Managing Patient Dose in Computed Tomography (CT)

8/14/2019 Managing Patient Dose in Computed Tomography (CT)

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INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION

Managing Patient Dose inManaging Patient Dose in

Computed Tomography (CT)Computed Tomography (CT)


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International Commission

on Radiological Protection

Information abstracted from

ICRPPublication 87

Available at www.icrp.org

Task Group: M.M. Rehani, G. Bongartz, S.J. Golding,

L.Gordon, W. Kalender, T. Murakami, P. Shrimpton,

R. Albrecht, K. Wei


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Use and disclaimer

This is a PowerPoint file

It may be downloaded free of charge

It is intended for teaching and not forcommercial purposes

This slide set is intended to be used withthe complete text provided in ICRPPublication 87


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Situation analysis

CT continues to evolve rapidly despite manyadvances in other imaging modalities

It is one of the most important radiological

examinations worldwide The frequency of CT examinations is increasing

rapidly from 2% of all radiological examinationsin some countries a decade ago to 10-15 % now

Patient doses in CT have not decreased incontrast to radiography where nearly 30%reduction has been documented in last decade


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UNSCEAR 2000


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Why increased frequency?

20 years ago, a standard CT of the thorax tookseveral minutes while today similar informationcan be accumulated in a single breath hold

making it attractive, patient & user friendly Advances in CT technology have made possible

CT fluoroscopy and interventional procedures,in some cases replacing ultrasound guidedinterventions

Recently CT screening is picking up


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Why increased dose

Unlike radiography where over-exposureresults in blackening of film, better imagequality is obtained with higher exposures inCT

There is a tendency to increase the volumecovered in a particular examination

Modern helical CT involves volume scanningwith no inter-slice gap and with possibility ofoverlapping scans

Repeat CT examinations


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Why increased dose (contd)

Same exposure factors used for children as for

adult

Same exposure factors for pelvic (high contrast

region) as for abdomen (low contrast region)


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What is the dose from CT?

How high? The effective dose in chest CT is in the order of

8 mSv (around 400 times more than chestradiograph dose) and in some CT examinations

like that of pelvic region, it may be around 20mSv

The absorbed dose to tissues from CT can

often approach or exceed the levels knownto increase the probability of cancer asshown in epidemiological studies


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Effective doses in CT

and radiographic examinations

CTexamination

Effective dose(mSv)

Radiographicexamination

Effective dose(mSv)

Head 2 Skull 0.07

Chest 8 Chest PA 0.02

Abdomen 10-20 Abdomen 1.0

Pelvis 10-20 Pelvis 0.7

Ba swallow 1.5

Ba enema 7


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Organ doses in CT

Breast dose in thorax CT may be as much

as 30-50 mGy, even though breasts are not

the target of imaging procedure

Eye lens dose in brain CT, thyroid in brain

or in thorax CT and gonads in pelvic CT

receive high doses


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Tissues in the field although they are not

the area of interest for the procedure

Lens of the eye Breast tissue


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Typical doses in mGy during CT

in adults (Shrimpton et al. 1991)

Examination Eyes Thyroid Breast Uterus Ovaries Testes

Head 50 1.9 0.03 * * *

Cervical spine 0.62 44 0.09 * * *

Thoracic spine 0.04 0.46 28 0.02 0.02 *

Chest 0.14 2.3 21 0.06 0.08 *

Abdomen * 0.05 0.72 8.0 8.0 0.7

L. spine * 0.01 0.13 2.4 2.7 0.06

Pelvis * * 0.03 26 23 1.7

The symbol * indicates that dose is < 0.005 mGy


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Does multi-slice CT impart more

or less radiation dose?

An increase by 10-30% may occur withmulti-slice detector array


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Some observations

Most doctors including many radiologists havea feeling that modern CT scanners which arevery fast give lesser radiation dose

Unfortunately time and radiation dose arenot proportional in such a situation

Over the years the x-ray tubes are becoming

more and more powerful such that they cangive high bursts of x-rays which can givesatisfactory image in shorter exposure time


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What can be done toWhat can be done to

manage patient dose in CT?manage patient dose in CT?


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What can operators do?

Limit the scanned volume

Reduce mAs values Use automatic exposure control by adapting

the scanning parameters to the patient cross

section. 10-50% reduction in dosedocumented, without any loss of image quality


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What can operators do (contd)

Use of spiral CT with pitch factor>1 and

calculation of overlapping images instead of

acquiring overlapping single scans

Shielding of superficial organs such as thyroid,

breast, eye lens and gonads particularly inchildren and young adults. This results in 30-

60% dose reduction to the organ


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What can operators do (contd)

Separate factors for children. Can reduce doseby a factor of 5 or more

Use of partial rotation e.g. 270 degree in Head

CT (refer figure on next slide) Adequate selection of image reconstruction

parameters

Use of z-filtering with multi-slice CT systems Record of dose, exposure factors


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Dose distribution (in relative units) through the section of the skull

(face-up) for 270o scan omitting the frontal 90o. Minimum dose

occurs in the region of the eyes. The doses are slightly higher on left

side since in this unit x ray tube rotates by an additional 20oc

(clockwise) for patient movement (adapted from Robinson 1996).


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Actions for manufacturers

Introduce automatic exposure control

Be conscious of high doses in CT

Include safety features to avoid unnecessarydose

Display of dose

Convenience in using low dose protocols

Draw attention of users to selecting separateprotocols for paediatric patients


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Actions for physician &

radiologist Justification: Ensure that patients are not irradiated

unjustifiably

Request for CT examination should be generated onlyby properly qualified medical or dental practitionersdepending upon national educational and qualificationsystem. The physician is responsible for weighing the

benefits against risks

Clinical guidelines advising which examinations areappropriate and acceptable should be available toclinicians and radiologists


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Actions for physician & radiologist

(contd)

Consider whether the required information be

obtained by MRI, ultrasonography

Consider value of contrast medium

enhancement prior to commencing

examination

CT scanning in pregnancy may not be

contraindicated, particularly in emergency situations,although examinations of the abdomen or pelvis

should be carefully justified


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(contd) CT examination should not be repeated without

clinical justification and should be limited to the areaof interest

Clinician has the responsibility to communicate tothe radiologist about previous CT examination of the

patient

CT examination forresearch purpose that do not

have clinical justification (immediate benefit to theperson undergoing the examination) should be subjectto critical evaluation by an ethics committee


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(contd)

CT examination of chest in young girls and

young females needs to be justified in view of

high breast dose Once the examination has been justified,

radiologist has the primary responsibility for

ensuring that the examination is carried outwith good technique


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Web sites for additional information

on radiation sources and effects

European Commission(radiological protection pages):europa.eu.int/comm/environment/radprot

International Atomic Energy Agency:www.iaea.org

International Commission on Radiological Protection:www.icrp.org

United Nations Scientific Committee on the Effects of

Atomic Radiation:www.unscear.org

World Health Organization:www.who.int

Documents

Managing Patient Dose in Computed Tomography (CT)