radiation dose optimization techniques in mdct era: from

Radiation Dose Optimization Radiation Dose Optimization Techniques in MDCT Era: From Techniques in MDCT Era: From

Basics To PracticeBasics To Practice

Chang Hyun Lee, MDChang Hyun Lee, MDJM Goo, MD, HJ Lee, MD JM Goo, MD, HJ Lee, MD

EJ Chun, MD, CM Park, MD EJ Chun, MD, CM Park, MD JG Im, MDJG Im, MD

Seoul National University HospitalSeoul National University Hospital

Purpose

1. To discuss the importance importance of radiation dose modulation in MDCT

2. To review the characteristics characteristics of of radiation dose modulationradiation dose modulation techniques in different MDCT scanners

3. To explain how to use and apply how to use and apply this techniquesthis techniques to the patients during MDCT scan

Contents1. Basic concepts in CT dose indexCT dose index

- Fundamental dose parameters- CT dose index- Parameters affecting CTDI

2.2. Radiation dose increaseRadiation dose increase in MDCT scanner- Radiation dose in MDCT- Radiation risk

3.3. Effective doseEffective dose in various CT examination- Effective dose- Calculation and estimates of effective doseCalculation and estimates of effective dose- Radiation dose summary

4. Radiation dose modulation techniques in different MDCT scanners – Reference mAs – Reference noise index – it’s advantages and disadvantages

5.5. Practical tipsPractical tips for optimizing radiation dose in MDCT

Fundamental Dose Parameters

BACK TOBACK TOCONTENTSCONTENTS



10 mSv = 1 rem

Fundamental Dose Parameters• ExposureExposure

– Roentgens (C/kg) or air kerma (J/kg – mGy)– Ionization in air per unit mass or amount of energy impart

ed per unit mass– Related to intensity of radiation at point of measurement– Irradiated area, penetrating power, tissue sensitivity 에

대한 고려가 없음 .

Fundamental Dose Parameters• Absorbed doseAbsorbed dose

– Joules/kg or mGray (mGy)– Energy absorbed by material per unit mass– Depends on radiation type, energy & material irradiated– Quoted locally or averaged over area e.g. organ

• Absorbed doseAbsorbed dose – amount of radiation energy deposited in the patient’s body as a result of exposure

• Radiation exposureRadiation exposure: radiation source-related termsource-related term• Radiation doseRadiation dose: : body-related termbody-related term

D D ~ 3D

Fundamental Dose Parameters• Effective dose

– mSv– Measure of radiation risk to patientMeasure of radiation risk to patient– Attempts to reflect equivalent whole body doseequivalent whole body dose that

results in same stochastic risk– Applies organ sensitivity factorsorgan sensitivity factors– Enables risk comparisonrisk comparison between different procedures

and modalities


• Absorbed doseAbsorbed dose,, however, does not account for differing sensitivities of the organs to radiation damage.

• Equivalent doseEquivalent dose in a tissue is a product of the tissue type and the radiation weighting factor

• Equivalent dose has the same numerical value as abssame numerical value as absorbed doseorbed dose and is measured in sievert or rem.

• Effective doseEffective dose is computed by summing the absorbed summing the absorbed doses in the organsdoses in the organs weighted by their radiation sensitivity.

• Effective dose estimates the whole-body doseestimates the whole-body dose that would be required to produce the same risk as partial-body dose delivered by a localized radiological procedure - useful in evaluating potential biological riskin evaluating potential biological risk of a specific radiologic examination.

NOTES

CT Dose Index



CT dose index

• CTDI was developed originally by Shope et al. back in 1981.

• The basic radiation dose parameter in CT is the CT dose index (CTDI).

• Represents absorbed radiation doseabsorbed radiation dose in in aa CT dose phantomCT dose phantom, measured in the grgray or raday or rad.

CT dose index: CTDI

• CTDI has been defined for use with single-detector single-detector row CT scanners.row CT scanners.

• CTDI is the total energy absorbed within a dose profile deposited within one nominal collimation.

CT dose index

• Three derivatives of the CTDI• CTDICTDI100100: radiation exposure measured by

means of an ionization chamber with a length of 100 mm.

• CTDICTDI100w100w: weighted radiation dose in axial (nonhelical) CT scan

• CTDIvolCTDIvol:: volume CTDI = CTDIw/pitch

CT dose index: CTDI

(Multiple-scan average dose)

Measurement equipment: CTDI

• Ionization chamberIonization chamber

• Thermoluminescent Dosimeters (TLD)

• X-ray film

Ionization chamber: CTDI

CTDI100

Average dose in scan plane: CTDIw

• Weighted average CTDI represents the average dose in scan plane of Perspex phantom

CTDIw = [2/3 CTDI100 (periphery) + 1/3 CTDI100 (center)] x 33.7

Average dose in scanned volume: CTDIvol

• AxialAxial: CTDIvol=CTDIw x (slice width/couch inc.)

• HelicalHelical: CTDIvol = CTDIw / PitchNoncontiguous exposure along z-axis

CTDI

• Advantages of CTDI as a dose descriptor

• Disadvantages of CTDI

Parameters affecting CTDI



Effect of scan parameters on CTDIvol

• mA and scan time (mAs per rotation)mA and scan time (mAs per rotation)

• CTDIvol increase linearly with mA and scan time

• E.g 2 x mAs = 2 x CTDIvol

Variation of CTDIvol with kVp

• CTDIvol increases with kVp

• Approx kVp∝ 2

ImPact 2005

CTDI and slice width

• CTDI increases if irradiated width does not match nominal width

T1 T2 T3

Z-axis

CTDI = Area / nT

Variation of CTDIvol with no. of slices

• Number of slices

• CTDIvol is independent of number of slices– Absorbed dose: energy absorbed per unit

mass

Effect of pitch on CTDIvol

• CTDIvol is inversely proportional to pitch

• E.g. doubling pitch halves the CTDIvol

• … but only if mA remains constant

• On some systems mA automatically adjusted for pitch so CTDIvol is constant

Effect of patient size• For same scan parameters (mAs, kV) the dose increa

ses as phantom/patient size decreases• For pediatrics CTDI can underestimate dose by ~ x 2

if measured in standard sized phantoms

Radiation dose in MDCT



Radiation dose in MDCT

• ~ 60 million CTs per year – doubled in 5 years

• > 60,000 CT examinations at University of Alabama at Birmingham, USA

• ~ 67% total radiation exposure is from CT

Radiation Dose in MDCT

Y Imanishi et al. Eur Radiol (2005) 15:41-46

CT examination is increasing in Japan

0

20,000

40,000

60,000

80,000

100,000

1979 1989 2000

(x10,000)年スキャン数(x1,000)年検査件数

CT装置数

No. of image (x10,000)No. of exam/year (x1,000)No. of CT

Nishizawa, Acta Radiol Jap 2004; 64:151-158

Nishizawa, Acta Radiol Jap 2004; 64:151-158

Scan area is also increasing

Does multi-slice CT impart more or less radiation dose?

• An increase by 10-30% may occur with multi-slice detector array

ICRP (International Commission on Radiological Protection) Publication 87

Doses from Chest CTHelical CT MDCT (4) LDCT (single)

kV 120 120 120mA100-210 300-350 25Rotation time (s/r) 1-1.5 0.5 2

Table feed (mm/s) 10 7-11 20/2s/rFOV (cm) 27 35 30

Organ dose (mGy)Bone marrow 5.91 7.19 2.51Lung 20.94 19.59 3.09Stomach 8.59 19.83 1.41Breast 18.24 20.20 2.41Liver 0.43 19.62 1.64Esophagus 18.12 18.16 2.90Thyroid 8.23 23.70 2.41

Effective dose (mSv) 7.62 11.0 1.40

Nishizawa, Acta Radiol Jap 2004; 64:suppl

Radiation Risk



Audience Response Question

• What is the conventional estimate of the long term risk of death from cancer for 10 mSv whole body exposure for an average person? A. Negligible B. 1:20,000 C. 1:2,000 D. 1:200


• What is the conventional estimate of the long term risk of death from cancer for 10 mSv whole body exposure for an average person? A. Negligible B. 1:20,000 C. 1:2,000C. 1:2,000 D. 1:200


• The radiation dose from a chest radiograph is approximately what fraction of the dose from yearly natural background radiation? A. 1:100 B. 1:10 C. 1:1 D. 10:1


• The radiation dose from a chest radiograph is approximately what fraction of the dose from yearly natural background radiation? A. 1:100A. 1:100 B. 1:10 C. 1:1 D. 10:1


• The radiation dose for One CT scanOne CT scan versus One chest radiographOne chest radiograph? A. CT < CR B. CT > CR, CT < 10 CR C. CT > CR, 10 CR < CT < 100 CR D. CT > CR, CT = 100 ~ 250 CR E. CT > CR, CT > 500 CR


• The radiation dose for One CT scanOne CT scan versus One chest radiographOne chest radiograph? A. CT < CR B. CT > CR, CT < 10 CR C. CT > CR, 10 CR < CT < 100 CR D. CT > CR, CT = 100 ~ 250 CRD. CT > CR, CT = 100 ~ 250 CR E. CT > CR, CT > 500 CR

Relative Dose Beliefs

Diagnostic CT Scans: Assessment of Patient, Physician, and Radiologist Awareness of Radiation Dose and Possible Risks. Lee CL et al. Radiology 2004

Cancer Risk Beliefs

Diagnostic CT Scans: Assessment of Patient, Physician, and Radiologist Awareness of Radiation Dose and Possible Risks. Lee CL et al. Radiology 2004

Radiation Cancer Risk Conventional Theory

• 5% excess cancer deaths per Sv (100 rem)

• 1:2,000 (0.05%) excess cancer deaths per 10 mSv (1 rem)

Brenner et al. Estimated Radiation Risks Potentially Associated with Full-Body CT Screening. Radiology 2004

Age-Dependent Cancer Risk

Brenner et al. Estimated Radiation Risks Potentially Associated with Full-Body CT Screening. Radiology 2004

Radiation Risk in Context

• Baseline risk of cancer in life time 20-25%

• Younger patients at higher risk• Late middle aged adult getting

average CT has life time risk of cancer increased from ~20% to ~20.01%~20% to ~20.01%– Not measurable

Radiation – Common Doses and Risks

• Chest radiograph (adult): 0.02 mSvChest radiograph (adult): 0.02 mSv 0.0001 – 0.000002% deaths

• UGI/BE: 1-7 mSv 0.005-0.035% deaths

• MammographyMammography 2.6 mGy, but– Effective Dose 0.13 mSv0.13 mSv (so cancer risk

much lower)

• Natural background: 3 mSv/yrNatural background: 3 mSv/yr• Medical population average 0.3-0.6

mSv/yr

Comparable Non-Radiation Risks

• Assume 10 mSv (1 rem) CT scan– Smoking 140 cigarettes in a lifetime

(lung cancer)– Spending 7 months in New York City

(air pollution – lung cancer)– Driving 4,000 miles in a car (accident)– Flying 250,000 miles in a jet

(accident)


• A single CT scan has a lifetime risk of death from cancer similar to:

A. Smoking 7 cigarettesB. Driving 40,000 miles in a carC. Having a standard IVUD. Flying into low earth orbit


• A single CT scan has a lifetime risk of death from cancer similar to:

A. Smoking 7 cigarettesB. Driving 40,000 miles in a carC. Having a standard IVU

D.D. Flying into low earth orbitFlying into low earth orbit

Effective dose



Effective dose

• Effective doseEffective dose– Estimate of stochastic radiation risk

• Dose Length Product (DLP)Dose Length Product (DLP)– Related to stochastic radiation risk

Random CarcinogenesisGenetic mutation

Calculation of Effective dose

• Direct approach impractical• Usual approach

– Mathematical anthropomorphic phantomMathematical anthropomorphic phantom– Computer simulated irradiation using Monte Monte

Carlo techniquesCarlo techniques• Statistical calculations of photon interactions

Estimates of Effective Dose

• Effective dose = DLP x Conversion factor (mSv)

CTDI radiation risk

Diagram shows algorithm for the estimation of radiation exposure risk from CT

European DRLs for CT

Radiation dose - Summary

KT Bae. JMRI 2004

Radiation dose - Summary

• CTDI100 is the fundamental CT dose parameter

• CTDIw characterises a CT scanner in terms of dose

• CTDIvol is used to represent average absorbed dose to irradiated area

• DLP for a given examination type is roughly proportional to stochastic radiation risk

• Effective dose is used for more accurate risk estimates

How ?

ALARA(as low as possible reasonably achievable)

Radiation Dose Modulation Techniques



Automatic Exposure Control (AEC)

• Automatic exposure control systems are now available in all MDCT scanners

• Each system operates on different basis, using a range of control methods

• Tube current (mA) adjusted relative to patient attenuation


•Patients size AEC

•Z-axis AEC•Rotational AEC

•Combination


• Patient size AECPatient size AEC: adjust the tube current based upon the overall size of the patient. The same mA is used for an entire examination or scan.

• Z-axis AECZ-axis AEC: tube current is adjusted for each rotation of the x-ray tube. The mA would be low through the thorax and higher through the abdomen.

• Rotational AEC:Rotational AEC: The tube current is decreased and increased rapidly (modulated) during the course of each rotation. The amplitude of mA modulation during rotational AEC reflect the patient asymmetry.


• Three levels of automatic exposure control. A) patient size AEC: higher mA is used for larger patient, b) z-axis AEC: higher mA used at more attenuating z-axis positions, c) rotational AEC: the degree of modulation depends on asymmetry at each z-axis position, d) combined effects of using all three levels of AEC.

Benefits of AEC

• Consistent image qualityConsistent image quality– Depending on its capabilities, there is less

image noise variation between patients and within a single scan series. With rotational AEC, there is also a slight reduction in the variation of noise across the field of view.

• Reduction in photon starvation artifactReduction in photon starvation artifact- Tube current is varied during the course of

rotation, it can be increased for the most attenuating scan angles (e.g. laterally through the shoulders)

Benefits of AEC

• Potential for dose reduction Potential for dose reduction through exposure optimizationthrough exposure optimization– AEC systems by themselves will not

automatically lead to a reduction in patient dose. However, when used correctly, their introduction should generally tend to result in reduced dose.

Benefits of AEC

• Reduced tube loading (extended Reduced tube loading (extended scan runs)scan runs)– MDCT generally have fewer problems

with tube cooling, but in general, if the radiation exposure to the patient is reduced by lowering tube currents, the heating of the x-ray tube is also reduced.

Methods for AEC

• Standard deviation (SD) based AECStandard deviation (SD) based AEC - by specifying image quality in terms of

SD of pixel values. - High SD values noisy image- Low SD values low noise image- set the tube current to achieve the

requested SD on an image by image basis

Methods for AEC

• Standard deviation (SD) based AECStandard deviation (SD) based AEC– AdvantageAdvantage:: image quality resulting from protocols f

orm different scanners can be compared more easily.

– DisadvantageDisadvantage:: easily to enter an SD which is lower than would be needed, resulting in higher patient doses than the ones achieved without AEC

– Image noiseImage noise is inversely proportional to the square of the tube current, so halving the SD results in an increase in the mA, and therefore the patient dose, by a factor of 4.

Methods for AEC

• Reference mAs AEC controlReference mAs AEC control– Uses the familiar concept of setting an mA

(or mAs) related value– Assesses the size of the patient cross-secti

on beig scanned, and adjusts the tube current relative to the reference value.

Methods for AEC

• Reference mAs AEC controlReference mAs AEC control– AdvantagesAdvantages:: permits more flexible adjustmemore flexible adjustme

nt of tube current according to patient sizent of tube current according to patient size than with SD AEC control. With SD based systems, the AEC response to different patient sizes is pre-defined.

– Can vary their response depending on the image quality requirements.

– Users are familiar with typical mAsUsers are familiar with typical mAs values for their scanners

Methods for AEC

• Reference ImageReference Image AEC control AEC control– Uses reference image that has Uses reference image that has

previously beenpreviously been scannedscanned and judged to be of appropriate quality, and then attempts to adjusts the tube current to match the noise in the reference image.

– Advantage: the required image quality is required image quality is expressed using an existing clinical expressed using an existing clinical imageimage rather than an abstract value of SD

– Disadvantage: temptation to pick a temptation to pick a ‘pretty’ image‘pretty’ image. This lead to use higher doses than are necessary. It is also difficult difficult to compare scan protocolsto compare scan protocols, as there is no value associated with the image quality in the reference image.

AEC systems on MDCT scanners

Impact 2005


• GEGE • AutomA:AutomA: set a desired image quality by

entering a “Noise Index” (NI).• Aims to achieve the same level of noise

in each image• SmartmA:SmartmA: varies the tube current sinuso

idally during the course of rotation


• PhilipsPhilips• DoseRight ACSDoseRight ACS (automatic current selector) pr

ovides patients based AEC, by use of a reference image

• DoseRight DOMDoseRight DOM (Dose Modulation)– Z-Dom: Z-axis AEC– D-Dom: Tube current is set so that 90% of images

will have equal or lower noise than the reference image, with remaining 10% of images in a series having equal or higher noise than the reference image.

• Use feedback from the previous rotation to asses the amplitude of mA modulation used for rotational AEC


• SiemensSiemens• CARE Dose 4DCARE Dose 4D use “image quality reference m

As”.• Adjust the tube current, setting a value that is

higher or lower than the reference mAs depending upon the patient attenuation, relative to Siemens’ reference patient size.

• The degree to which the tube current is adjusted for patient size can be selected, using ‘weak’, ‘average’, or ‘strong (high degree of mA adjustment)’, compensation settings.


• ToshibaToshiba• SureExposureSureExposure in helical scanning only• Operated by selecting a target image SDselecting a target image SD form

a drop down list.• AEC setup allows tube current to be limited by

max. and min. values.• Variable length reference position selectorreference position selector to

highlight a particular region. The AEC uses the mean attenuation within this scan region to calculate the tube current through this region.


Automatic Exposure ControlAutomatic Exposure Control

Report 06013, 32 to 64 slice CT scanner comparison report version 1. Feb 2006, www.pasa.nhs.uk/cep.

Clinical Use of AEC system

• Clinical use of AEC system requires careful consideration

• Generally lead to reduced patient doses• However, it is possible to increase doses

operating an AEC system• Dose reduction generally accompanied

by reduction of image quality• Goal is to achieve diagnostic image

quality without an impact upon clinical usefulness

Protocol optimization

• The key is to use an appropriate image noise level, reference mAs or reference image in the AEC setup.

• This process is not a straightforward• One approach is to focus image

quality assessment upon the European Guidelines on Quality Criteria for Computed Tomography

The Effect of Protocol Modification

• There are significant differences from one system to another

• Changing kVp will not affect the tube current on Siemens CARE Dose, but it will do on other AECs.

• Changing the reconstruction kernel will alter the tube current Toshiba SureExposure, but not by others

• It is important that users are aware of the behavior of their system, and the effect that varying scan and reconstruction parameters has upon the AEC.

Monitoring patient dose

• Use CTDIvol and DLP for monitoring • Before and after the introduction

AEC, radiation dose can be assessed.• AEC exposure levels can be

modified, although the effect of changing any other parameters such as beam collimation or kV should also be accounted for.

AEC Future

• Introduction of the AEC into clinical practice should be approached carefully

• There is a need for education of users by the manufacturer

• Scanner need to have default AEC protocols that draw a sensible compromise between the demand of image quality and radiation dose.

• Common method for operation of the system would be of great help and it leads to the protocol uniformity and radiation dose optimization.

Practical Tips



Clinical Issues in Radiation with MDCT

• Pediatric exposure• Breast radiation for chest MDCT• Cumulative dose from follow-ups in

some applications, e.g. renal stones• Scanning in pregnancy• Screening• Automatic Exposure Control• Low kVp to reduce dose and contrast

volume

Pediatric Exposure

• Effective Doses in children may be 50% higher with common techniques

• Strategy for dose reduction;– Be selective– Alternative diagnostic strategies– Reduce technique

Breast Radiation with Chest MDCT

• Breast radiation for chest CT• Especially scans for pulmonary

embolism in low-risk young women• Dose to breasts may be up to 19

times that from a mammogram• Strategy for dose reduction: be

selective

Cumulative Dose

• Rescanning (e.g. renal stones)• Often in young patients without

serious conditions• Indication “creep” (lower threshold)• Up to 18 follow-ups recorded in one

case!• Strategy: low dose, educate

clinicians, be alert and recommend alternatives (e.g., radiography or ultrasound)

Scanning in Pregnancy

• 2nd-15th week gestation exposure– No concern < 50 mGy– Moderate concern 50-150 mGy– High concern > 150 mGy

• Risk overestimated by physicians– 5% of obstetricians, 6% of family

physicians recommend abortion after CT

Scanning in Pregnancy

• Principles of pregnancy policy Identify probability of pregnancy Minimize dose, consider alternative Document dose Low exposure – perform exam without

delay High exposure – informed decision,

awareness of riskQuestionnaire, consent form

Screening MDCT

• Whole-body screening may increase cancer risk about 1%

• Limit screening CT to proven applications

• Use low-dose technique for chest, colon, coronary calcification scans

Automatic Exposure Control

• System adapts to changes in patient thickness: x-y and z-axis

• Can reduce dose by 20-40%• Implementation differs for each vendor• Not yet perfected – may cause

unsatisfactory increase in noise in some areas and patients

• Should use with caution in patients with prosthesis

Low kVp

• Low kVp increases image contrast for IV contrast

• Allows lower IV contrast doses• Low kVp increases noise at constant mAs • Probably satisfactory for vascular studies

and small patients (increase mAs)• Not yet adequately clinically validated for

many applications

Strategies For Dose Reduction

• Be selective, consider risk (young, sensitive)

• Minimize technique ( mAs, pitch with 4 or fewer slice scanner)

• Use AEC, low kVp when possible• Limit follow-up scans• Alternative diagnostic strategies, esp. Children and pregnancy• Be able to counsel for risk

Actions for physician & radiologist…

• JustificationJustification:: Ensure that patients are not irradiated unjustifiably.

•Consider whether the required information be obtained by MRI, ultrasonography

•Consider value of contrast enhancement or omitting pre-contrast scan

•CT scanning in pregnancy may not be contraindicated, particularly in emergency situations, although examinations of the abdomen or pelvis should be carefully justified

ICRP Publication 87

Actions for physician & radiologist (cont’d)

• CT examination shouldshould not be repeatednot be repeated without clinical justification and should be limited to the area of interest• Clinician has the responsibilityresponsibility to

communicate to the radiologist about previous CT

examination of the patient• CT examination for researchresearch purpose that do purpose that do

not have clinical justificationnot have clinical justification (immediate benefit to the person) should be subject to critical evaluation by an ethics committee

ICRP Publication 87

• CT examination of CT examination of chestchest in young girls and young females needs to be to be justified in view of high breast dosejustified in view of high breast dose

• Once the examination has been justified, radiologist has the primary radiologist has the primary

responsibilityresponsibility for ensuring that the examination is carried out with good technique

Actions for physician & radiologist (cont’d)

References• ImPACT, www.impactscan.org• National conference on dose reduction in CT, with an emphasis on pediatric patients.

AJR Am J Roentgenol. 2003 Aug;181(2):321-9. Linton OW, Mettler FA Jr; National Council on Radiation Protection and Measurements.

• A new pregnancy policy for a new era.AJR Am J Roentgenol. 2003 Aug;181(2):335-40. El-Khoury GY, Madsen MT, Blake ME, Yankowitz J.

• Physicians' perceptions of teratogenic risk associated with radiography and CT during early pregnancy.AJR Am J Roentgenol. 2004 May;182(5):1107-9. Pole M, Einarson A, Pairaudeau N, Einarson T, Koren G.

• Dose reduction in pediatric CT: a rational approach. Boone JM, Geraghty EM, Seibert JA, Wootton-Gorges SL. • Estimated radiation risks potentially associated with full-body CT screening. Radiology. 2004 Sep;232(3):73

5-8. Brenner DJ, Elliston CD. • ICRP, Publication 87, 2000• JRS, Guidelines for pediatric CT, 2005• Nishizawa K, Acta Radiologica Jap, 2004National Research Counsil, 2005

Thank you !Thank you !

• Risk versus BenefitRisk versus Benefit• For patientsFor patients• ResponsibilityResponsibility

radiation dose optimization techniques in mdct era: from

Documents

average dose

mdctradiation dose

dose profile

weighted radiation dose

ct dose index ctdi

mdct scanner radiation

basic radiation dose

partialbody dose