introduction - aapm

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu1

MahadevappaMahesh,MS,PhD,FAAPM,FACR,FACMP,FSCCT.

ProfessorofRadiologyandCardiology

JohnsHopkinsUniversitySchoolofMedicine

ChiefPhysicist– JohnsHopkinsHospital

JointAppointment- JohnsHopkinsSchoolofPublicHealth

Baltimore,Maryland,USA

CardiacCTPrinciplesandRadiationDose

AAPMClinicalSpringMeeting,LasVegas,NV*April7-10,2018

ContactInfo:email- mmahesh@jhmi.edu Phone:410-955-5115(O)

Introduction

• CardiacCTstudiesweredeemedhighdoseCT

proceduresinthebeginning

• However,frompastfewyears,radiationdoses

havedecreasedconsiderably

• Technologicalandoperationalfactorsareaidingin

loweringthedose

EssentialsforCardiacImaging

• HighTemporalResolution:toimage

coronarysegmentsproximaltoheart

• HighSpatialResolution: toimageproximal

coronarysegments(RCA,LAD,CX)ofsub-

millimetersize

• HighContrast-to-noiseratio:toresolvesmall

structuressuchasplaques

• HighLow-contrastresolutionwithlimited

radiationexposure withshorterexposure

timeiskey

LAD

RCA

DiastolicPhaseversusHeartRate

406080100120

600

400

0

0Heartrate(bpm)

msExposuretime

Diastole

500ms

250ms

100ms

• Leastcardiacmotionisobserved

duringdiastolicphase

• Diastolephasenarrowswith

increasingheartrate

• Desiredtemporalresolutionfor

motionfreecardiacimaging

• ~250msforheartrates~70bpm

• ~150msforheartrates~100bpm

• Motion-freeimagingneeds

temporalresolution~50ms

CardiacPhaseReconstruction

At10%,therightcoronaryarteryisinastateofheavymotion

At70%itisalmoststationary(right)

10%ofRàR 70%ofRàR

0% 90%

NumberofCTproceduresinUS

IMVBenchmark2016

18.4 19.7 21.6 22.625.8

2933.1

37.941.4

44.347.2

5155.7

58.461.1

63.9 66.162.3

58.562.3 62.7

2.62.9

3.5 3.7

4.8

5.9

6.5

7.5

8.79.6

10.4

11

13

14.7

16.4

1819.2

18.3

17.5

18.9 16

2122.6

25.126.3

30.6

34.9

39.6

45.4

50.1

53.9

57.6

62

68.7

73.1

77.5

81.985.3

80.6

76

81.278.7

0

10

20

30

40

50

60

70

80

90

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

NumberofProcedures(m

illions)

Non-HospitalHospital

MDCT

2011:85.3million

2012:80.6million

2013:76.0million

2014:81.1million

2015:78.7million

2016:82.0million

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu2

Pelvis & Abdomen23%

Brain21%

Chest16%

Head & Neck9%

Spine8%

CT Angiography (non-cardiac)

7%

Guided Procedures

5%

Lower Extremities3%

Upper Extremities3%

CT Angiography (cardiac)

2%

Calcium Scoring1%Other

Cardiac1%

Whole Body

Screening0%

Virtual CT Colonography0%

Other0%

IMV2015HCAP:~76%ofallCTprocedures

CategoriesofCTprocedures(78.7millionin2015)

CT ProcedureCategories

Total 2016 CT Procedures (M)

% of All CT Procedures

% of CT Sites Performing

Head & Neck 22.5 27% >90%

Chest, Abdomen& Pelvis

38.3 47% >95%

Calcium Scoring 1.2 1% 30%

CT Angiography 2.2 3% 26%

Total 2015 CT Procedures

82.0 100%

IMV2016

CardiacCTusageinUS

CoronaryCTAngiographyvolumesinEmergencyDepartments

(Medicareprograms,2006-2015)

AJR2018

ScanParametersthatimpactsDoseandImageQualityinCT

• TubeCurrent(mA)

• TubeVoltage(kV)

• ScanTime

• Pitch

• ScanAcquisitionType

• ScanFieldofView(SFOV)

• DisplayFieldofView(DFOV)

• BeamCollimation

• ReconstructedSliceWidth

• ReconstructionInterval

• ReconstructionAlgorithms

• PatientSize

• PatientMotion

• GeometryandDetectorEfficiency

• Trainingandexperience

PrimaryFactors SecondaryFactors

OtherFactors

MaheshM.,MDCTPhysics:TheBasics…,Lippincott,2009

TubeCurrent(mA)

• Amountofx-raysproducedinx-raytube

• Indicate‘Quantityofx-rays’

• RadiationdosevarieslinearlywithmA

• Decreasingtubecurrentby50%

– Decreasesradiationdoseby½

– Increasesimagenoiseby√2

TubeCurrent(varywithclinicalindications)

CalciumScoreImage

Range:25to200mA

CTAImage

Range:200to800mA

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu3

TubeVoltage(kV)

• Potentialdifferencebetweenanodeand

cathodeofx-raytube

• Qualityofx-rays- affectsimagecontrast

• 120kV– mostcommon

– OtherkVstations– 140/135,100/110and80kV

– 100kVor80kV– thinpatients

• Dosevarieswithtubevoltage(kV2)

Pitch and Dose

I - Tablefeed(mm/rotation)W- Beamwidth(mm)

Pitch=IW

T- SingleDASchannelwidth(mm)

N- NumberofactiveDASchannels

† IEC Part 2-44, 2003

I

T

W

MDCT

Dose 1

Pitch(mAs/rotation)µ

WhyCardiacCTprotocolsuseLowPitch†?

Slope:Tablefeedspeed

• Higherpitchproducesgaps

• Highquality3Dwithminimal

artifactsrequiresdataoverlap

• Typicalpitch:0.20- 0.4

• Sincepitchislow,radiation

dosetendstobehigh

Time

Z-position

Z-position Helical

scandirection

Datagaps

withhigherpitch † IECPart2-44,2003

Dose 1

Pitch†(mAs/rotation)µ

RetrospectiveECGGating

ECG

� � � � � �

ContinuousrecordingofspiralscanandECG

Time/Pos.

movingcouch-top

TemporalResolution

Radiationdosehigherthan

prospectivetriggering

RetrospectiveECGGating

Time

Z-Position

Recon

Recon

Recon

Recon

Recon

N*T:64*0.625– 40mmscancoverage

ProspectiveECGTriggering

ConventionalAxial� PartialScan� (StepandShoot)

ECG

moving

couch-topPreset

Delay

Temporalresolution

Radiationdoseminimized

Limiteddataset

X-rayON X-rayON

Preset

Delay

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu4

ProspectiveECGTriggering

Time

Z-Position

Scan

Scan

Scan

Scan

Scan

N*T:64*0.625– 40mmscancoverage

CTA Dose: Prospective vs Retrospective

PROTECTIONIstudy,AJR,2010

Helicalmode:11.2mSv

Sequentialmode:3.6mSv

Sequential

Helical

CoronaryCTAngiography:ProspectiveTriggeredvs HelicalRetrospectivegated

Effectivedose

forCTAportion:

4-6mSv

Effectivedosefor

CTAportion:

12-15mSv

Javadi M,MaheshM,etal.,JNucl Cardiol 2008

MotivationforadvancementinCTtechnology

Goal

• ToimageentireheartinsingleCTgantryrotation

– Achievedbywide-detectorCTsystems

• Toimageentireheartinasingleheartbeat

– Achievedwithhigh-pitchscanusingdualsourceCT

Scancoverage- 320vs 64sliceMDCT

Aquillion 64- 32mmbeamwidth

Aquillion One- 320sliceMDCT- 160mmbeamwidth

320slice64slice

MDCTPhysics:TheBasics…,Lippincott,2009

Toshiba

AdvantagesofwidedetectorCT

• WidedetectorCTsystemshavelargescanregions

– Scanrangesupto160mm

• Minimizespatientmotion

• Requireslesscontrast

• Reducesoverallexamtime

• Step-and-shootscanningwithminimaloverlap

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu5

Exposure

Exposure

WithoutECG

DoseModulation*

WithECG

DoseModulation

320MDCT:CardiacCTAProtocol

SingleHeartBeatProtocol(forHR≤65bpm)

Toshiba

SingleHeartBeat

Protocol(forHR≤65bpm)

2-HeartBeat

Protocol(forHR≥65bpm)

Exposure WithoutECG

DoseModulation*

StudieshavereportedcardiacCTdosesof3.2mSvand5.7mSv

DualSourceCT:DefinitionFLASH*

Definition– FLASH

2nd Detectorsetstillsmallerthan1st butlargerthanDefinitionSFOV:1st detector– 50cm,2nd detector– 34cm

X-rayTube A

X-ray Tube B

Detector B

Detector A

* Siemens JohnsHopkins– May2009

SingleSourcevs DualSourceCT*

64SliceMDCT~190ms DSCT~90ms

180� DataAcquisition 90� DataAcquisitionpertube

*Siemens

Temporalresolution:~1/3rd to1/4th ofgantryrotationtime

DataAcquisitionwithDSCT-Flash

• Tablespeed:430mm/s

• Pitch:3.2

• Gantryrotationtime:0.28sec

• Beamwidth:38.4mm

• Maximumslices:128

• Scanrange:120mm

• Scantime:280ms

HighPitchCardiacCTScanwithDSCTFLASH*

• Interleavedspiralpathfromdual

sourceisusedinimagereconstruction

• High-pitch(>3)scansenablesdata

acquisitionwithinsingleheartbeat

• Highdemandonpatientselection

(<60bpmdesired)

*AchenbachS,JCCT,3:117-121,2009

CTADose:Conventionalvs HighPitch

PROTECTIONIVstudy,JCCT,2015

Conventional:4.7± 4.8mSv

Highpitch(DSCT):2.0± 2.4mSv

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu6

UnderstandingCTdosedisplay

• Pre-scandisplay

– Allowsverificationofset

parametersarenottoooffthechart

• Post-scandisplay

– Detailsonscanseries

– Scanparameters

– DLPforeffectivedoseestimation

– Nostandardformats

– DICOMheadersformoredetails

available

CTDIvol

Phantom

Type

www.radiologyinfo.orgDoseModulation

RadiationDoseReport– CTAngiographyExam

IntheBeginning

(2007)

Around2011

Effectivedose

(mSv)

2.7mSv

15.0mSv

17.7mSvTotaleffectivedose(mSv)

1.1mSv

8.2mSv

9.3mSvTotaleffectivedose(mSv)k=0.014mSv/mGy-cm

RadiationDoseOptimizationStrategies

• Minimizescanrange

• Heartratereduction

• ECGgatedtubecurrentmodulation

• Reducedtubevoltageinsuitablepatients

• Performcalciumscoringonlyifneeded

• SequentialScanning– Prospectivetriggeringmethods

• Iterativereconstructionmethods

JCardiovasc Comput Tomogr.2011,5(4):198-224

CTDoseModulation

Temporal

DoseModulation

Spatial

DoseModulation

AutomaticTubeCurrentModulation(ATCM)

• Spatialmodulation:Basedonmodulatingtube

current(mA)atdifferentspatialprojections

• UtilizedinmostroutinebodyCTprotocols

• Temporalmodulation:Basedonmodulatingtube

current(mA)atspecifiedtimepointsofan

electrocardiographicallygated(ECG)signal

• UtilizedincardiacCTprotocols

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu7

McCollough,C.H.etal.Radiographics 2006

SpatialDoseModulation

• Bodyprotocols

• Tubecurrentvariationsbasedonspatial

variations(patientthickness)

TemporalDoseModulation

withRetrospective-GatedAcquisition

• Constanttubecurrent

throughentireR-Rcycle

canbemodulated

• Tubecurrentislowered

outsidediastolicregion

enablingdosereduction

duringcardiacCT

TemporalDoseModulation

withProspectivelyTrigerred acquisition

• Constanttubecurrent

throughentireR-Rcycle

canbemodulated

• Tubecurrentislowered

outsidediastolicregion

enablingdosereduction

duringcardiacCT

CTADose:AxialversusHelical

PROTECTIONIIIstudy,JACC,2012

A- Axial:3.5� 2.1mSv

B- Helical:11.2� 5.9mSv

TemporalDoseModulationwithSinglebeatorSingleGantryRotationAcquisition

• Constanttubecurrent

throughentireR-Rcycle

canbemodulated

• Tubecurrentislowered

outsidediastolicregion

enablingdosereduction

duringcardiacCT

TemporalDoseModulation

forcoronaryCTAngiography

• Constanttubecurrent

throughentireR-Rcycle

canbemodulated

• Tubecurrentislowered

outsidediastolicregion

enablingdosereduction

duringcardiacCT

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu8

ScanModes

JCCT,2011

TubeVoltageModulation

• Lowertubevoltageimprovesimagecontrastandreducedose

• Astubevoltagedecreases,tubecurrentmayhavetobe

increasedtomaintainimagenoise

Radiology2012;264(2)::567-580

InfluenceofTubeVoltageonCTADose

PROTECTIONIIstudy,JACC,2010

EffectiveDoseEstimation

A- 100kV:8.4± 3.6mSv

B- 120kV:12.2± 4.4mSv

TubePotential

JCCT,2011

IterativeReconstruction

• ObjectiveistoenableusertoacquireCTdataatlowdoseandimproveimage

qualitywithiterativeprocess

•Mostiterativereconstructionalgorithms

actas‘BLACKBOX’duetomanufacturer

proprietaries

CTADose:ImpactofIR

PROTECTIONVstudy,JACC,2015

ReducedmA+IR:2.2(1.6– 3.3)mSv

StandardmA+FBP:3.1(2.0– 4.5)mSv

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu9

EstimatingEffectiveDose

• Computersoftware

– BasedonMonteCarlosimulations

– IMPACTdosecalculator

• k-factorsbasedonDLP– E=DLP*k–wherekinmSv/mGy-cm

EffectiveDose=k*DLP

AAPMTaskGroupReportNo.96,2008

Quickestimationofeffectivedoseformostcardiac

CTprotocols:~1.4%ofDLP

Ex:DLP=1000mGy *cm,E=14mSv

DLP=500mGy *cm,E=7mSv

where,‘k’– standardconversionfactorsofhead,neck,

chest,abdomenandpelvisCT

Newk-factorforcardiacCT

• Currentk-factor

– 0.014- 0.017mSv/mGy-cm

• Newconversionfactors

• AccountsforICRP103weighting

factorsandcurrentCTtechnology

• Proposednewk-factor

– 0.026mSv/mGy-cm(0.020to0.035)

JACCCardiovasc Imaging.2018,11(1):64-74

CTDoseCheck

• USFDAhassuggested

CTalertvalueforCTDIvolof1Gy (1000mGy)

CTNotificationValues*

CTDoseAlert

*MaheshM,JACR2015**AAPMDoseCheckGuidelines,2011

*NEMAXR25-2010

CardiacCTA– submSv studies

• 107patients– 27.3BMI

– 100kVfor97patients

• Widevolumecoverage

(320*0.5mm- Toshiba320)

• IterativeReconstruction

• Automaticexposurecontrol

• Radiationdose:0.93mSv

(0.58– 1.74mSv)

ChenMY,etal.,Radiology2013

CardiacCTA

JohnsHopkinsonToshiba320CT

100kV

6.6mGy

92.40mGy-cm

1.3mSv

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu10

DoseOptimizationinClinicalPractice

JCCT,2011

DoseMonitoring

JCCT,2011

CardiacCTA- UnsteadyHRandUltraLowDose

Courtesy- Toshiba

HR41-96bpm

1.1mSv

HR47bpm

0.27mSv

RaffGL,JCCT,2010

Conclusions

• CardiacCTimaginghasbeenthedrivingforcebehind

manytechnicaladvancesinMDCT

• Noveltechnologicaladvanceshaveaidedinlowering

cardiacCTdose

• RadiationdosesinCardiacCThasbeendecreasing

• Importanttounderstandandutilizetechniquesto

lowercardiacCTdoseinroutinepractice

ApologiestoDr Stewart

4/8/18

©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT

JohnsHopkinsUniversity mmahesh@jhmi.edu11

Raff,G.L.etal.JAMA2009;301:2340-2348.

PercentageofPatientsAchievingTargetDoseofLessThan15mSv byBimonthlyIntervals

Distributionof

Patientsby

Estimated

RadiationDose

Impactafterimplementingradiationreductiontechniques

Cademartiri F,Eur Radiol,2013

CardiacCT:RadiationDoseStatus

Radiation Risks versus other causer

FletcherJG,etal.,AbdominalImaging,2012