Download 3 JCI dosimetry for CT

21/03/60
MAHIDOL UNIVERSITY
Wisdom of the Land
Dosimetrymethodsfor
ComputedTomography
Napapong Pongnapang, Ph.D.
Department of Radiological Technology
Faculty of Medical Technology
Mahidol University
Why CT?
Outline
• CTTechnology
• Currentissues
• CTdosedescriptor
• Dosimetrymethods
• Patientdoseaudit
Inside CT scanner
• Fast
• Multi plane imaging
• Good spatial
resolution
• Good temporal
resolution
• Quantitative
possible
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CT Data Acquisition and
Image Reconstruction
CT Data Acquisition and
Image Reconstruction
X-ray beam
intensity
Detector number
A CT Projection
First CT Scan (1972)
Simple Back-projection
Filtered Back-projection
Technological advances in CT
Half second scanning
Slip ring scanning
1s scan
Dual slices scanning
8 slices scanning
85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10
256,320 slices scanning
Helical scanning
16 slices scanning
4 slices scanning
80 x 80 matrix size, 4 min/rotation, 8 grey level,
overnight reconstruction
640 slices scanning
64 slices scanning
111213141516
Dual Energy/Spectral CT
Low dose CT
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TRS-457Standard
NewJointCommissionStandard
(July2015)
CTRadiationDose— DocumentingPatientDoseIndicesandAnalyzingDose
Incidents
TherevisedTJCrequirementsalsospecifythattheradiationdoseofeveryCT
exammustberecorded,andthathighradiationdoseincidentsmustbe
evaluatedagainstindustrybenchmarks:
The[criticalaccess]hospitaldocumentsinthepatient’srecordthe
radiationdoseindex(CTDIvol,DLP,orsize-specificdoseestimate[SSDE])on
everystudyproducedduringadiagnosticcomputedtomography(CT)
examination.Theradiationdoseindexmustbeexamspecific,summarizedby
seriesoranatomicarea,anddocumentedinaretrievableformat.
The[criticalaccess]hospitalreviewsandanalyzesincidentswhere
theradiationdoseindex(CTDIvol,DLP,orsize-specificdoseestimate[SSDE])
fromdiagnosticCTexaminationsexceededexpecteddoseindexranges
identifiedinimagingprotocols.Theseincidentsarethencomparedtoexternal
benchmarks.
CTDosimetryMethods
• TLDMeasurement
TLDMeasurement
• isthedirectmethodbyusingananthropomorphicphysicalphantom
• ComputerBasedSimulationMethod
• DoseMeasurementusingIonizationChamber
• dosesaremeasuredinthelocationof
organsortissuesofinterestbyusingTLDs
• effectivedosecanthenbecalculated
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DoseMeasurementusingIonization
Chamber
• basedontheFDAdefinitionofCTDI
• normallyusedforQAmeasurement
• doesnotprovideadirectassessmentof
therisktothepatient
TLDMeasurementMethod
ComputerBasedSimulationMethod
• Doseestimatesarebasedon
MCcalculationsfor
anthropomorphicmathematical
phantomsorusingdata
previouslycollectedfromthe
survey
• Thephantomswerebasedon
theoriginalMIRDphantomwith
specificationofallorgans
andweremodifiedasAdam and
Evaincludingsex-specific
characteristics
Adam
Eva
TLDMeasurementMethod
• DoseMeasurementinPhantom
- insertedTLDsintoeachpointintheslicesofphantom
andpointforscatteredradiation
- scanthephantomforeachexaminationaccordingto
theprotocols
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TLDMeasurementMethod
• DataAnalysis
- theaveragedsignalreading(nC)fromeverypointof
measurementarecorrectedforsensitivitybycorrectionfactor
forsensitivity andthenmultipliedbycalibrationfactor for
convertingthemtoabsorbeddose
- DependsonhowyoucalibratetheTLD,iftheenergyor
beamcharacteristicsaredifferent,correctionfactorfor
energyresponseisthenapplied
TLDMeasurementMethod
• DeterminationofOrganorTissueDose
- theeffectivedosewascalculatedfromtheorganor
theorganortissuedosesaccordingtotheICRP103(2007)
ET =DT,R xWR xWT
whereET istheeffectivedosetoanorganortissuetypeT(Sv)
DT,R istheabsorbeddosetoanorganortissueT
deliveredbyradiationtypeR(Gy)
WR istheradiationweightingfactorforradiationtypeR
WTisthetissueweightingfactorforanorganortissuetypeT
ComputerBasedSimulationMethod
• DoseestimatesarebasedonMC
calculationsforanthropomorphic
ComputerBasedSimulation
Method
mathematicalphantoms
• Thephantomswerebasedonthe
originalMIRDphantomwith
specificationofallorgansand
weremodifiedasAdam andEva
includingsex-specificcharacteristics
Adam
Eva
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Softwarepackages
ComputerBasedSimulationMethod
• AllorgansconsideredinICRPPublicationsNo.26andNo.60(Most
updatedisNO.113,released2007)
• Foragivenscanrange,therespectivecontributionsoriginating
fromeachirradiatedsectionhavetobesummedup
• Theorgandoesvaluesperunitofprimaryradiationisspecified
bythekermavalueinairattherotationaxis
• Theyhavetobenormalizedtothescanner-specifickerma
valueandtochosenscanparameters
Softwarepackages
1.CT-Expo
Consist of
CT-ExpoisaMicrosoftExcelapplication
writteninVisualBasicforpatientdose
calculationinCTexamination.
• WeightedCTDI(CTDIW)
V2.3
• VolumeCTDI(CTDIvol)
• Doselengthproduct(DLP)
• Organdose
• Effectivedose(accordingtoICRP60and103)
1. CT-Expo
2. ImpactMC
3. ImpactDose
4. XCATdose
Softwarepackages
• Calculationmodule
1.CT-Expo
Consist of
• Calculationmodule
• Standardmodule
• Standardmodule
• ‘Light’module
• ‘Light’module
• Benchmarkingmodule
• Benchmarkingmodule
Allowsevaluatingdosefor
patient-specificdatasuchas
age,sex,andscanrangewith
graphicalinputfacilities.
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Softwarepackages
Softwarepackages
1.CT-Expo
Consist of
• Calculationmodule
1.CT-Expo
Consist of
• Standardmodule
• ‘Light’module
• ‘Light’module
• Benchmarkingmodule
• Benchmarkingmodule
canperformonlyinadult
caseandscanrangeis
automaticallydefined.
Softwarepackages
1.CT-Expo
Consist of
• Calculationmodule
• Standardmodule
UsingofCTDIvol andDLP
valuesfromthemachine
tocalculate
Softwarepackages
• Calculationmodule
2.ImpactMC
• Standardmodule
• ‘Light’module
• Benchmarkingmodule
usingMonteCarloalgorithmsanditcan
calculate3Ddosedistribution.
• WeightedCTDI(CTDIW)
Threedimensionalgridofvoxels
• VolumeCTDI(CTDIvol)
• Organdose
• Effectivedose(accordingtoICRP26,60and103)
enablesyoutobenchmark
thescanprotocolsforthe
entiresetof14standard
CTexaminations
Userdefinedspectrum
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Softwarepackages
Softwarepackages
3.ImpactDose
4.XCATdose
usingtabulatedMonteCarlocalculationresults
forthestandardanthropomorphicphantom
(ImpactMC).
isacalculatortopredicteddosetotheadult
patientsforvarietyofroutinelyusedCT
examinations,worksonmobileapplication
whichavailableoniOS andAndroid.
• WeightedCTDI(CTDIW)
• VolumeCTDI(CTDIvol)
• Doselengthproduct(DLP)
• Organdose
• SSDE
• Effectivedose(accordingtoICRP60and103)
ModifiedORNLmale/femalephantoms.
• Organdose
• SSDE(accordingtoAAPMreportNo.204)
• Effectivedose
ICRPmale/femalephantom.
SoftwareComparison
Software
package
Basis andAlgorithm
Information
provided
1.CT-ExpoV2.3
Calculatedbyacomputational
methodonMicrosoftExcel
application.
• CTDIW,CTDIvol
• DLP
• Organdose
• Effectivedose
2.ImpactMC
MonteCarloalgorithms.
• CTDIW ,CTDIvol
• Organdose
• Effectivedose
3.ImpactDose
UsingtabulatedMonteCarlo
calculationresultsforthe
standardanthropomorphic
phantom(ImpactMC).
• CTDIW ,CTDIvol
• DLP
• SSDE
• Organdose
• Effectivedose
4.XCATdose
Calculatedbyacomputational • SSDE
• Organdose
methodoniOS, Andriod.
• Effectivedose
RadiationProtectionDosimetryJournal,2011
Dependson…
ü Scannerinuse
ü%CV
Sexes
ü Bodypartofexamination
ü Software’salgorithm
CT-ExpoV.1.5
ImpactDose V.0.99x
WinDose V.2.1a
Exam
%CVof SSCT
%CVof MSCT
female
male
female
male
Head
23.4
3.3
17.5
43.8
Chest
8.5
8.9
22.1
21.0
Abdomen
9.7
12.5
16.4
19.4
Pelvis
8.6
7.3
22.2
10.6
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Flowchart
Start
Dose Measurement, fill in forms
Make a copy of the form, keep in file (JKN)
CTExpo– MalaysiaExample
Send completed forms to MOH HQ
Use CT Expo software to obtain CTDI and
Effective Dose
Analysis
End
Protocol
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Refertotheform:UNSCEAR:Forms/Diagnostic/CT/1.0.
Fillinthehospitalnameandroomnumber/name.
Fillindateandtypeofexamination,refertoTable4-3.
RecordthepatientID,ethnicgroup,sex,age,weightandheight.
RecordthestudyparametersasdisplayedontheCTconsolemonitor,suchasthescan
area,kV,mA,Time(s)or(mAs),FieldofView(FOV),Matrixsize:eg.512x512,1024x1024,
TableFeed/Rotation(TF)(mm),Pitch(P),Collimation/DetectorSelection(eg.16x0.625,
4x2.5),ReconSliceThickness(mm),ScanLength(cm),Spiralmode(YesorNo),Remark
(eg.Numberofphasesformultiplescanswithsameparameters)
Iftherewasmorethanonescanarea,recordalltheparameters(asmentionedabove)in
thesametable.
ThecollecteddatawillbesentbacktotheUNSCEARsecretariatforcomputationofthe
CTDIandEffectiveDose.
ThecalculationoftheCTDIandEffectiveDosewillbecarriedoutusingtheCTExpo
software.
RefertoexampleofthefilledUNSCEAR:Forms/Diagnostic/CT/1.0inappendix.
Markthenumberofcasesdoneforeachtypeofexaminationonthe
UNSCEAR:Forms/Diagnostic/Checklist/1.0.
Type&No.ofcases
Computed Tomography (CT)
Brain
30
Spine/Musculo-skeletal (Cervical, thorax,
lumbo-sacral spine)
30
Chest
30
Abdomen
30
Pelvis
30
Cardiac CT
30
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DoseMeasurement
usingPencilshapedChamber
UNSCEAR:Forms/Diagnostic/CT/1.0.
Measurement of CTDI and effective dose per study
Hospital:
Room:
Date
Type of examination
Patient Data:
Patient ID
Ethnic Group
Information:
FOV
Matrix Size
TF (Table Feed)
P
Collimation/ Detector Selection
M/C/I/O
Sex
Male / Female
Age
Weight
kg
Height
cm
No. of Phases
Field of View (cm x cm)
eg. 512 x 512 , 1024 x 1024
distance/rotation (mm)
Pitch
Eg. 16 x 0.625, 4 x 2.5.
For single sliced CT, leave this empty.
Number of scan with same parameters (only for
multiple phases)
*Study Parameters:
Series
No.
Scan Area
kV
mA
mAs
Time
(s)
FOV
Matrix
size
TF
(mm)
P
Collimation/
Detector
Selection
Recon
Slice
Thickness
(mm)
Scan
Length
(cm)
Spiral
Mode
(Y/N)
Remark
(e.g. No.
of Phases)
1
2
3
4
5
*Do not record scout scan
DEFINITIONSOFQUANTITIESFOR
ASSESSINGDOSEINCT
•
•
•
•
•
•
Computed Tomography Dose Index (CTDI)
Weighted CTDI (CTDIW)
Volume CTDI (CTDIVOL)
Dose-Length Product (DLP)
Limits to CTDI Methods
Effective Dose (E)
Computed Tomography Dose Index (CTDI)
Averageabsorbeddose alongthez–
axisfromseriesofcontiguousirradiation
inoneaxialCTscanoronerotationofxraytubewithinthecentralregionofa
scanvolume
TRS457=CTairkermaindex,Ca,100(inair)or
CPMMA,100(inphantom)
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Computed Tomography Dose Index
(CTDI, Ca,100orCPMMA,100)
Computed Tomography Dose Index (CTDI, Ca,100or
CPMMA,100)
CTDI100 =
1
NT
50 mm
ò D( z )dz
-50 mm
• D(z) = the radiation dose profile along
the z-axis
ComputedTomographyDoseIndex
Weighted CTDI (CTDIW)
• Appliestosingleaxialscanonly
• AverageCTDIacrossfieldofview
• Measuredonaxisofscannerusingpencil
ionisationchamber
• Usefulindicatorofscannerradiationoutput
foraspecifickVpandmAs
• CTDI twohigheratthesurfacethanatthe
centerofFOV
• Calculatedasintegralofairkermaalong
chamberdividedbynominalslicethickness
• Severaldifferentversions!!
• TRS457=Cw
CTDI w =
1
2
CTDI100,center + CTDI100,edge
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3
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Volume CTDI (CTDIVOL)
• Absorbedradiationdoseoverthex,yandz
directionwithinscanvolumeforstandardized
Phantom,TRS=Cvol
• Dosetostandardizedphantomforaspecific
scanprotocol
• Takegaporoverlapsbetweenthex-raybeam
fromconsecutiverotationsofthex-raysource
CTDI vol =
N ´ T ´ CTDI w
I
I=thetableincrementperaxialscan(mm)
Dose-Length Product (DLP)
LimitationofCTDIVOL
• Does not represent the average dose
for objects ofdifferent
- size
- shape
- attenuation
• Does not indicate the total energy
deposited into the scan volume because it
is independent of the length of the scan
DifferenceinCTDIvol andDLP
Overall energy delivered by a given scan protocol, the
absorbed dose integrated along the scan length , TRS = P
KL,CT
Air Kerma Length Product
• Change in technique (mAs/rotation) affects the CTDIvol and DLP
• Change in acquisition length (at the same
technique) is only reflected by the DLP
DLP (mGy - cm) = CTDI vol (mGy ) ´ scan length (cm)
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Effective Dose (E)
Effective Dose (E)
Biologicaleffectsfromradiationdependon
– Radiationdose
– Biologicalsensitivity
EstimationofdosetopatientsundergoingCTexams
- Directmeasurementoforgandoseinphantom
- CalculationbyMonteCarlomethod
• Comparebiologicaleffectsbetween
diagnosticexamofdifferenttypes
• Communicationwithpatient regardingthe
potentialharm
E(mSv)=kxDLP
k=conversionfactor
error10-15%
InSummary
EffectiveDose
AAPM Report No.96
• Diagnosticdoserangesbetween0.2-20mSv
• Highdoseproceduresincludeinterventional
radiologyandCT
• Doseoptimizationisneededtoreduce
stochasticrisksofradiation
• PatientdosimetryinCTiscomplexbut
manageable
• NewJointCommissionstandardrequiresCT
dosereporting,trackingandaudit
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AnyQuestions?
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