a certified medical physicist.

Although older systems required significant userinvolvement for quality control and calibration, mostmodern PET/CT systems and an increasing fraction ofcurrent SPECT and SPECT/CT systems have built-inautomated daily quality control procedures. With thesesystems, all that is required of the user is to place a sealedsource phantom in the appropriate location and launchthe process (some systems even include a quality controlsource that automatically moves into the field of view).The acquisition and data analysis is automatic and yieldseither a passing or failing result. These processes havebecome so simplified that there is no valid excuse for notperforming quality control and calibration checks as pre-scribed by the manufacturer.

Although uniformity is important for planar imaging, itis particularly important for SPECT as any non-uniformities will be propagated and magnified by SPECTimage reconstruction algorithms. SPECT quality controlmust also include verification of the center of rotation.

9.4.2.1. Conventional Gamma Cameras

Gamma cameras, including those used to acquire SPECTdata should have daily performance assessments. Thisincludes “peaking” detectors to verify accuracy of thephoton energy window and obtaining uniform flood fieldsto verify uniform responsiveness over the imaging field.Ideally, a scanner should be re-peaked just prior to eachdata acquisition. Weekly tests include center of rotation,bar phantom imaging to verify spatial resolution, andhigh count flood acquisition. Quarterly quality assurancetests include evaluation of tomograms acquired andreconstructed using standardized multipurpose phan-toms of Plexiglass and radioactive water, which typicallyinclude spheres and rod sections of various sizes, fromwhich image contrast, spatial resolution, and field uni-formity are quantified. Reconstruction of a point source isused quarterly to verify consistency of tomographicspatial resolution. For SPECT cameras that use translatingradioactive rod sources for attenuation correction, dailylow-count floods and weekly high-count floods arerequired.

9.4.2.2. PET Systems

Daily quality control involves testing all of the detectormodules by acquiring a blank scan or phantom scan.Sensitivity should be quantified weekly by cross


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referencing to dose calibrator readings, and at leastannual testing should be performed of accuracy, scatterfraction, and attenuation correction accuracy. Quarterlymeasurements typically are performed of standardizedmultipurpose phantoms of Plexiglass and radioactivewater, which typically include refillable cylinderinserts and rod sections of various sizes, from whichstandard uptake value, spatial resolution, and field uni-formity are quantified.

9.4.2.3. CT Component of SPECT/CT and PET/CT Units

Additional considerations must be given to equipmentquality assurance and radiation issues related to the CTcomponent of SPECT/CT and PET/CT imaging. For the CTcomponent, daily tube warm-up and air calibrationchecks are recommended, and weekly or monthly waterphantom checks of slice thickness, accuracy, and posi-tioning. Appropriate registration between SPECT and CTor PET and CT transaxial images are verified along withattenuation correction accuracy at least quarterly, if notmore frequently.

At a minimum, a daily uniformity flood must be ac-quired to verify that count acquisition is uniform. Spatialresolution should be assessed weekly by imaging a barphantom. It is particularly critical that the plannedmaintenance includes photopeak calibration for all iso-topes employed by the facility.

9.4.3. Nuclear Scintigraphy Attenuation Correction Equipment

Quality and Calibration

Attenuation correction greatly improves diagnostic accu-racy and is an extremely valuable adjunct to conventionalSPECT and PET imaging. It provides the potential toremove attenuation-related artifacts and/or permitquantification. However, attenuation correction adds atransmission scan imaging procedure to the nuclearacquisition with the potential to add importantly to thetotal patient radiation dose. Different transmission scan-ning protocols involve different incremental exposures. Itis important to select the protocol that causes the smallestpatient exposure.

9.4.4. Nuclear Cardiology Patient Radiation Exposure Monitoring

As discussed in the previous text, the patient dose fromnuclear studies derives from the radiopharmaceuticaldose combined with the dose from any coacquiredtransmission scan. For studies that include a rod sourcetransmission scan, this is generally negligible. For studiesincluding a CT, the CT dose ideally should be monitoredin the same manner as doses for standalone diagnostic CTstudies (albeit with different targets). For the radiophar-maceutical dose, individual patient monitoring andmeasurements are not performed. In lieu of this, eachdepartment should review, at least on an annual basis,

their procedure manual and prescribed radiopharmaceu-tical doses. The review should include a comparison withpublished practice guidelines to ensure that the dosesused are as low as reasonable to ensure diagnostic imagequality and in line with those recommended by consensusstatements/published guidelines. However, it is impor-tant to note that different instrumentation may havesubstantially different sensitivity and other parameters,and so one cannot simply choose the lowest dose sug-gested in guidelines as this may be insufficient for diag-nostic image quality on some systems.

9.4.5. Nuclear Cardiology Clinical Personnel Exposure Protection

and Monitoring

Because staff in nuclear cardiology are routinely in closeproximity to radioactive materials, as well as to patientswho have been administered radiopharmaceuticals, it iscritical that all staff members be monitored with personaldosimeter badges. These must be worn in a reproduciblefashion and stored in a location with typical backgroundradiation. Most staff members require a whole-bodydosimeter badge. Additionally, staff who handle dosesyringes should also wear a ring dosimeter to enableseparate calculation of whole body and extremity dose.

The frequency of badge exchange may be tailored toexpected exposure, with those with very low expectedexposure needing perhaps quarterly badges. For moststaff in nuclear cardiology, monthly monitoring isappropriate. Dosimeter results should be available to allstaff and should be reviewed by the radiation safety of-ficer. It is good practice to set trigger levels (e.g., 10% ofthe annual dose limit, discussed in Section 5), and anystaff members who exceed the trigger level in a givenmonth or quarter should have their practice patternsreviewed to ensure that their doses adhere to the ALARAprinciple. Of note, because many nuclear cardiac imagingcenters are multidisciplinary facilities with technologists,nurses, physicians, and so on, staff-monitoring protocolsshould be based on actual work duties and likelihood ofradiation exposure rather than making assumptions basedon a given staff member’s training or job title. Monitoringshould err on the side of over monitoring rather thanunder monitoring.

Occupational radiation exposure to radiopharmaceuti-cals is different from exposure to x-ray beams in thatexternal shielding is less protective. Since the potentialfor uterine exposure is more prominent in nuclear cardi-ology, there are specific procedures and lowered exposurelimits for pregnant staff members. Of note, these do notapply unless a staff member has declared in writing thatshe is pregnant. Upon declaration of pregnancy, addi-tional monitoring is appropriate with a separate fetalbadge that may need expedited processing and poten-tially more frequent exchange. The facility radiation



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safety officer is responsible for determining the maximumpermissible fetal dose for the pregnant staff member andfor ensuring that she is properly monitored, with report-ing occurring in a time frame that accurately assesses herfetal and overall exposure, thus ensuring that accumu-lated exposure is within guideline limits.

10. PATIENT AND MEDICAL PERSONNEL

RADIATION DOSE MONITORING AND

TRACKING: PROGRAMMATIC AND

INDIVIDUAL CONSIDERATIONS

Currently, technology exists to measure and track cumu-lative radiation dose for both patients and occupationallyexposed healthcare workers. The importance of trackingthe accumulated radiation dose of occupationally exposedhealthcare workers is indisputable as it is important fortheir protection and can indicate a need for correctiveaction when necessary. However, compared with occu-pational dose tracking, patient medical exposure dosetracking is more complex and nuanced. This is in partbecause of uncertainty surrounding the actual healthimpact of current medical radiation practices (146,147).

Now that current imaging equipment and electronicmedical records make patient dose monitoring and tabu-lation feasible, the question has been raised as to whethertracking cumulative individual patient medical radiationexposure is a worthwhile undertaking that has practicalclinical value. Patient-level tracking technologies havebeen proposed such as an electronic “smart card” thatwould be updated following each medical exposure (148).The FDA together with the ACR and other societiesdeveloped the “Image Gently” educational and socialmarketing program, which was followed by the “ImageWisely” campaign. This program offers a medical imaginghistory card that patients can use to track an estimate oftheir personal accumulated radiation exposure (149,150).The FDA has posted a white paper on its website titled“White Paper: Initiative to Reduce Unnecessary RadiationExposure from Medical Imaging,” in which the agencyadvocates for a national radiation exposure tracking sys-tem (127).

10.1. Requirements for Dose Monitoring and Tracking

For cumulative radiation dose data acquisition to bemeaningful, it must be compiled rigorously andcompletely. To track a patient’s radiation dose, it is crit-ical that all exposures be monitored, reported, andrecorded in a standardized manner. To do so, it would benecessary to:

1. Record radiation exposure from all medical tests andprocedures involving radiation. This requires that ra-diation exposure metrics be included in all test andprocedure reports in a standardized manner using

discrete, uniform parameters that allow for the calcu-lation of cumulative radiation exposure.

2. Compile standardized exposure data from allradiation-based modality procedures automatically inmanagement systems and electronic medical records/electronic health records.

3. For occupationally exposed workers, compile occupa-tional exposure data to include medical exposure data.

10.2. Program-Level Dose Monitoring and Tracking

Program-level patient dose tracking provides potentiallyvaluable quality assessment data. It has value in moni-toring both:

1. A program’s overall radiation-based cardiovascularprocedures utilization rates.

2. The range of administered per-procedure radiationdoses for comparison to national norms and identifi-cation of outliers.

Consequently, for quality assessment and improve-ment purposes, a program should track, compile, andanalyze its radiation exposure data.

10.3. Patient-Level Dose Monitoring and Tracking

Currently, a medical procedure’s radiation exposure dataare not systematically recorded. Consequently, data arenot uniformly stored in electronic health records in aformat that permits searching exposure values and tabu-lating accumulated exposures. In addition, patientsfrequently access care in multiple health systems.Consequently, at this time there is no infrastructure toenable compiling comprehensive patient-based radiationdose data that would permit rigorous assessment of totalaccumulated medical radiation exposure.

There is uncertainty about the value to a medicallyexposed patient of tracking his/her cumulative exposure(7). For cumulative exposure tracking to provide clinicalmanagement value, the patient’s radiation exposure his-tory and accumulated dose must be relevant to the deci-sion to conduct a future radiation-based examination orprocedure. Since a patient’s tissue reaction risk is linkedto prior, and, particularly, recent exposures, exposurehistory has a clear relevance for tissue reaction risk andmight potentially influence the choice to conduct a sub-sequent procedure.

On the other hand, although compiling an estimate oftotal accumulated exposure provides a potential estimateof a patient’s future stochastic risk, the linear-no thresholdconcept indicates that the incremental (above baseline)cancer risk that would be conferred by an individualradiation-based procedure would be independent of thepatient’s prior accumulated exposure. Thus, based on thisconcept, prior exposure history should not be a factor in


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determining a proposed procedure’s appropriateness. Thedecision to conduct the procedure should be based onoverall appropriate use guidelines for the procedure in thecontext of the patient’s current health circumstance.

There are complex considerations regarding the addedvalue of a patient having knowledge of his/her accumu-lated medical radiation dose. Although one’s accumulatedradiation dose is an important element of health infor-mation to which any person is entitled, it is challenging tointerpret such information. Interpretation of a given pa-tient’s accumulated radiation dose requires advancedunderstanding of the radiation safety knowledge base andmust be interpreted in the context of the patient’s age,overall state of health, and appropriateness of proposedmedical procedures. Thus, physicians need to be suffi-ciently informed about radiation safety to be able tointerpret a patient’s own clinical circumstance, assist apatient in understanding the significance and context ofhis/her accumulated radiation dose, and make thoughtfulevidence-based recommendations concerning the appro-priateness of additional radiation-based procedures.

Overall, based on the current radiation safety knowl-edge base, the importance for quality assessment of fa-cilities and institutions tracking and monitoringprocedure and patients’ total radiation exposure is clear.On the other hand, a program of rigorously tracking apatient’s lifetime cumulative radiation exposure appearsto provide little clinically important value for that pa-tient’s future clinical management.

11. SUMMARY, CONCLUSIONS, AND

RECOMMENDATIONS

11.1. The Issue

During the past 2 decades, there has been an explosivegrowth in the capabilities of both diagnostic examinationsand therapeutic procedures that employ ionizing radia-tion. This growth has been fueled by a combination ofimproved imaging capability resulting from refinedradiological equipment engineering and of extendedprocedure capability and complexity. As a result, therehas been both an increase in procedure utilization ratesand an escalation of their complexity. These de-velopments have increased medical radiation exposureboth at the population and the individual level, creatingthe potential for greater radiation-induced harm both dueto individual patient tissue reaction radiation injury riskand, potentially, to increased population-level cancerrisk. Both patients and occupationally exposed healthcareworkers are potentially at an increased cancer risk.

Radiation-related hazard presents an additional vari-able to weigh when considering whether to undertake agiven procedure, when making choices among alternative

procedure modalities, and when weighing procedureconduct decisions. The healthcare profession is respon-sible to be aware of this issue and to work to optimize therisk-benefit balance both for the individual and at thepopulation level.

11.1.1. Patient Participation in Clinical Imaging Decisions

Patients should participate with their physicians in thedecision to undertake any medical procedure that in-volves risk, and radiation risk is no exception. Patientsshould understand the radiation component of a proced-ure’s overall risk and interpret it in the context of theprocedure’s overall risk-benefit relationship. Thus, phy-sicians are responsible to explain the radiation compo-nent of a procedure as well as the other aspects of amedical procedure. These explanations, which should becomprehensive and rigorous, can be challenging toconduct given that many patients have limited under-standing of radiation metrics and effects (7). Current un-derstanding is that the incremental stochastic riskassociated with a particular procedure is almost always anextremely small component of the procedure’s total riskand can generally be discounted.

11.2. Clinical Value of Radiation-Based Imaging Studies andRadiation-Guided Therapeutic Procedures

A procedure’s risk-benefit relationship determines itsoverall clinical value (and, consequently, its appropri-ateness). The risk conferred by radiation exposure is acomponent of a procedure’s overall risk complement. Theestablished benefit to patient health outcomes ofradiation-based cardiovascular diagnostic and therapeu-tic procedures is substantial. The radiation-associatedhazard is generally very small in absolute terms andcertainly in comparison to hazards from other compo-nents of a procedure. In appropriate circumstances,radiation-based cardiovascular procedures have substan-tial clinical value that justifies the attendant risk,including the small associated radiation-associated haz-ard. Nonetheless, the radiation component is real andshould be assessed in considering a proposed procedure’sappropriateness.

The risk of radiation-caused cancer is not uniformacross the entire population. In addition to being line-arly related to dose magnitude, patient characteristics—particularly age, gender, and comorbidities—modulatethe risk associated with a particular radiation exposure.Because most radiation-induced cancer requires a min-imum of 5 years to emerge (some as early as 2 years[12]), the potential for radiation-induced cancer is lessrelevant in patients with shorter life expectancies. Therisk is the least important for elderly patients who haveimportant comorbidities and most important for



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children and young adults with a long life expectancy,particularly females, in addition to those with congen-ital heart disease who have an ongoing need for evalu-ation and risk an increased lifetime cumulativeexposure.

11.3. Individual Patient Risk and Population Impact(Including Occupationally Exposed Workers)

Medical radiation cancer risk has 2 potential impacts: acategorical effect on the exposed individual, and anaggregate probabilistic effect on the exposed population.

Individual patient risk is linearly related to the totaleffective dose. The risk estimate that accompanies aparticular total dose may be further refined by calculatingthe effective dose in mSv (taking into account the indi-vidual organ exposure magnitude) and also by consid-ering patient characteristics including age, gender, andlife expectancy.

The overall population risk is the potential for an in-crease in population cancer rates caused by the pop-ulation’s aggregate medical radiation exposure. Thiseffect is difficult to detect because of cancer’s largebackground frequency and the comparatively smallmagnitude of accumulated medical exposures. Nonethe-less, now that the U.S. population’s aggregate medicalradiation exposure is greater than background, there isreason for concern that medical radiation may become acontributor to overall cancer incidence.

11.4. The ALARA Principle

The ALARA principle has long been the guiding principlegoverning medical radiation exposure. It is based on thelinear-no threshold model of radiation cancer risk andstates that medical radiation exposure should beemployed judiciously and that healthcare professionalsare responsible for minimizing radiation exposure both topatients and to healthcare personnel.

11.5. The Potential to Minimize Exposure to Patients andPersonnel

Physicians have 2 options available to minimize patientmedical radiation exposure:

1. Choice of procedure modality2. Choice of procedure conduct

11.5.1. Imaging Modality Choice

Procedure modality choice is based on the purpose andgoals for the procedure. Often more than a single mo-dality can be employed to address a clinical issue. Ifalternative modalities provide truly comparable diag-nostic utility, a modality that does not employ radiationwould be preferable to a radiation-based modality. For

example, for some stress testing circumstances, bothnuclear perfusion imaging and echocardiographic imag-ing can have equivalent utility. On the other hand,depending on patient characteristics affecting echocar-diographic image quality, the ability of a patient to exer-cise, or on the clinical question to be addressed, nuclearperfusion imaging may be sufficiently superior to echo-cardiographic imaging to offset the small risk of attendantradiation exposure.

11.5.2. Procedure Conduct Choice

Procedure conduct choices can also have a substantialimpact on the attendant radiation dose. With currentimaging technology, the best image achievable mayactually be better than needed. In some circumstancesemploying a smaller radiation dose, although degradingimage quality somewhat, can still yield images of suffi-cient quality for diagnosis. Examples include x-ray fluo-roscopy at lower detector doses and/or slower framingrates, CT scanning at lower detector doses, and nuclearperfusion imaging employing smaller tracer doses.

11.5.3. Protecting Occupationally Exposed Workers

It is important to be vigilant to minimize healthcareworker occupational exposure. Healthcare workers whowork in radiation environments are typically young andtherefore more susceptible. Someone working in a radia-tion environment for an extended period has the poten-tial to accumulate a substantial exposure. Healthcareworker occupational exposure is greatest in an x-rayfluoroscopy environment. There is a synergistic incentiveto minimize patient x-ray fluoroscopy exposure becausethe same practices that decrease patient exposure alsodecrease occupational exposure. X-ray fluoroscopic en-vironments also have ample opportunity to controloccupational exposure through standard protectivepractices.

11.6. Physician Responsibilities to Minimize Patient Exposure

All physicians, whether or not they work in a radiationenvironment, have a responsibility to minimize patientexposure. This responsibility falls into 3 domains: pro-cedure selection, procedure conduct, and facilitymanagement.

11.6.1. Case Selection

When selecting a diagnostic or therapeutic procedure, aphysician is responsible to understand the procedure’scomplete risk-benefit relationship and, when there arealternative procedures to choose among, select the mostappropriate procedure. Radiation exposure is an impor-tant consideration that must be weighed in this choice. Itis important to consider the patient characteristics that


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modulate that risk. In particular, the patient’s risk factors,including patient age, comorbidities, and natural life ex-pectancy, should be considered. For younger patientswithout comorbidities, radiation-based imaging is lesspreferred than for older patients with limited life expec-tancies. Physicians should employ the ACC appropriateuse criteria as a point of departure in making thesejudgments. When feasible, particularly in younger pa-tients, an alternative imaging procedure that does not useionizing radiation may be preferable (e.g., cardiovascularmagnetic resonance or echocardiography).

11.6.2. Procedure Conduct

Physicians who perform radiation-based procedures areresponsible for understanding the variables that deter-mine patient dose and adjusting procedure conduct toachieve successful diagnosis or therapy while employingthe minimal necessary dose. In x-ray fluoroscopy, thisbegins with attention to beam-on time, beam collimation,and system positioning to minimize dose to both the pa-tient and to nearby clinical personnel. In addition, phy-sicians should select the imaging protocol (detector dose,frame rate) that minimizes dose while providing diag-nostic quality images. In x-ray CT, it is important to selectthe lowest dose imaging protocol that will yield diag-nostic quality images. In addition, care should be taken tolimit the examination to the body region of interest.Similarly, in radionuclide scintigraphy, which is one ofthe largest radiation exposure procedures in cardiovas-cular medicine, it is important to consider radiation doseand endeavor to minimize it when selecting a protocol. Itis important to select the radionuclide species that de-livers the least radiation exposure while best answeringthe clinical question(s) at hand, and to administer thesmallest radiopharmaceutical activities likely to ensurediagnostic image quality. Stress-first imaging, which hasthe potential to deliver substantially less dose, ispreferred for subjects who have a reasonable pre-testprobability of a normal study and who are good imagingsubjects (see Section 7). Rest-first imaging is appropriatefor subjects who are more challenging to image and arelikely to have abnormal studies. Thus, both physiciansordering and physicians conducting nuclear cardiologystudies should individualize the protocol according to thepatient’s characteristics with a goal of minimizing patientdose without compromising diagnostic quality.

11.6.3. Facility Management

Physicians who manage facilities that use ionizing radia-tion are responsible for ensuring that those facilitiesgenerate high-quality images at minimal radiation expo-sure to patients and personnel. These responsibilitiesinclude radiological equipment selection, calibration, andmaintenance; establishing imaging protocols that

optimally balance image quality and exposure; andfostering a culture of minimizing patient radiation expo-sure and maximizing personnel protection.

Radiological equipment should be capable of gener-ating diagnostic quality images at minimal dose. A unitthat is in good operating order but requires a greater thancurrent state-of-the-art dose to generate quality imagesshould be considered obsolete. Such a unit should beeither renovated or replaced. The facility’s managing di-rector should collaborate with the radiological equipmentcompany’s service engineers and the institution’s radio-logical physicist to verify that equipment is optimallycalibrated. The equipment should provide user control ofimaging dose parameters so that operators can select theimaging protocol that best balances image quality anddose.

The facility’s physician director should monitor thefacility’s overall radiological performance by tabulatingpatient procedure doses and personnel doses to ascertainthat these doses are within guideline levels. Individuallarge outlier exposures should be investigated andexplained, and corrective action should be taken ifindicated.

11.6.4. Imaging Equipment Renovation and Replacement

The past decade has seen prodigious engineering effortsby equipment manufacturers to improve image qualitywhile decreasing radiation dose. Current imaging unitsgenerate vastly better images with less radiation andincorporate features that permit operators to select lower-dose imaging techniques when appropriate. If an olderimaging unit requires larger radiation doses than thecurrent state of the art, it should be considered obsoleteeven if it is in good working order and should be eitherreplaced or, if feasible, renovated to bring its performanceup to current standards.

11.7. Patient Radiation Dose Tracking

Technology now exists that could be applied to create acomprehensive patient dose tracking system. There hasbeen some advocacy to create such systems. However,current understanding of the biological basis of cancerinduction by radiation does not support a clinical utilityto the patient of longitudinal tracking. Based on thelinear-no threshold concept, the incremental cancer riskassociated with a particular medical exposure is inde-pendent of prior exposure magnitude. Knowing a pa-tient’s lifetime accumulated radiation exposure providesno additional information of clinical decision-makingvalue with respect to the incremental stochastic riskthat would be conferred by a contemplated radiationexposure (although knowledge of prior exposure can aidin predicting the tissue reaction risk). The radiation-basedrisk that should be weighed when deciding whether to



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conduct a given procedure is the incremental risk that theprocedure’s exposure adds to the patient’s backgroundrisk. Consequently, knowing the amount of prior expo-sure is not a factor in the decision to conduct a proposedprocedure. Accordingly, tabulating a patient’s aggregateradiation exposure adds little practical clinical value. Theprincipal value of a radiation tracking program would beto provide data for future clinical research to more pre-cisely define the dose-stochastic risk relationship fordoses in the medical range.

11.8. Need for Quality Assurance and Training

Properly conducted quality assurance is essential forconsistent facility operation, and for providing reliablehigh-quality imaging while minimizing radiationexposure.

Quality assurance requires verifying equipment per-formance and calibration and monitoring metrics of pa-tient and personnel exposure. Proper training of allpersonnel is essential. Good training ensures that all

clinical personnel have the requisite understanding ofradiation physics, radiation biology, and radiation pro-tection. In addition, training should create a culture ofrespect for radiation hazard and a commitment to mini-mize exposure and maximize protection.

PRESIDENT AND STAFF LIST

American College of Cardiology

Mary Norine Walsh, MD, FACC, PresidentShalom Jacobovitz, Chief Executive OfficerWilliam J. Oetgen, MD, MBA, FACC, Executive Vice

President, Science, Education, and QualityJoseph M. Allen, MA, Senior Director, Clinical Policy and

PathwaysLea Binder, Team Leader, Clinical Policy and PathwaysAmanda Ladden-Stirling, Associate, Clinical Policy and

PathwaysShira Klapper, Publications Manager, Clinical Policy and

Pathways

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KEY WORDS ACC Expert Consensus Document,nuclear cardiology, positron emissiontomography radiation, radiation risk, radiationsafety, single-photon computed tomography,x-ray computed tomography, x-ray fluoroscopy

























http://scct.org/page/CBCCTCertification

http://scct.org/page/CBCCTCertification

http://www.cccvi.org/cbcct/content_200.cfm?navID=15






https://www.nrc.gov/reading-rm/doc-collections/cfr/part035/part035-0200.html





http://www.fda.gov/Radiation-EmittingProducts/RadiationSafety/RadiationDoseReduction/ucm199994.htm



http://www.acr.org/Quality-Safety/National-Radiology-Data-Registry/Dose-Index-Registry

http://www.acr.org/Quality-Safety/National-Radiology-Data-Registry/Dose-Index-Registry

http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?CFRPart=812&showFR=1
























http://www.jointcommission.org/assets/1/18/Radiation_Overdose.pdf






http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX-Rays/ucm115331.htm




http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/cfrsearch.cfm?fr=1020.33



https://www.acr.org/-/media/ACR/Files/Practice-Parameters/CT-Equip.pdf



http://www.acraccreditation.org/%7E/media/ACRAccreditation/Documents/CT/CT-Accreditation-Testing-Instructions.pdf?la=en





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http://www-pub.iaea.org/MTCD/Publications/PDF/Pub1393_web.pdf











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http://www.imagewisely.org/Patients.aspx

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Institutional,

APPENDIX A. AUTHOR RELATIONSHIPS WITH INDUSTRY AND OTHER ENTITIES (RELEVANT): 2018 ACC/

HRS/NASCI/SCAI/SCCT EXPERT CONSENSUS DOCUMENT ON OPTIMAL USE OF IONIZING RADIATION IN

CARDIOVASCULAR IMAGING–BEST PRACTICES FOR SAFETY AND EFFECTIVENESS

CommitteeMember Employment Consultant

SpeakersBureau

Ownership/Partnership/Principal

PersonalResearch

Organizational,or OtherFinancialBenefit

ExpertWitness

John W.Hirshfeld,Jr. (Chair)

Hospital of the University ofPennsylvania Cardiac

Catheterization Lab—Professor ofMedicine

None None None None None None

Victor A. Ferrari(Co- Chair)

Hospital of the University ofPennsylvania—Professor ofMedicine and Radiology


Frank M. Bengel Medizinische HochschuleHannover—Direktor, Klinik Fur

Nuklearmedizin

n GE Healthcaren Siemens Medical

Solutions

None None n Siemens MedicalSolutions*

None None

Lisa Bergersen Boston Children’s Hospital—Associate in Cardiology; Harvard

Medical School—AssociateProfessor of Pediatrics

n 480 Biomedical Inc None None None None None

Charles E.Chambers

Penn State Milton S. HersheyMedical Center—Professor of

Medicine & Radiology


Andrew J.Einstein

Columbia University MedicalCenter—Associate Professor of

Medicine (in Radiology)

n InternationalAtomic EnergyAgency

n Radiation EffectsResearchFoundation(Hiroshima, Japan)

n SpectrumDynamics

None n GE Healthcare*n Philips Medical

Systems*n Toshiba America

Medical Systems*

None None

Mark J.Eisenberg

Jewish General Hospital McGillUniversity—Professor of Medicine


Mark A. Fogel Children’s Hospital of PhiladelphiaDivision of Cardiology—AssociateProfessor of Pediatrics, Cardiology

& Radiology

None None None n EdwardsScientific

n Kerios None

Thomas C.Gerber

Mayo Clinic, Rochester—Professorof Medicine and Radiology


David E. Haines Willliam Beaumont Hospital—Director, Heart Rhythm Center

None None None n Boston Scientificn Medtronic

None None

Warren K. Laskey University of New Mexico Schoolof Medicine—Professor ofMedicine Chief, Division of

Cardiology


Marian C.Limacher

University of Florida—Professor ofMedicine, Division of

Cardiovascular Medicine


Kenneth J.Nichols

Northwell Health, Division ofNuclear Medicine and MolecularImaging—Senior Physicist; HofstraUniversity—Professor of Radiology

n Syntermed Inc.* None None None None None

Daniel A. Pryma Perelman School of Medicine atthe University of Pennsylvania—

Associate Professor of Radiology &Radiation Oncology; Chief, Nuclear

Medicine & Clinical MolecularImaging

None None None n Advanced Accel-eratorApplications†

n Bayer HealthcarePharmaceuticals†

n ProgenicsPharmaceuticals†

n Siemens†

None None

Continued on the next page

CommitteeMember Employment Consultant

SpeakersBureau

Ownership/Partnership/Principal

PersonalResearch

Institutional,Organizational,

or OtherFinancialBenefit

ExpertWitness

Gilbert L. Raff William Beaumont Hospital—Medical Director, AdvancedCardiovascular Imaging


Geoffrey D.Rubin

Duke University Medical Center—Professor of Radiology and

Bioengineering

n Fovian GE Healthcare

None n TerareconHeartFlow

None None None

Donnette Smith Mended Hearts—Executive VicePresident


Arthur E.Stillman

Emory University Hospital—Professor of Radiology & ImagingSciences; Professor of Medicine


Suma A. Thomas Cleveland Clinic Heart andVascular Institute—Vice-Chairman

Strategic Operations


Thomas T. Tsai Denver VA Medical Center;University of Colorado Denver—

Director Interventional Cardiology;Assistant Professor; CCOR Group-

Investigator


Louis K. Wagner University of Texas/HoustonMedical School Department of

Radiology—Professor


L. Samuel Wann University of Wisconsin, Madisonand Medical College of Wisconsin,Milwaukee— Clinical Professor of

Medicine

n AstellasPharmaceutical

n United Healthcare

None None None None None

This table represents the relationships of committee members with industry and other entities that were determined to be relevant to this document. These relationships werereviewed and updated in conjunction with all meetings and/or conference calls of the writing committee during the document development process. The table does not necessarilyreflect relationships with industry at the time of publication. A person is deemed to have a significant interest in a business if the interest represents ownership of $5% of the votingstock or share of the business entity, or ownership of $$5,000 of the fair market value of the business entity; or if funds received by the person from the business entity exceed 5% ofthe person’s gross income for the previous year. Relationships that exist with no financial benefit are also included for the purpose of transparency. Relationships in this table aremodest unless otherwise noted. According to the ACC, a person has a relevant relationship IF: a) the relationship or interest relates to the same or similar subject matter, intellectualproperty or asset, topic, or issue addressed in the document; or b) the company/entity (with whom the relationship exists) makes a drug, drug class, or device addressed in thedocument, or makes a competing drug or device addressed in the document; or c) the person or a member of the person’s household, has a reasonable potential for financial, professionalor other personal gain or loss as a result of the issues/content addressed in the document.*Significant relationship.†No financial benefit.

CCOR ¼ Colorado Cardiovascular Outcomes Research; VA ¼ Veterans Affairs.

APPENDIX 1. CONTINUED


e349



e350

APPENDIX B. PEER REVIEWER INFORMATION: 2018 ACC/HRS/NASCI/SCAI/SCCT EXPERT CONSENSUS

DOCUMENT ON OPTIMAL USE OF IONIZING RADIATION IN CARDIOVASCULAR IMAGING: BEST

PRACTICES FOR SAFETY AND EFFECTIVENESS

This table represents the individuals, organizations, andgroups that peer reviewed this document. A list of

Reviewer Representation

Lyndon Box Official Reviewer—SCAI W

James Case Official Reviewer—ASNC

Manuel D. Cerqueira Official Reviewer—ASNC Cle

Panithaya Chareonthaitawee Official Reviewer—SNMMI M

Mina K. Chung Official Reviewer—HRS Cle

Mehmet Cilingiroglu Official Reviewer—SCAI

B. Kelly Han Official Reviewer—SCCT Chil

Jacobo Kirsch Official Reviewer—NASCI Cl

Dharam Kumbhani Official Reviewer—ACC Task Force onClinical Expert Consensus Documents

Thomas H. Schindler Official Reviewer—SNMMI Joh

Todd C. Villines Official Reviewer—ACC Board of Governors

Paul C. Zei Official Reviewer—HRS

James A. Arrighi Content Reviewer—CompetencyManagement Committee

Noel G. Boyle Content Reviewer—EP Council UCLA

Rami Doukky Content Reviewer—Imaging Council C

Jonathan Halperin Content Reviewer—CompetencyManagement Committee

Mahadevappa Mahesh Content Reviewer—SCCT Johns

Joseph E. Marine Content Reviewer—ACC Task Force onClinical Expert Consensus Documents

Jo

Pamela B. Morris Content Reviewer—ACC Task Force onClinical Expert Consensus Documents

Me

William A. Van Decker Content Reviewer—Health AffairsCommittee

Robert Vincent Content Reviewer—ACPC Council C

ABIM ¼ American Board of Internal Medicine; ACC ¼ American College of Cardiology; ACPC ¼Association; ASNC ¼ American Society of Nuclear Cardiology; HRS ¼ Heart Rhythm Societydiovascular Angiography and Interventions; SCCT ¼ Society of Cardiovascular Computed TomoTexas.

corresponding comprehensive healthcare-related disclo-sures for each reviewer is available as an Online Appendix.

Employment

est Valley Cardiology—Interventional Cardiologist; Cardiac CatheterizationLaboratory Director

Cardiovascular Imaging Technologies—Chief Scientific Officer

veland Clinic Foundation—Chairman, Department of Molecular and FunctionalImaging

ayo Clinic—Consultant and Associate Professor of Medicine, Department ofCardiovascular Medicine

veland Clinic Lerner College of Medicine of Case Western Reserve University—Associate Professor of Medicine

Arkansas Heart Hospital—Professor of Medicine

dren’s Hospitals and Clinics of Minnesota and the Minneapolis Heart Institute—Director of Advanced Congenital Cardiac Imaging

eveland Clinic Florida—Professor; Harper University Hospital, Detroit MedicalCenter—Director, Center for Hospital Specialties

UT Southwestern Medical Center—Assistant Professor of Medicine

ns Hopkins University—Director of Cardiovascular Nuclear Medicine; AssociateProfessor of Medicine and Radiology

Walter Reed National Military Medical Center—Director, Cardiovascular CT

Stanford University School of Medicine—Clinical Professor of Medicine,Electrophysiology, and Cardiovascular Medicine

Rhode Island Hospital—Director, Graduate Medical Education

Medical Center, Cardiac Arrhythmia Center—Professor of Medicine; Director ofCardiac EP Labs & Fellowship Program

ook County Health and Hospitals System—Professor of Medicine, PreventiveMedicine, and Radiology; Chairman, Division of Cardiology

Mount Sinai Medical Center—Professor of Medicine

Hopkins Hospital—Professor of Radiology and Radiological Science; Professor ofMedicine, Cardiology, Chief Physicist

hns Hopkins University School of Medicine—Associate Professor of Medicine

dical University of South Carolina—Director, Seinsheimer Cardiovascular HealthProgram; Women’s Heart Care Co-Director

Temple University Hospital—Assistant Professor of Medicine

hildren’s Healthcare of Atlanta, Sibley Heart Center—Pediatric Cardiologist

Adults With Congenital Heart Disease and Pediatric Cardiology; AHA ¼ American Heart; NASCI ¼ North American Society for Cardiovascular Imaging; SCAI ¼ Society for Car-graphy; SNMMI¼ Society of Nuclear Medicine and Molecular Imagine; UT ¼ University of



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APPENDIX C. ABBREVIATIONS

ALARA ¼ as low as reasonably achievable

AU ¼ Authorized User

CT ¼ computed tomography

CTDI100 ¼ is a measure of the dose delivered along a
100-mm scan length
CTDIw ¼ weighted or average CTDI given across the
field of view
DLP ¼ dose length product

eV ¼ electron volts

Gy ¼ gray

FDA ¼ Food and Drug Administration

ICRP ¼ International Commission on Radiation Protection

KAP ¼ Kerma-area product

Kerma ¼ kinetic energy released in material

keV ¼ kiloelectron volts

ms ¼ millisecond

mSv ¼ millisieverts

MIRD ¼ medical internal radiation dose

PET ¼ positron emission tomography

RWI ¼ relationship with industry

Sv ¼ sievert

uSv ¼ microsieverts (1/1,000 of a millisievert, 1/1,000,000
of a sievert)