CARDIAC CT ANGIOGRAPHY CARDIAC CT ANGIOGRAPHY

RADIATION DOSE: HOW LOW RADIATION DOSE: HOW LOW

CAN WE GO?CAN WE GO?

NEW YORK LANGONE UNIVERSITY

JILL E. JACOBS, M.D., FACRJILL E. JACOBS, M.D., FACRProfessor of RadiologyDepartment Of RadiologyNYU Langone Medical Center

USA MEDICAL RADIATION EXPOSURE: NCRP

•• 1982: per capita dose 0.54 mSv; collective dose 124,000 1982: per capita dose 0.54 mSv; collective dose 124,000 personperson--SvSv

•• 2006: per capita dose 3.0 mSv (600% ); collective dose 2006: per capita dose 3.0 mSv (600% ); collective dose 900,000 person900,000 person--Sv (700% )Sv (700% )

Mettler, et al. Health Phy 2008; 95 (5): 502-507

= background radiation= background radiation

USA MEDICAL RADIATION EXPOSURE

•• largest dose contributions are from CT & nuclear largest dose contributions are from CT & nuclear med studies (75% of collective effective dose)med studies (75% of collective effective dose)

Mettler, et al. Health Phy 2008; 95 (5): 502-507

USA MEDICAL RADIATION EXPOSURE

Mettler, et al. Health Phy 2008; 95 (5): 502-507

CT SCANNER AND MANUFACTURER TECHNOLOGY

4-row MDCT 16-row MDCT

64-row MDCT

32-row MDCT + z-FFS DSCT

64-rows high definition MDCT

128-rows + z-FFS MDCT

320-rows MDCT

64-rows + z-FFS DSCT

1st generation

1998

2nd generation

2000-02

3rd generation

2004-05

4th generation

2006-08

Adapted from: Earls, et al. Radiol Clin N Am 2010; 48: 657-674

TO ACHIEVE OPTIMIZED MOTION-FREE IMAGING OF THE

HEART AND CORONARY VESSELS FOLLOWING THE “ALARA”

PRINCIPLE

USING AS LOW A DOSE AS USING AS LOW A DOSE AS REASONABLY ACHIEVABLE WHILE REASONABLY ACHIEVABLE WHILE MAINTAINING DIAGNOSTIC IMAGE MAINTAINING DIAGNOSTIC IMAGE

QUALITYQUALITY

PARAMETERS INFLUENCING PATIENT DOSE

•• Tube currentTube current

•• Tube voltageTube voltage

•• Gantry rotation timeGantry rotation time

•• Collimated detector slice widthCollimated detector slice width

•• ZZ coveragecoverage•• ZZ--coveragecoverage

•• Beam pitch (helical acquisition)Beam pitch (helical acquisition)

•• Image spacing (axial acquisition)Image spacing (axial acquisition)

RADIATION DOSE IS PROPORTIONAL TO TUBE CURRENT, EXPOSURE TIME, AND THE SQUARE OF TUBE VOLTAGE

AND IS INVERSELY PROPORTIONAL TO PITCH

TUBE CURRENT

•• Radiation dose is Radiation dose is proportionalproportional to tube current to tube current

•• reflects the number of electrons flowing from the reflects the number of electrons flowing from the negative to the positive electrode in an Xnegative to the positive electrode in an X--ray tuberay tube

•• expressed in expressed in milliamperesmilliamperes ((mAmA) )

•• increasing the Xincreasing the X--ray tube current increases the ray tube current increases the number of electrons striking the target, resulting in a number of electrons striking the target, resulting in a higher Xhigher X--ray flux (the number of electrons produced ray flux (the number of electrons produced per unit time).per unit time).

•• resultant increase in photon penetration and resultant increase in photon penetration and patient dosepatient dose

TUBE VOLTAGE

•• Radiation dose is Radiation dose is proportionalproportional to the square of the to the square of the tube voltage tube voltage

•• increasing peak Xincreasing peak X--ray tube voltage increases the ray tube voltage increases the number and energy of Xnumber and energy of X--rays produced rays produced

•• resulting in increased Xresulting in increased X--ray penetration and ray penetration and di tidi tiradiation exposureradiation exposure

•• in addition to decreasing radiation dose, lowering the in addition to decreasing radiation dose, lowering the tube voltage results in decreased Compton scattering tube voltage results in decreased Compton scattering and increased photoelectric effect which increases and increased photoelectric effect which increases vascular vascular opacificationopacification

SCAN LENGTH

•• Radiation dose is Radiation dose is proportionalproportional to proscribed scan to proscribed scan length length

•• the longer the scan length, the higher the patient dosethe longer the scan length, the higher the patient dose

PITCH

•• Radiation dose is Radiation dose is inversely proportional inversely proportional to pitch to pitch

•• for a spiral scan, pitch is the longitudinal distance in for a spiral scan, pitch is the longitudinal distance in mm that the table travels during one rotation of the Xmm that the table travels during one rotation of the X--ray tube divided by the collimation of the xray tube divided by the collimation of the x--ray beam ray beam

•• decreasing the pitch results in increased overlap decreasing the pitch results in increased overlap b t d t i itib t d t i itibetween data acquisitionsbetween data acquisitions

•• increases scan time and Xincreases scan time and X--ray exposureray exposure

WHAT IS THE EFFECT OF CHANGING EACH PARAMETER??

•• Decreasing tube current Decreasing tube current •• decreases dose but increases image noisedecreases dose but increases image noise

•• Decreasing voltageDecreasing voltage•• decreases dose but increases image noisedecreases dose but increases image noise

•• Faster gantry rotation time Faster gantry rotation time •• improves temporal resolution but increases image noiseimproves temporal resolution but increases image noise

•• Decreasing zDecreasing z--collimated detector slice widthcollimated detector slice widthii i ti l l ti b t i i ii ti l l ti b t i i i•• improves zimproves z--axis spatial resolution but increases image noiseaxis spatial resolution but increases image noise

•• Increasing ZIncreasing Z--axis coverage per rotationaxis coverage per rotation•• decreases scan time & # heartbeats but potentially increases decreases scan time & # heartbeats but potentially increases

exposure of tissue outside the desired imaging range (zexposure of tissue outside the desired imaging range (z--overscanningoverscanning))

•• Increasing pitch (helical acquisition)Increasing pitch (helical acquisition)•• decreases scan time; decreases dosedecreases scan time; decreases dose

•• Increasing image spacing (axial acquisition)Increasing image spacing (axial acquisition)•• decreases scan overlap; decreases dosedecreases scan overlap; decreases dose

HOW DO WE DESCRIBE RADIATION DOSE??

•• CTDICTDIvolvol (in (in GyGy) )

•• averages radiation dose over the x, y, & z directions averages radiation dose over the x, y, & z directions & expresses the average dose to the scan volume & expresses the average dose to the scan volume for a CT examfor a CT exam

•• Dose length product (DLP) in Dose length product (DLP) in mGymGy x cm x cm

•• == CTDICTDI times scan lengthtimes scan length•• = = CTDICTDIvolvol times scan length times scan length

•• indicates integrated radiation dose for the scanindicates integrated radiation dose for the scan

•• Effective dose (E) in Effective dose (E) in mSvmSv

•• reflects the varying reflects the varying radiosensitivityradiosensitivity of the tissues of the tissues within the acquisitionwithin the acquisition

•• = DLP x chest conversion coefficient (k) where k= = DLP x chest conversion coefficient (k) where k= 0.014 0.014 mSvmSv mGymGy --11cm cm --11

BASIC PILLARS OF DOSE REDUCTION

Dose Reduction Technology

AppropriateUtilization TechnologyUtilization

APPROPRIATE UTILIZATION

•• Confirm the test is indicatedConfirm the test is indicated

•• does it follow appropriateness guidelinesdoes it follow appropriateness guidelines

•• is there an appropriate risk/benefit ratiois there an appropriate risk/benefit ratio

•• will it change patient managementwill it change patient management

•• are there other tests that can give the sameare there other tests that can give the same•• are there other tests that can give the same are there other tests that can give the same info with less or no radiation (ie MRI)info with less or no radiation (ie MRI)

APPROPRIATE PATIENT SELECTION & PREPARATION

•• is the patient able to cooperate & tolerate the is the patient able to cooperate & tolerate the scanscan

•• is the patient body is the patient body habitushabitus appropriate for the appropriate for the scan and chosen protocolscan and chosen protocol

•• is there an appropriate heart rate/rhythm for is there an appropriate heart rate/rhythm for scanner and protocol scanner and protocol

•• ??? beta??? beta--blockblock

VENDOR CT DOSE REDUCTION FEATURES

•• Adaptive preAdaptive pre--patient zpatient z--collimatorscollimators

•• Cardiac specific XCardiac specific X--ray filters and Xray filters and X--ray beam ray beam shaping filtersshaping filters

•• More efficient detector materialsMore efficient detector materials

•• Automatic tube current adaptation along x, y, Automatic tube current adaptation along x, y, and z directionsand z directions

•• Automatic tube voltage adaptationAutomatic tube voltage adaptation

•• Automated arrhythmia rejection methodsAutomated arrhythmia rejection methods

•• postpone acquisition until stable HRpostpone acquisition until stable HR

AUTOMATIC TUBE VOLTAGE ADAPTATIONPROSPECTIVE ECG –TRIGGERING

OFF ON

WHAT MANUAL ADJUSTMENTS CAN WE MAKE TO HELP

MINIMIZE PATIENT DOSE???MINIMIZE PATIENT DOSE???

PROTOCOL MODIFICATION FOR DOSE REDUCTION

•• adjust number of series (based on clinical question)adjust number of series (based on clinical question)

•• how many series are necessary?how many series are necessary?

•• different protocol for follow up vs. initial workup?different protocol for follow up vs. initial workup?

•• use small FOV (improves spatial resolution)use small FOV (improves spatial resolution)

•• adjust scan lengthadjust scan length•• adjust scan lengthadjust scan length

•• adjust tube current and voltageadjust tube current and voltage

•• adjust reconstruction methodadjust reconstruction method

•• adjust type of EKG synchronizationadjust type of EKG synchronization

•• change type of reconstructionchange type of reconstruction

PROTOCOL MODIFICATION FOR DOSE REDUCTION

•• series adjustment (? clinical question)series adjustment (? clinical question)

Coronary CTA

Calcium Scoring

•• Prospective gating with 320Prospective gating with 320--MDCTMDCT

6.5 mSV

4.33 mSV

SCAN LENGTH ADJUSTMENT

* Khan, et al. AJR 2011; 196: 407-11

3.47 mSV

EKG SYNCHRONIZATION

•• Retrospective gatingRetrospective gating

•• Prospective triggeringProspective triggering

RETROSPECTIVE ECG GATING•• ADVANTAGES:ADVANTAGES:

•• spiral mode: volumetric data acquired thru whole cardiac cyclespiral mode: volumetric data acquired thru whole cardiac cycle

•• data from specific parts of cardiac cycle retrospectively data from specific parts of cardiac cycle retrospectively referenced to ECG signal for image reconreferenced to ECG signal for image recon

•• can perform LV functional analysis and 4D evaluationcan perform LV functional analysis and 4D evaluation

•• less dependent on regular heart rhythm (capability to edit)less dependent on regular heart rhythm (capability to edit)

DISADVANTAGEDISADVANTAGE•• DISADVANTAGE:DISADVANTAGE:•• higher radiation dose (9higher radiation dose (9--21 mSv)21 mSv)

Shuman, et al. Radiology 2008;248:431

PROSPECTIVE EKG TRIGGERING

•• ADVANTAGES:ADVANTAGES:

•• “step and shoot”; predefined acquisition point in cardiac cycle“step and shoot”; predefined acquisition point in cardiac cycle

•• significant dose reductions c/w retrospective gatingsignificant dose reductions c/w retrospective gating

•• 22--5 5 mSvmSv

•• DISADVANTAGES:DISADVANTAGES:

•• vulnerable to cardiac motion artifacts with high orvulnerable to cardiac motion artifacts with high or irregirreg HRsHRsvulnerable to cardiac motion artifacts with high or vulnerable to cardiac motion artifacts with high or irregirreg HRsHRs

-- beta blockbeta block

•• not possible to perform accurate LV functional analysisnot possible to perform accurate LV functional analysis

Shuman, et al. Radiology 2008;248:431

PROSPECTIVE GATING: 5 mSv

LAD RCA

PADDING

SSCT min current duration ≈

½ rotation time + fan <

LaBounty, et al. AJR 2010; 194: 933-37

45% increase in radiation dose for

every 100-millisecond increase in

padding Earls, et al. Radiology 2008; 246: 742-53

Bischoff, et al. AJR 2010; 194: 1495-1499

How does this translate into cancer risk??

Huda, et al. AJR 2011; 196: W159-165

RISK IS GREATER WITH YOUNGER AGE!!

Huda, et al. AJR 2011; 196: W159-165

How does this translate into cancer risk??

PROSPECTIVELY ECG-TRIGGERED HIGH-PITCH SPIRAL WITH DSCT

25 ptsMean eff dose

1.0 mSv

Lell, et al. Eur Radio 2009; 19: 2576

HIGH PITCH SPIRAL DSCT

•• HR slow and regular (<60 BPM)HR slow and regular (<60 BPM)

•• NonNon--obese patientsobese patients

1 mSv

TUBE CURRENT ADJUSTMENT

PROTOCOL study:PROTOCOL study:every 100every 100--mA tube current reduction mA tube current reduction was associated with a 20% reductionwas associated with a 20% reduction

LaBounty, et al. Circulation 2009; 120: S334

was associated with a 20% reduction was associated with a 20% reduction in radiation dosein radiation dose

100%

20%

µA

EGC-GATED TUBE CURRENT MODULATION

* Irregular & variable HRs require widening of nominal dose window

Radiation dose is decreased up to 50% depending HR, minimum tube current, and duration of max current

EFFECT OF INCREASING HR ON ECG MODULATION

Low HR

High HR

TUBE VOLTAGE ADJUSTMENT

•• Radiation exposure Radiation exposure ≈≈ proportional to the square of the proportional to the square of the

tube voltagetube voltage

••

-- reducing from reducing from results in results in

and and dose dose

reduction but increases noisereduction but increases noise

PROTECTION STUDIES:PROTECTION STUDIES:53%53%

31% 31% (PROTECTION II) (PROTECTION I) 120 to 100 kV120 to 100 kV

•• Reducing tube voltage also affects tissue contrast Reducing tube voltage also affects tissue contrast

•• increases photoelectric effect and decreases Compton increases photoelectric effect and decreases Compton

scatteringscattering

•• NYU BMI based protocols:NYU BMI based protocols:

•• 120 kV for BMI >30120 kV for BMI >30

•• 100 kV for BMI 23100 kV for BMI 23--3030

•• 80 kV for BMI < 2380 kV for BMI < 23

increased vascular opacificationincreased vascular opacification

Retrospective Gating; 100 kV: 7 mSv

AJR 2011; 196:W550–W557

RECONSTRUCTION METHOD

•• Filtered back projection (FBP)Filtered back projection (FBP)

•• Iterative reconstruction Iterative reconstruction

•• method varies per vendormethod varies per vendor

-- Adaptive statistical iterative reconstructionAdaptive statistical iterative reconstruction

(ASIR) and model based (MBIR): GE(ASIR) and model based (MBIR): GE

-- Iterative reconstruction in image space Iterative reconstruction in image space

(IRIS) and (IRIS) and SinogramSinogram Affirmed Iterative Affirmed Iterative

Reconstruction (Safire): SiemensReconstruction (Safire): Siemens

-- Adaptive iterative dose reduction (IARD): ToshibaAdaptive iterative dose reduction (IARD): Toshiba

-- ii--Dose: PhilipsDose: Philips

ITERATIVE RECONSTRUCTION

•• Assumes initial attenuation coefficients for all Assumes initial attenuation coefficients for all voxelsvoxels

and uses these coefficients to predict projection dataand uses these coefficients to predict projection data

•• predicted projection data are compared to actual predicted projection data are compared to actual projection data and projection data and voxelvoxel attenuations are modified attenuations are modified until the error between estimated and measured until the error between estimated and measured projection data is acceptableprojection data is acceptable

•• IR algorithms reduce image noiseIR algorithms reduce image noise

•• thereby allowing tube current reductionthereby allowing tube current reduction

•• Produce equivalent signalProduce equivalent signal--toto--noise ratios at lower noise ratios at lower

radiation doses without loss of spatial resolutionradiation doses without loss of spatial resolution

•• Require more time for image reconstruction than FBPRequire more time for image reconstruction than FBP

Leipsic, et al. AJR 2010; 195: 655-60

FBP IR

σ = 20.7 σ = 14.8* Image noise = standard deviation of CT density in ROI

FBP IR FBP IR

COMBINE DOSE REDUCTION TECHNIQUES WHEN APPROPRIATE

JAMA 2009; 301 (22): 2340-8

100 kV; PROSPECTIVE TRIGGERING: 2 mSv

80 kV; High Pitch Spiral DSCT: 0.7 mSv

62 yo man with chest pain; BMI 21 67 yo woman with chest pain; BMI 23; mean HR 61 BPM

80 kV; High Pitch Spiral DSCT: 0.4 mSv

80 kV; High Pitch Spiral DSCT: 0.2 mSv14 yo boy with syncope after exercise; BMI 19

IN CONCLUSION…..

WHEN CONSIDERING DOSE REDUCTION WE SHOULD BEAR

IN MIND THE WORDS OF LEONARDO Da VINCI

“I have been impressed with the urgency of doing. Knowing is not enough; we must apply.

Being willing is not enough;

-- Leonardo da Vinci

g g g ;we must do.”