introduction - aapm
TRANSCRIPT
4/8/18
©Dr M.Mahesh– MS,PhD,FAAPM,FACR,FACMP,FSCCT
JohnsHopkinsUniversity [email protected]
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- [email protected] 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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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 [email protected]
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