IAEA International Atomic Energy Agency Radiation Protection in Paediatric Radiology Radiation Protection of Children During Computed Tomography L06

IAEAInternational Atomic Energy

Agency

Radiation Protection in Paediatric Radiation Protection in Paediatric RadiologyRadiology

Radiation Protection of Radiation Protection of Children During Children During

Computed TomographyComputed TomographyL06L06

IAEA Radiation Protection in Paediatric Radiology L06. Radiation protection in computed tomography

2

Educational objectives

At the end of the programme, the participants

should:

•Recognize that CT is a relatively higher dose imaging procedure.

•Understand dose management strategies for computed tomography in children.


3

Answer True or False

1. Reduction of kVp in CT reduces the dose.2. CT contributes 60-70 % of the dose from

radiological examinations in developed countries.

3. The same CT protocol used for children and adults will result in a higher dose to adults.


4

Contents

• Overview of CT systems: SDCT and MDCT.• Dose levels in CT and risk attributable to paediatric CT.• Importance of application of justification in paediatric

CT.• Optimization of image quality and patient dose in

paediatric CT.• Selection of appropriate technical parameters. • Use of shielding devices in paediatric CT.• Dose management strategies in paediatric CT.• Requirements for staff: experience and training.


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Computed Tomography

• Computed tomography (CT) is the method that extends the clinical capabilities of X-ray imaging:• High contrast sensitivity for visualizing soft tissues.• Production of configurable data sets.

• Three-dimensional (3D) representations• Multiplanar depictions• “Volume” CT

• Dynamic (e.g. perfusion, cardiac) information• Tissue characterization (dual energy technology)

• Advances in computed tomography (CT) technology have continued to improve existing and open new clinical applications.


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Computed Tomography

• Since 1972; then…

Hounsfield

Cormack

Nobel prize for medicine 1979


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Computed Tomography

• ..and now…


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Modern CT Scanners

• Modern CT scanners are 3rd generation, that is the tube and detectors rotate together around the patient

• Slip ring technology allows for spiral hence volume scanning

M. Mahesh, MDCT physics, the basic technology, image quality and radiation dose, Wolters Kluwer, 2009

Principle of spiral CT. Patient is transported

trough the gantry, x-ray tube traces spiral path

around the patient when acquiring data


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Single Detector (SDCT) vs Multi-detector (MDCT) Computed

Tomography


SDCT and MDCT design. The difference is the presence of multiple-row detectors

in the longitudinal direction with MDCT yielding multiple slice options for single rotation


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Multi-detector (MDCT) Computed Tomography


MDCT detectors


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CT and Paediatric Radiology

• The patient dose in CT is an important issue for children.

• In some centres, the exposure factors used for scanning children are the same as for adults.

• CT scanning contributes most to collective dose from exposures from medical imaging due both to relatively high dose per exam and to the increasing use of this modality.


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Facts About CT…

• Facts about CT…•69 million CT examinations per year for all

ages in USA in 2007.•Approximately 10% growth rate per year•7 million CT examinations per year in children•40-50 % increase in paediatric CT from

2005/06.•Up to 31% of paediatric body CT examinations

are multiphase in some reports


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Facts About CT…

• The frequency of CT examinations is evenly distributed at all ages:•33% are performed in children under age

of 10

• Repeated examination: •30% of adults and children have three or

more CT scans METTLER, F.A., et al., J. Radiol. Prot. 20 4 (2000) 353-

359


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CT as a Dose Contributor

CT examinations:• comprise only 17% of all radiological

examinations, but...• contributes to 49% of the effective dose all

radiological examinationsRadiological examinations

CT17%

Rest83%

Mettler et al. Helath Phys 2008, 95:502-7

Collective dose

CT49%

Rest51%


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Amount of Radiation Resulting From CT

Examination Effective Dose (mSv) Chest X-ray Equivalents

3-view ankle radiography 0.0015 0.07

2-view chest radiography 0.02 1

Radionuclide cystogram 0.18 9

Flouroscopic cystogram ~0.33 ~16

Radionuclide bone scan ~5 ~250

Brain CT 2 100

Chest CT up to 3 up to 150

Abdominal CT up to 5 up to 250

Frush D, et al, CT and Radiation Safety: Content for Community Radiologistswww.imagegently.org


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Radiography CT

Why is this so?


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Why is this so?

Dose distribution**in relative units


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Risk of CT Examination

• Unique consideration in children:• Life time to manifest the bioeffects• More radiosensitive tissues• Dose is considered cumulative over time• Risk is higher for females and younger age

groups

• From a single abdominal CT in paediatric age, lifetime estimated risk for fatal cancer is 1: 1000 - 1: 2000.


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Risk Versus Benefit

• Important to distinguish between individual risks and collective, public-health risks

• The individual risks are small, so the benefit / risk ratio for any child will generally be very large,

• …but the exposed population (~7.0 million children/yr in the US) is large

• Even a very small individual radiation risk, when multiplied by a large (and increasing) number of children, is likely to produce a significant long-term public health concern


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CT in Paediatric Radiology

• The frequency of paediatric CT examinations has been increasing over the past 20 years• Reduced requirements for sedation and allowance

of examination of younger, sicker and less co-operative children

• Increased speed of acquiring diagnostic information

• Increased number of multiple scans• Attention must be given to adapting protocols

to suit children taking into account that they are more sensitive than adults


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In Paediatric Radiology…

• If identical CT head examination protocol is used:

•Adult dose: 1.5 mSv•Child dose: 6 mSv

Huda et al. Radiology, 1997, 203:417-22


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In Paediatric Radiology…

• It estimated that between a third and half of the examinations occurring have questionable indications.

• Many are conducted using inappropriate technical factors.

Frush, RSNA, 2006, Berenner Pediatr. Radio.l 32 (2002) 228 –

231, Oikarinen et al. Eur Radiol 19 (2009)

1161-5


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Justification and CT

• It is very important that each examination is rigorously justified, thus…


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Justification for CT: Practical Advice

• Justify CT examination rigorously and eliminate inappropriate referrals.

• Perform only necessary CT examinations.• Reduce the number of multiple phase scans.• Work to account for previous procedures.


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Justification for CT: Practical Advice

• Use referral guidelines and appropriateness criteria when available

• Use alternative approaches, such as ultrasound, MRI where appropriate

• Include justification in clinical audit


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How to achieve the objective?

• Respect age-specific pathology and its prognosis.

• Consider potential contribution of the scan to patient management and outcome.

• Consider the patient’s medical imaging record with respect to ionizing radiation


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How to Achieve the Objective?

• Respect cost and radiation exposure.• Replace CT by examination with no or with

lower radiation exposure (e.g. US, MRI).• Delay/cancel follow-up examination unless a

decision based on scan is needed now.


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Optimisation and CT

One size does not fit all...


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Optimisation and CT

For paediatric CT examinations, the use of specific radiographic technical parameters should be promoted as:

•Child size the kVp and mA.

•One scan (single phase) is often enough.

•Scan only the indicated area.


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General Recommendation

• You must use paediatric protocols to reduce the dose for the same image quality as in adults

• Make sure there are no inappropriate high (e.g. adult) parameter settings behind the name paediatric protocols

• Plan paediatric scans according to patient’s size and age


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Generic Requirements for Optimisation

• Inform and prepare the patient and accompanying person(s).

• Be familiar with CT dose descriptors.• Realise lower noise usually means higher

doses; accept noise if scan is diagnostic.• Make sure operating conditions balance

image quality and radiation exposure.


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Generic Requirements for Optimisation

• Optimize scan parameters within the axial plane.

• Optimize a set of tube current settings for paediatric examinations.

• Optimize scan parameters for volume coverage.

• Scan minimal length and minimise repeated scanning at identical areas.


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Equipment, Protocol, Dose and Image Quality

• In most children a tube voltage of 80–100 kVp will suffice, especially in children with a body weight <45 kg.

• In adolescents, a tube voltage of 100 kVp for the thorax and 120 kVp for the abdomen is usually sufficient

• Recent studies with phantoms suggest that the optimal tube voltage in children may be even lower (60kVp) at least for some indicationsNievelstein, Pediatr Radiol,

2010


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• Spiral or helical scanning is preferable in paediatrics as an entire volume is imaged

• Short tube rotation times reduce movement artefacts and provide more detailed cardiac imaging

• One main benefit for MDCT scanners is speed of acquisition rather than dose reduction



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• An increase in pitch can result in a shorter scan time and (in some scanner types) in a dose reduction

• In modern MDCT scanners this may not be the best option (due to overranging)

• If effective mAs is used, an increase in pitch will result in an increase in the tube current

• Therefore, it is usually more dose efficient to keep the pitch as low as possible (<1) and if needed manually decrease the tube current


Nievelstein, Pediatr Radiol, 2010


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• Multi-slice scanners have potential to deliver higher dose•by having a wider beam irradiating a

number of detector rows to achieve multiple slices simultaneously

•as well as owing to more extensive clinical use

• However.…



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• Strategies for dose reduction in MDCT:

•Hardware improvements•Software improvements, as tube current

modulation, image reconstruction algorithms, …



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• Modern scanners give automatic or semiautomatic correction of tube current (mA) for patient size (mA modulation).

• Significant dose reduction

(20–50%) without appreciative loss of image quality.



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1. Image thickness:• Should be chosen depending on the size

of the child and the application • Use maximal acquisition collimation

(assuming this would result in scanning at lower mA) appropriate for specific diagnosis

• Narrow collimation in MSCT and 1 mm slices on some SDCT result in a higher dose (increase in mAs to maintain image quality)


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2. Pitch:• SDCT: a pitch factor 1.5 is recommended

for most examinations• 25% reduction in dose compared with

using a pitch of 1• MDCT: reduction in dose due to greater

pitch may not be achieved• tube current (mA) can be automatically

adjusted to keep the dose and noise the same


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3. Tube potential (kVp)

• There are few advantages to using a high tube potential (kV).

• Without a reduction in tube current (mA) this leads to a significantly higher dose.

• 100 kVp or 80 kVp is usually adequate for children.• Lowering of kVp enhances contrast• 10 kg patient transmits 3-4% while an adult

transmits less than 0.1%.• Be aware that images with high noise, even if they

do not look very crisp, may provide the diagnostic information.


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4. Lower tube current (mA):• Lower tube current (mA) should be used for

scanning kids.• High tube current is required only when there is a

need for high image detail ( in low contrast settings)

• Decrease of mA according to body diameter and use of exposure charts if AEC is not available (dose reduction 70-80%), Lucaya, et al, 2000, AJR 175:895-92

• Use of tube current modulation technology results in dose reduction by 60% for paediatric scanning, Kalra et al, 2004, Radiology, 233:649-57


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5. Gantry Tilt•A straight gantry results in irradiation of a

smaller volume of tissue compared with a tilted gantry and is recommended.

•Exception: tilt is used to avoid unnecessary exposure of sensitive tissues, e.g. in brain CT for avoiding the orbits.


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6. Scan Length•Scan the minimum length required and be

restrictive in defining upper and lower limits.

•Optimise scan parameters for volume coverage by using representative volume sample(s) when the entire volume is not needed (by sequential scans with gaps) to reduce dose-length productVock and Wolf , Dose Optimization and Reconstruction in

CT of children, in Radiation Dose from Adult and Paediatric MDCT, Springer, 2007


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7. Reconstruction Algorithm•Appropriate reconstruction algorithms,

window levels and window settings should be used


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Equipment, Protocol, Dose and Image Quality8. Dose Indices• Protocols must be adjusted by the operator to take

into account the patient's age and weight (size).• Newer scanners indicate the volumetric CT dose

index (CTDIvol ) and Dose-length product (DLP) on the console (Requirement from IEC 60601-2-44).

• This allows the user to automatically:• See the relative effect on dose owing to changes

in kVp, mA, collimation and pitch,• Estimate the effective dose to patient.


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Radiation Dose Indices for CT

Dose displays on modern multislice scanners:•Volume CTDI (CTDIvol)

•Dose Length Product (DLP)


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Dose Indices for CT

• CTDI is a local per scan dose and is dependent on kVp, mAs and slice collimation.

• DLP is an integral dose over the scan length and number of series and depends on pitch and dose


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Computed Tomography Dose Indices

• Effective dose, E, provides risk estimate which depends on the body size and organs imaged as well as on the integral dose.

• E is calculated as the product of DLP and conversion factors

Shrimpton et al, BJR (2006) 79, 968-980


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Typical Doses in Paediatric CT

Exam type Relevant

organ

Range of absorbed

organ doses (mGy)

Range ofeffective doses

(mSv)

Head unadjusted* (200 mAs)

Brain 23- 49 1.8 - 3.8

Head adjusted (100 mAs)

Brain 11 - 25 0.9 - 1.9

Abdomen unadjusted

(200 mAs)

Stomach

21 - 43 11 - 24

Abdomen adjusted (50 mAs)

Stomach

5 - 11 6 - 12

NCI: www.cancer.gov/cancertopics/causes/radiation-risks-pediatric-CT

*"Unadjusted" refers to using the same settings as for adults. "Adjusted" refers to settings adjusted for body weight.


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9. Viewing Conditions:• Make sure windows levels and settings are

adequate and that the monitors are calibrated.


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10. Shielding:

• Lead shielding can be place over the male gonads if:• the edge of the volume of investigation is less

than 10-15cm away• it does not interfere with the image

Dauer, et al, BMC Medical Imaging 2007, 7:5 doi:10.1186/1471-2342-7-5


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• The use of reusable bismuth attenuation shields is possible for sensitive organs such as the eyes, gonads, breasts and thyroid.

10. Shielding:


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Shielding• The bismuth eye shield is simple

to place and covers only the eye• In-plane shields are associated

with greater image noise and streak artifacts. However, shields reduce radiation dose. Automatic exposure control did not increase radiation dose when using a shield.Karla et al, Korean J Radiol. 10:156-63, 2009

• This adult patient has a 3 layer bismuth latex eye shield in place. While artefact is seen into the globe, no artefact is transmitted into the brain. Standoff pads can reduce surface artefact Hopper KD, et al, Am J Neuroradiol 22:1194–1198,2001


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11. Training•The examination should always be

supervised by a radiologist experienced in paediatric imaging

If all listed factors are taken into consideration, significant dose reduction can be achieved


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Training

• Following options are available on modern scanners • Tube current modulation (mA, mA/slice, effective

mAs), pitch, noise level setting, field-of-view for bow tie filter, kVp, beam (vs slice) collimation…

• This requires a skilled operator:• Who knows well the model of the scanner using• Trained in paediatric imaging to adjust the

examination parameters according to examination type, age and/or size of the child


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Radiologists, Physicists and Technologists’ Responsibilities

• Improve awareness of need to decrease CT radiation dose to children.

• Be committed to make a change in daily practice by team work between radiologists, technologists, referring healthcare providers and parents.

• Medical physicists, radiologists, technologists and department managers should review vendor or other CT protocols and “down-size” them for children.


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Radiologists, Physicists and Technologists’ Responsibilities

• Single phase scans are often adequate• Pre- and post-contrast or delayed scans

rarely add additional information in children, but can double or triple the dose.


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Radiologists, Physicists and Technologists’ Advice

• Scan only the indicated area. If a patient has a possible small dermoid on ultrasound, there may not be a need to scan the entire abdomen and pelvis.

• Be involved with your patients. Be the patient’s advocate. Ask the questions required to ensure that you “child-size” the scan.


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http://rpop.iaea.org/RPoP/RPoP/Content/index.htm


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http://www.pedrad.org/associations/5364/ig/


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Summary

• CT can be a relatively high dose diagnostic imaging procedure

• Rigorous justification of CT for children is required• Good practice in paediatric CT:

• Optimisation of the CT examination protocol based on patient size (lower kVp and mA)

• Acceptance of images with greater noise• One scan (single phase) is often enough - Reduce

repeat scanning of identical body areas• Scan only the indicated area• Use of shielding devices• Trained and experiences staff


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1. Reduction of kVp in CT reduces the dose.2. CT contributes 60-70 % of the dose from

radiological examinations in developed countries.

3. The same CT protocol used for children and adults will result in a higher dose to adults.


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1. Тrue - Reduced kVp reduce the dose in children while maintaining image quality.

2. Тrue - It is a high dose modality and with 10% contribution to number of all radiological examination it gives 60-70% of dose.

3. False- It is opposite, the same protocol will give a few time higher dose to children.


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References

• BRENNER, D.J., ELLISTON, C.D., HALL, E.J., BERDON, W.E., Estimated risks of radiation-induced fatal cancer from paediatric CT, Am. J. Roentgenol. 176 (2001) 289-296.

• BRENNER, D.J., Estimating cancer risks from paediatric CT: going from the qualitative to the quantitative, Pediatr. Radio.l 32 (2002) 228 – 231.

• FRICKE, B.L., et.al., In-plane bismuth breast shields for pediatric CT: effects on radiation dose and image quality using experimental and clinical data, Am. J. Roentgenol. 180 (2003) 407 – 411.

• HOPPER, K.D.,et al, The breast: in-plane x-ray protection during diagnostic thoracic CT - shielding with bismuth radioprotective garments, Radiology 205 (1997) 853 – 858.

• KILJUNEN, T., JÄRVINEN, H., SAVOLAINEN, S., Diagnostic reference levels for thorax X-ray examinations of paediatric patients, Br. J. Radiol. 80 (2007) 452-9.

• BOONE, J.M., et. al., Dose reduction in paediatric CT: a rational approach, Radiology 228 (2003) 352-360.

• LUCAYA, J., et. al., Low-dose high-resolution CT of the chest in children and young adults: dose, cooperation, artefact incidence and image quality, Am. J. Roentgenol. 175 (2000). 985-992.

• INTERNATIONAL ATOMIC ENERGY AGENCY, Dose Reduction in CT while Maintaining Diagnostic Confidence: A Feasibility/Demonstration Study, IAEA-TECDOC-1621, IAEA, Vienna, (2009).

• KALRA, M.K., et. al., Techniques and applications of automatic tube current modulation for CT, Radiology 233 (2004) 649-657.

• INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, ICRP Publication 102: Managing Patient Dose in Multi-Detector Computed Tomography (MDCT), Annals of the ICRP Volume 37/1, Elsevier, (2007).

• D. Tack,Pierre A Gevenois, Radiation Dose from Adult and Pediatric Multidetector Computed Tomography, Springer, 2007

• Karla et al, In-plane shielding for CT: effect of off-centering, automatic exposure control and shield-to-surface distance, Korean J Radiol. 2009 Mar-Apr;10(2):156-63..

IAEA

Additional material


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Practical Optimisation in Paediatric CT (I)

• Reduce mAs according to body weight/diameter or composition and/or

• Use dose modulation (angular/longitudinal)• Use maximal slice reconstruction thickness

to reduce noise and potentially dose appropriate for specific diagnosis.

• Decrease kVp for thin (small) patients and high contrast exams (CT angiography, chest, musculoskeletal )


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Practical Optimisation in Paediatric CT (II)

• Normally use shortest rotation time available.

• Use representative volume sample when entire volume is not needed.

• Use spiral scan with pitch greater than 1 (eg.: 1.5), provided this does not automatically increase the mA.

• Use newer dose reduction strategies such as iterative reconstruction and adaptive modulation (to reduce over ranging)


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Practical Optimisation in Paediatric CT (III)

• Be restrictive in defining upper-most and lower-most scan range

• Use localising projection scan extending just minimally beyond scan limits.

• Consider low kVp and single AP topogram• Reconstruct additional thick noise-reduced

slices without increase in exposure.• Avoid major overlap when scanning adjacent

areas with different protocols


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Practical Optimisation in Paediatric CT(IV)

• Avoid additional non-enhanced scans unless specifically justified.

• Optimise the protocol to obtain all the information requested during one scan.

• Minimise the number of scans in multi-phase scanning.

• In case of multi-phase scanning use shorter scan length for additional scans.

• Use lower dose for non-enhanced or repeat scans unless high quality is needed.


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Practical Optimisation in Paediatric CT (V)

• Minimise length of scans and fluoroscopy time in interventional applications.

• Use low mA with CT fluoroscopy• Replace test bolus/bolus triggering by

standard can delay unless timing is very critical.

• Use additional protection devices where indicated such as bismuth shields (lens, thyroid, breast, gonads).


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Tube current modulation:• Based on patient's size• Longitudinal (z-axis)• Angular (xy-axis)• Combined

Thick patientThin patient

Tube Current Modulation Options


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Tube Current Modulation Options

•Dose reduction based on patient anatomy.

•Lower mA in AP, higher mA in lateral directions.

1 GE, 2 Toshiba and 3 Siemens MDCT

200 mA

150 mA130 mA150 mA180 mA210 mA200 mA

170 mA

180 mA

Methods

• Patient attenuation measured during scout scan (AP & Lat) and alter mA for each gantry rotation (Smart mA1, Real AEC2) or “on-the-fly” (Care dose3)


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Tube potential (kVp)

Decreasing kVp significantly reduces dose, typically:

• 80 kV – 0.5 mSv•100 kV – 1 mSv•120 kV – 1.6 mSv•140 kV – 2.3 mSv

kV = dose


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Tube potential (kVp)

• CT examinations with a high intrinsic contrast (chest, bones) justify lowering the tube voltage to 80–100 kVp

• However, bony examinations can be performed with very low current 25-70mA

Nievelstein, Pediatr Radiol, 2010

Cook, Imaging, 2001


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Guidelines

• FDA Public Health Notification: Reducing Radiation Risk from Computed Tomography for Paediatric and Small Adult Patients, November 2nd, 2001

• National Cancer Institute: Radiation Risks and Paediatric Computed Tomography (CT): A Guide for Health Care Providers, http://www.cancer.gov/cancertopics/causes/radiation-risks-pediatric-CT

• Image Gently: http://www.pedrad.org/associations/5364/ig/


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A Practice QualityImprovement (PQI)Program in CT Scans inChildren: • The PQI module capture

how your practice performs CT scans in children, and allows you to compare your practice to “safe practice” guidelines in the literature.

How to Develop CTProtocols for Children?• Provide guidance in

developing CT protocols for children and periodically verifying that your current protocols are appropriate


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Example of successful story I

Arch and Frush, AJR 2008;191:611–617:• Since 2001, kVp and mA settings, two principal parameters

determining radiation dose, have decreased significantly for paediatric body MDCT

• It is a reasonable assumption that these changes are due to efforts to increase awareness about the risks of radiation

Paediatric chest CT


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Example of successful story II

Wallace, et al. Proceedings of IRPA 12, Buenos Aires, 2008, FP0227:• Eight paediatric hospitals• Training and seminars on optimisation• Dose reduction greater than 50%

Documents

IAEA International Atomic Energy Agency Radiation Protection in Paediatric Radiology Radiation Protection of Children During Computed Tomography L06