indice de exposicion

AAPMREPORTNO.116

ANEXPOSUREINDICATORFORDIGITALRADIOGRAPHY

REPORTOFAAPMTASKGROUP116

JULY2009

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TheAAPMdoesnotendorseanyproducts,manufacturers,orsuppliers.Nothinginthispublicationshouldbeinterpretedasimplyingsuchendorsement.

2009byAmericanAssociationofPhysicistsinMedicine

DISCLAIMER:Thispublicationisbasedonsourcesandinformationbelievedtobereliable,buttheAAPM,theeditors,andthepublisherdisclaimanywarrantyorliabilitybasedonorrelatingtothecontentsofthispublication.

TheAAPMdoesnotendorseanyproducts,manufacturers,orsuppliers.Nothinginthispublicationshouldbeinterpretedasimplyingsuchendorsement.

ISBN:978-1-888340-86-0ISSN:0271-7344

2009byAmericanAssociationofPhysicistsinMedicine

Allrightsreserved.Nopartofthispublicationmaybereproduced,storedinaretrievalsystem,ortransmittedinanyformorbyanymeans(electronic,mechanical,photocopying,recording,orotherwise)withoutthepriorwrittenpermissionofthepublisher.

PublishedbyAmericanAssociationofPhysicistsinMedicineOnePhysicsEllipseCollegePark,MD20740-3846

AAPMREPORTNO.116


REPORTOFAAPMTASKGROUP116

CO-CHAIRS

S.JeffShepardJihongWangImagingPhysicsDepartment#056DivisionofDiagnosticImagingTheUniversityofTexasM.D.AndersonCancerCenterHouston,TX77030

TASKGROUPMEMBERS:

MichaelFlynn,HenryFordHospital,Detroit,MIEricGingold,ThomasJeffersonUniversity,Philadelphia,PALeeGoldman,HartfordHospital,Hartford,CTKerryKrugh,ToledoHospital,ToledoOHDavidL.Leong,AnalogicCorporation,Peabody,MAEugeneMah,MedicalUniversityofSouthCarolina,Charleston,SCKentOgden,SUNYUpstateMedicalUniversity,Syracuse,NYDonaldPeck,HenryFordHospital,Detroit,MIEhsanSamei,DukeUniversity,ChapelHill,NCCharlesE.Willis,TheUniversityofTexasM.D.AndersonCancerCenter,Houston,TX

III

ACKNOWLEDGMENTS

TG-116wouldliketorecognizecontributionsfromindustryliaisonstotheTGeffort.Thisacknowledgmentwasinadvertentlyleftoutoftheoriginalreport.Manyoftheseindividualscontributedmateriallytowritingtheappendicesandallofthemparticipatedinmeetingsandcontributedtodiscussions.Theseindividualswere:

StephenBalter,ColumbiaMedicalCenter(IECLiaison)PatrickC.Brennan,UniversityCollege,DublinMartinDarms,SwissrayMedicalUriFeldman,ICRCompanyBernhardGeiger,SiemensMedicalSolutionsKadriJabri,GEHealthcareDavidLeong,AnalogicCorp.StephenW.Meyer,CanonMedicalSystemsChristopherR.Mitchell,ALARA,Inc.UlrichNeitzel,PhilipsMedicalSystems,DIN/NAR,IECWG43RalphSchaetzing,AgfaCorporation(nowwithCarestreamHealth,Inc.)RobertA.Uzenoff,FujifilmMedicalSystemsUSA(Liaison:Fujifilm,MITACR&DRcommittee)RichVanMetter(FormerlyCarestreamHealth,Inc.)StephenVastagh,NationalElectricalManufacturersAssociationDarrenWerner,KonicaMinoltaMedicalImagingUSARobinWinsor,ImagingDynamicsCompanyJohnYorkston,CarestreamHealth,Inc.WeiZhao,StateUniversityofNewYork

CONTENTS

Abstract.............................................................................................................................1I.Introduction....................................................................................................................1II.DefinitionofTerms..........................................................................................................4III.Recommendations...........................................................................................................7IV.StandardRadiationExposureConditions....................................................................8A.StandardBeamSpectrum.........................................................................................8B.StandardBeamGeometryandCalibrationVerificationProcedure..........................9V.AssessmentofIndicatedEquivalentAirKerma(KIND)................................................12VI.ReportingDeviationIndex(DI).....................................................................................14VII.ClinicalUseoftheDeviationIndex(DI).........................................................................15VIII.ExposureIndicatorandRadiographicTechniques......................................................16IX.KINDandAutomaticExposureControl(AEC)Systems...............................................18X.InappropriateClinicalUseofDI....................................................................................19XI.RecommendedOptionalFeatures..................................................................................19XII.ApplicationtoDedicatedChest,Mammography,Veterinary,andDentalRadiography.................................................................................................20References.................................................................................................................................21AppendicesARQA5vs.TG-116StandardBeamConditions,XSPECT3.5bComputationSimulation...............................................................23BComparisonofPureAluminumvs.CommerciallyAvailableTypes1100and1190AluminumandaCopper/AluminumAlternativeforRQA5.................................................................................................................33CCurrentStatusofExposureIndices.......................................................................41DAgfaCR...................................................................................................................47EFujifilmFCR..........................................................................................................49FKodakCR(nowCarestreamHealth,Inc.)............................................................51GKonicaMinoltaCR.................................................................................................59HImagingDynamics.................................................................................................61IPhilipsDigitalDiagnostExposureIndex..............................................................63

V

CONTENTS(CONT.)

JGEHealthcare........................................................................................................67KAlaraCR.................................................................................................................69LSiemensMedicalSystems......................................................................................71MiCRco.......................................................................................................................73NCanonMedicalSystems.........................................................................................77OSwissVisionDoseIndicator....................................................................................85

VI

ABSTRACT

Digitalradiographicimagingsystems,suchasthoseusingphotostimulablestoragephosphor(PSP),amorphousselenium,amorphoussilicon,charge-coupleddevice(CCD),andmetaloxidesemicon-ductor-fieldeffecttransistor(MOSFET)technology,canproduceadequateimagequalityoveramuchbroaderrangeofexposurelevelsthanthatofscreen/filmimagingsystems.Inscreen/filmimaging,thefinalimagebrightnessandcontrastareindicativeofover-andunderexposure.Indigitalimaging,brightnessandcontrastareoftendeterminedentirelybydigitalpost-processingoftheacquiredimagedata.Over-andunderexposuresarenotreadilyrecognizable.Asaresult,patientdosehasatendencytoincreasegraduallyovertimeafteradepartmentconvertsfromscreen/film-basedimagingtodigitalradiographicimaging.Thepurposeofthisreportistorecommendastandardindi-catorwhichreflectstheradiationexposurethatisincidentonadetectoraftereveryexposureeventandthatreflectsthenoiselevelspresentintheimagedata.Theintentistofacilitatetheproductionofconsistent,high-qualitydigitalradiographicimagesatacceptablepatientdoses.Thisshouldbebasednotonimageopticaldensityorbrightness,butonfeedbackregardingthedetectorexposureprovidedandactivelymonitoredbytheimagingsystem.Astandardbeamcalibrationconditionisrecom-mendedthatisbasedonRQA5,butusesfiltrationmaterialsthatarecommonlyavailableandsimpletouse.Recommendationsonclinicalimplementationoftheindicestocontrolimagequalityandpatientdosearederivedfromhistoricaltolerancelimitsandpresentedasguidelines.

Keywords:digitalradiography,directdigitalradiography,indirectdigitalradiography,computedradiography,photostimulablestoragephosphor,imagequality,imagenoise,exposureindex,qualitycontrol,qualityassurance,acceptancetesting.

I.INTRODUCTIONThechargeofTaskGroup116(TG-116),asapprovedbytheScienceCounciloftheAmericanAssociationofPhysicistsinMedicine(AAPM)wastoidentifyamethodofprovidingfeedback,intheformofastandardindex,tooperatorsofdigitalradiographicsystems,whichreflectstheade-quacyoftheexposurethathasreachedthedetectoraftereveryexposureevent.Thisreportistheanswertothatchargeandwillcoveralldigitalradiographicimagedetectorsystems,leavingdigitalfluorographyorfluoroscopy(radioscopy)forfutureconsideration.Unlikescreen/filmimaging,imagedisplayindigitalradiographyisindependentofimageacqui-sition.Inadequateorexcessiveexposureismanifestedashigherorlowerimagenoiselevelsinsteadofasalightoradarkimage.Thefinalbrightnessoftheimageiscontrollednotbytheexposuretothedetector,butbypost-processingappliedtotheacquiredimagedata.Consequently,overexposedimagesmaynotnecessarilybedark,andunderexposedimagesmaynotappearlight.Thismaybeanewandconfusingconceptforoperatorsofdigitalradiographicsystemswhoareaccustomedtoscreen/filmimaging.Formorethanadecade,thephenomenonofexposurecreepinphotostimulablestoragephos-phor(PSP)imaginghasbeenreported.[1,2,3]Thisisattributedtothefactthatdigitalimagingsystemscanproduceadequateimagecontrastoveramuchbroaderrangeofexposurelevelsthanscreen/filmimagingsystems.Thisbroaddynamicrangeisoneofthebenefitsofdigitaldetectors.However,ifthedetectorisunderexposed,highernoiselevelsmayobscurethepresenceofsubtledetailsinthe

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image.Excessivedetectorexposuresmayproducehigh-qualityimageswithimprovednoisecharac-teristicsbutattheexpenseofincreasedpatientdose.Inextremecases,excessivedetectorexposuresmayresultinartifacts.Asaresult,mostradiologiststendtocomplainaboutunderexposedimagesbutremainsilentwhenimagesareacquiredathigherdoselevelsunlessapparentsaturationhasoccurred.Technologistsquicklylearnthattheycanproduceimagesofbetterqualityiftheyincreasetheirexposuretechniques,resultinginlessnoisyimagesandavoidingradiologistcomplaintsaboutnoisyimages.Averageexposurelevelstendtocreepupovertimeifaclearindicatorofexposureisnotprovidedandroutinelymonitored.TechniquesrequiredtoachieveoptimalradiographicimaginginDigitalRadiography(DR)maybedifferentfromthoseusedforscreen/filmimaging.Inaddition,differentDRdetectorsmayrequiredifferenttechniquefactorsduetodifferencesintheenergydependenceofthedetectormaterialsinuse(seeFigure1).[4]ThesedifferencesintechniqueamongDRsystemsmaycauseconfusionandsuboptimalimagequalityatsiteswheremorethanonetypeofsystemisinuse.Operatorsneedaclearsetofrulestoproduceconsistent,high-qualitydigitalradiographicimaging,basednotonimagedensitybutonfeedbackregardingthedetectorexposureprovided.Severalmanufacturerscurrentlyuseanexposureindicatorthatparallelstheconceptofspeedorspeedclassusedbyfilmmanufacturers.Inaddition,manymanufacturersandusershavebecomeaccustomedtocharacterizingtheirsystemsasfunctioningwithinagivenspeedclass.Thishascreatedsomemisunderstandingsandscientificinaccuracies,whichhavebeendiscussedintheliterature.[5]

FIGURE1.Energydependenceofcommondetectormaterials.

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TG-116recommendsavoidingtheconceptofspeedclasswhenreferringtoDRsystemper-formance.ThedefinitionofradiographicspeedaccordingtoISO9236-1[6]isbasedontheradiationexposurerequiredtoachieveanetopticaldensityof1.0onthedevelopedfilm.WithDRthereisnofixedrelationshipbetweentheradiationexposureandtheresultantdensityintheimage.Withscreen/filmdetectorsachangeinspeedmayalsoresultinachangeinthespatialresolutionproper-tiesofthedetector.Thissamerelationshipdoesnotholdtruewithdigitaldetectorssincesharpnessisindependentoftheamountofexposureusedtoacquirethedigitalimage.Thecharacterizationofadigitalradiographicsystemasbeinginagivenspeedclassmaygivethefalseindicationthatitshouldalwaysbeoperatedataspecificexposurelevel.Itmayalsogivethefalseimpressionthattheresultingdigitalimagewillhavethesamenoiseandresolutioncharacteris-ticsasthatacquiredwithanequalspeedclassscreen/filmsystem.Thedigitalsysteminrealitycanbeoperatedoverabroadrangeofsensitivitysincetheamountofradiationexposuredeterminesonlythelevelofquantummottleandnotthebrightnessoftheimage.Fromthiscontextthelevelofradiationexposure,andthustheso-calledspeedclass,shouldbedependentupontheimagingtaskandupontheobserverstoleranceofimagenoise.Asageneralrule,theAsLowAsReasonablyAchievable(ALARA)conceptshouldprevailinthattheminimumamountofexposure,andhencethemaximumtolerablenoisecontent,shouldbeusedtoachievethenecessarydiagnosticinformation.[7]UsingthespeedclasscharacterizationforgivendigitalimagingsystemsmayincreasethepossibilitythatALARAisviolatedforsomeimagingtasks.Anindexofdetectorexposureisappropriatebecauseitisreflectiveofthenoisecontent,andthusthesignal-to-noiseratio(SNR)intheimage.ForDRsystems,theappropriateincidentexposureisvariablebasedonthedesiredSNRratherthanontheresultingopticaldensityofaradiograph.Differentdigitaldetectorsmayrequiremoreorlessradiationexposuretoachievethesamenoisecontentdependinguponthedetectivequantumefficiency(DQE)ofthedetectortechnologyinuse.Foragivensystem,theimagenoisecontentwilltrackinverselywiththedetectorexposure.Asradia-tionexposuretothedetectorincreases,imagenoisewilldecreaseandSNRwillincrease.AstandardizedindicatoroftheexposureincidentonaDRdetectorthatisconsistentfrommanu-facturertomanufacturerandmodeltomodelisneeded.ThiscouldbeusedtomonitordifferencesinexposurebetweenDRsystemsatagiveninstitution,tocomparetechniquesbetweeninstitutions,ortoestimatethequalityofimagesfromagivenradiographicsystem.Itcouldalsoprovidequalitycon-trol(QC)dataifsoftwareisprovidedtorecordandretrospectivelyanalyzeexposuredatafromallsystems.Astandardindicatorthatreflectstheradiationexposurethatisincidentonadetectoraftereveryexposureeventisappropriate.Thedetectorexposureindicatorisintendedtoreflectthenoiselevelspresentinimagedata.Anadequateexposureisonethatresultsinanappropriatenoiselevelintheimageasdeterminedbytheclinicwherethesystemisinuse.Thisreportdoesnotmakerecommen-dationsonexposureadequacy,nordoestheindicatorrepresentexposuretothepatient.TG-116considerstherecommendationsinthisreporttobeachievableandimportant.Itrecog-nizesthataparallelstandardwasrecentlycompletedwithintheInternationalElectrotechnicalCommission(IEC),designatedIEC62494-1Ed.1:Medicalelectricalequipment-Exposureindexofdigitalx-rayimagingsystems-Part1:Definitionsandrequirementsforgeneralradiography.ThisIECstandardspecifiesthedefinitionsandcalibrationconditionsforthedetectorexposureindicesofDRsystems.TheleadershipofTG-116participatedinthisIECeffortsinceitsinceptionandservedasU.S.NationalCommitteeexpertsinIECWorkingGroup43.Theconceptsandcalibrationconditions

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FIGURE2.Essentialprocessesintheacquisitionofadigitalradiograph.KINDandDIarecomputedfromQvaluesusingsegmentationinformation.

intheIECworkingdraftareconsistentwiththoseinthisreport.WhiletheterminologyanddefinitionsintheIECstandardmaydifferinscaleandnomenclaturefromthoseinthisreport,theIECstandardiscompletelyconsistentwiththisreport.Absoluteadherencetothenomenclature,symbols,andmulti-plicativefactorsofscaleinthisreportareinconsequentialtoachievingtheultimatebenefitoftheserecommendationsaslongasallmanufacturersadheretotheIECstandarddefinitions.UsersshouldbeabletorelyonamanufacturersclaimofconformancetotheIECstandardtoidentifyequipmentofferingastandardexposureindexasdescribedinthisreport.

II.DEFINITIONOFTERMSDRsystemsutilizeaseriesofcomputationalprocessestotransformtherawdataofthedetectorintoanimageintendedforpresentation.Theseprocessesincludethoseusedtoassesstheaverageresponseofthedetectoranditsrelationtotheincidentx-rayexposure.Theimageformationprocess(Figure2)beginswiththeextractionofrawdatafromthedetectorimmediatelyfollowinganexposureevent.Thatdatamustbecorrectedforimperfectionsinthedetectorarraysuchasthepresenceofbadpixelelements,darkcurrentcorrections,andgaincorrec-tionsthatmaybeappliedonapixel-by-pixelbasis.Afterthesecorrectionshavebeenapplied,theresultingpixelvaluesarereadytobeprocessedbythesystemandarereferredtoasfor-processingpixelorQvalues.Thesystemthenattemptstoidentifywhichofthesepixelscontaininformationthatisofinteresttotheuser,typicallythosethatcontaininformationrelevanttotheanatomybeingexamined.Thisprocessiscalledsegmentation.ItisfromthesegmentedimagevaluesthatTG-116

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proposestheexposureindicatorsbedetermined.Thefinalimagefordisplayresultsfromgrayscaletransformations,broadareaequalization,edgerestoration,noisereduction,orotherimage-relatedprocessesthatareperformedonthefor-processingQvaluesresultinginfor-presentationQPval-ues.QPvaluesaretypicallystoredinaPictureArchivingandCommunicationsSystem(PACS)andtransmittedtoaprinterorworkstationfordisplay.TheremainderofthissectiondefinesthetermsusedinthisdocumentthatrelatetotheDRprocessesjustdescribed.

DIGITALRADIOGRAPHY

Radiographicimagingtechnologyproducingdigitalprojectionimagessuchasthoseusingphoto-stimulablestoragephosphor(computedradiography,orCR),amorphousselenium,amorphoussil-icon,CCD,orMOSFETtechnology.

STANDARDIZEDRADIATIONEXPOSURE(KSTD)TheairkermaatthedetectorofaDRsystemproducedbyauniformfieldradiationexposureusinganominalradiographickVPandspecificaddedfiltrationresultinginaspecificbeamhalf-valuelayer(HVL).(SeesectionIV.StandardRadiationExposureConditions.)

FOR-PROCESSINGPIXELVALUES(Q)

For-processingpixelvaluesaretheimagepixelvaluesproducedbyaDRsystemafternecessarycorrectionshavebeenappliedtotheinitiallyrecordedrawdatatocompensateforthesetypesofeffects[seeIEC62220-1Ed.1foracompletedescriptionofappropriatecorrectionmethods].[7]Thefollowingcorrectionsmaybeapplied:1.Defectivepixelsmaybereplacedbyappropriatedata.2.Flat-fieldcorrection.3.Correctionforthegainandoffsetofsinglepixels.4.Geometricdistortion.TherelationshipbetweenQandKSTDmayvaryfordifferentDRsystems.ManufacturersareexpectedtoprovideaccesstoQdataandtoprovideinformationonthisrelationshipasapartofnor-malsystemdocumentation.ImageswithQvalueswouldtypicallybeprocessedbytheDRsysteminordertoproduceimagesforpresentation.

NORMALIZEDFOR-PROCESSINGPIXELVALUES(QK)Normalizedfor-processingpixelvalues,QK,arefor-processingpixelvalues,Q,whichhavebeenconvertedtohaveaspecificrelationtoastandardizedradiationexposure(KSTD).Qvaluesarecon-vertedtoQKusingtheDRsystemsrelationshipbetweenQandKSTD.AfterconversionofQtoQK,therelationshipbetweenairkermaattheinputsurfaceofthedetectorandtheQKvalueis

QK=1000*log10KSTDK0whereKSTDisinmicrograyunits,K0=0.001Gy,andKSTDK0.

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FOR-PROCESSINGIMAGEVALUES(QP)For-processingpixelvalues(Q)aretypicallymodifiedbyimageprocessingtoproduceanimagewithvaluessuitablefordisplay(QP).Thisprocessinggenerallydeterminestheusefulvaluesfordisplayandappliesagrayscaletransformation.Theprocessingmayalsoprovidebroadareaequal-ization,edgerestoration,ornoisereduction.

INDICATEDEQUIVALENTAIRKERMA(KIND)Anindicatorofthequantityofradiationthatwasincidentonregionsofthedetectorforeachexpo-suremade.Thevaluereportedmaybecomputedfromthemedianfor-processingpixelvaluesindefinedregionsofanimagethatcorrelatewithanexposuretothedetector.Themedianvalueisthenconvertedtotheairkerma,KSTD,fromastandardizedradiationexposurethatwouldproducethesamedetectorresponse,i.e.,resultinthesamemedianfor-processingsignalvalue,Q,inapre-definedregionofinterest(ROI).Theregionsfromwhichthemedianisdeterminedmaybedefinedindifferentways(seesectionV.AssessmentofIndicatedEquivalentAirKerma,KIND).Thevalueshouldbereportedinmicrograyunitswiththreesignificantfiguresofprecision.

IMAGEVALUESOFINTEREST(VOI)

Pixelvaluesintheoriginalimage(Q)thatcorrespondtotheregionintherecordedimageareaforaparticularbodypartandanatomicalview.KINDmaybecalculatedfromasubsetofpixelswithintheVOI.Notallpixelvaluesinanimageareassociatedwithobjectsthatareofinteresttotheviewerforthepurposesofdiagnosis.Thosethatareofinterestarereferredtoasthevaluesofinterest(orVOI).ThepixelsthatareassociatedwiththeVOIaretypicallyidentifiedbasedontheirphys-icallocationandtheirrelativesignalstrengthcharacteristics.Thisidentificationprocessisreferredtoassegmentation.Specificationofastandardmethodtobeusedforsegmentationisnotwithinthechargeofthistaskgroup.Further,torecommendonestandardmethodaboveallothersmayimpedethedevelopmentofmoresophisticatedmethodsthatyieldmorestableresultsinthefuture.Detectorvaluessuitableforpresentation(QP)aretypicallysenttodisplaydevices(printersorworkstations)orimagearchives.DigitalImagingandCommunicationsinMedicine(DICOM)standards,includingDICOMPS3.14,definetheseaspresentationvalues,orP-values.[8]

TARGETEQUIVALENTAIRKERMAVALUE(KTGT)TheoptimumKINDvaluethatshouldresultfromanyimagewhenthedetectorisproperlyexposed.KTGTvalueswilltypicallybeestablishedbytheuserand/orDRsystemmanufacturerandstoredasatablewithintheDRsystem.ThetableisreferredtointhisdocumentasKTGT(b,v)wherebandvaretableindicesforspecificbodypartsandviews.

DEVIATIONINDEX(DI)

Anindicatorastowhetherthedetectorresponseforaspecificimage,KIND,agreeswithKTGT(b,v).Relativeexposureindicesaretobereportedas:

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KINDDI=10*log10KTGT(b,v)

(2)

withonesignificantdecimalofprecision(i.e.,0.0,0.6,1.3,etc.).DIisintendedasanindi-catorforradiographersandradiologistsforwhetherornotthetechniqueusedtoacquirearadiographwascorrect.ThisdefinitionresultsinaDIof0.0whenthereportedKINDequalsKTGT(aperfectexposure).Theindexchangesby1.0foreach+25%/20%changeofthereportedKIND.

III.RECOMENDATIONSThisreportmakesthefollowingspecificrecommendationsregardingtheindicatorofexposureforDRsystems:ItisrecommendedthatallDRsystems(regardlessofdetectordesign)provideanindicatorofthex-raybeamairkerma(expressedinGy)thatisincidentonthedigitaldetectorandusedtocreatetheradiographicimage.ThisindicatorshallbecalledtheIndicatedEquivalentAirKerma(KIND).ItisfurtherrecommendedthattheDICOMstandardincorporateanewele-mentforDRthatisspecificallydefinedastheIndicatedEquivalentAirKerma.TheindicatorvalueshallbeincludedintheDICOMheaderofeveryimageasafloatingpointvaluewiththreesignificantfigures.InadditiontotheIndicatedEquivalentAirKerma,itisrecommendedthattherelativedevia-tionfromthevaluetargetedbythesystemforaparticularbodypartandviewbereported.Thisindex,theDeviationIndex(DI),shouldbeprominentlydisplayedtotheoperatoroftheDRsystemimmediatelyaftereveryexposureandimmediatelyafteranymodificationofthedetectedimageVOI.ItisfurtherrecommendedthattheDICOMstandardincorporateanewelementforDRthatisspecificallydefinedastheDeviationIndex.ThisindicatorvalueshallbeincludedintheDICOMheaderofeveryimageasasigneddecimalstringvaluebetween9.9and+9.9withonesignificantdigitafterthedecimal.TheIndicatedEquivalentAirKermaandtheDeviationIndexaredeterminedfromtheseg-mentationimagepixels(seesectionV.AssessmentofIndicatedEquivalentAirKerma,KIND).Itisrecommendedthatsystemsprovidedisplayfunctionstodelineatethesegmentedimagepixelsoptionallyasanoverlayontherecordedimagethatisotherwisenormallypre-sentedforapprovalbytheoperator.Additionally,thisoverlayregioncanbeincorporatedinanyimagesexportedforarchiveorviewingusingDICOMservices.DICOMSegmentationStorageSOPClass(Supplement111)formsthebasisforachievingthisfunctionality.[9]Alternatively,thiscouldbeaccomplishedwithoverlayandannotationsthatarepartofGrayScalePresentationStateStorageobjectsdescribedinDICOMSupplement33.[10]Vendorsshouldprovideappropriateanalyticaltools(seesectionXI.RecommendedOptionalFeatures)andallowfor-processingimagedata(Qvalues),orexposurevaluesnor-malizedtothestandardbeamconditions(QK),tobedisplayedandanalyzedonthesystemconsole.ItisalsoimportantthatthevendorallowthesedatatobeexportedinDICOMformatforoff-lineanalysis.Toaccomplishthis,allDRsystemsshouldprovideaccesstoimages

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containingfor-processingpixelvalues,Q.ThiscanbeprovidedbysupportforDICOMexportservicesofDXfor-processingimagescontainingnormalizedfor-processingvalues,QK.Alternatively,imagesofeitherQKorQcanbemadeavailableinDICOMPS3.10[11]for-matonamediastoragedevice.TherelationshipbetweenQKvaluesandthestandardizedradiationexposureincidenttotheDRdetectorisrequiredfortestsofsystemperformance.Itisrecommendedthatthisrelationshipbeprovidedbythesystemmanufactureroverthefullrangeofradiationexpo-suresthatthesystemiscapableofrecording.Fortestsofsystemperformance,itisusefultoviewandanalyzethefor-processingimagevaluesofacquiredtestradiographs.Itisrecommendedthatsystemsprovidefunctionstodisplayimageswithoutimageprocessing(i.e.,Qvalues)andtoreportthemean,median,mode,standarddeviation,andpixelcountofvalueswithingraphicallydefinedregions.Interactivelydrawncircularorrectangularregionsareappropriateforthispurpose.Fortestingofsystems,itisrecommendedthatmanufacturersprovidemethodstoremovetheanti-scattergridwithoutotherwisechangingthedetectorsresponseorprovidegridattenuationfactorstobeusedincalibration.

IV.STANDARDRADIATIONEXPOSURECONDITIONS

A.STANDARDBEAMSPECTRUM

AuniformfieldradiationexposuremadetothedetectorofaDRsystemisusedtoassesstherelationbetweenfor-processingimagevaluesrecordedbythedetector(Q)andthequantityofradiationincidentonthedetector(KSTD).Theradiographictechniqueusedtomaketheexposureisintendedtoprovideabeamqualitytypicalofthatformostexaminationsforwhichthesystemisused.Thisisdonebyusingadditionalfiltrationtoemulatethebeamhardeningofapatient.Thissectionrecommendsstandardizedradiationconditionstobeusedforthispurposeandaddressesonlygeneralradiographicsystems.TheIECandISOhavepreviouslymaderecommendationsforstandardradiationconditionsforuseintestingmedicaldiagnosticx-raysystems.[6,12,13]AvarietyofconditionswithdifferentbeamqualityarerecommendedandlabeledwithRQAprefixes.However,theseconditionsrequirethickfilterscomposedof99.9%aluminum(Al),whichisimpracticalforfieldmeasurements.Alloy1190fallsintothecategoryofscientificgrade(alsocalledultrapurealuminum)andisavailableonlythroughspe-cialtymetalscompaniesforahighprice,insmallquantities,andlimitedform.Alloy1100isa99.0%pureAlalloythatiswidelyavailableonthemarket.Theuseofcopper(Cu)asacomponentoftheaddedfiltrationisrecommendedinordertoreducetheoverallthicknessofaddedmaterial.Inpriorpublications,0.5mmofCuwasfoundtominimizethevariabilityintheresponseofaCRsystemaskVPwasvariedwithin80kVP10%.[14,15]TheadditionofAlmaterialallowsaHVLnearthedesirednominaltobeachievedwhilekeepingtheCuthicknessatavaluethatisreadilyavailablefrommetalfoilsuppliers.TheaddedAlmaterialshouldbeonthebeamexitsurfaceoftheCusothatanyCucharacteristicradiationisabsorbedinthealuminum.Typically,clinicaltubesinuseatmodernfacilitiescontainenoughinherent+addedfiltrationtoexceedtheIECopenbeamHVLspecificationof2.5mmAlat70kVP(RQR5).Ifthisisthecase,thefil-trationtobeaddedtothebeamshouldbereducedtosatisfyRQA5byremovalofallorpartofthealu-

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minum.IftheopenbeamHVLfallsbelowthespecificationof2.5mmAlat70kVP,thefiltrationtobeaddedtothebeamshouldbeincreasedtosatisfyRQA5byadditionofupto4mmAl.Iftheunfilteredbeamisgeneratedusingexoticaddedfiltration,itisrecommendedthataddedfiltrationbereplacedwithenoughaluminumtomeettherequirementsofRQR5.ThekVPmayalsobeadjusted,ifnecessary.Addedfiltrationwith0.5mmCuplus34mmAlissuitableforx-raytubeswithmodestintrinsicfiltration.Foranunfilteredx-raytubespectrawithHVLof2.58mmAlat70kVP(RQR5),computa-tionalsimulations(seeappendicesAandB)indicatethatasimilarbeamqualitywithHVL=6.8mmAlisobtainedusingaddedfiltrationofeither21mmofpurealuminumasspecifiedforRQA5,0.5mmCuplus3mmAl(type1100),or24cmofmuscle.Thefollowingtypesofbrass(30%50%zincwithtracesoftin)arealsoconsideredacceptabletobeusedinplaceofthecopper:admiraltybrass(30%zincand1%tin),alphabrass(lessthan35%zinc),alpha-betabrassorduplexbrass(35%45%zinc),arsenicalbrassorDZRbrass,betabrass(45%50%zinc),cartridgebrass(30%zinc),commonbrassorrivetbrass(37%zinc),highbrass(35%zinc),lowbrass(20%zinc),navalbrass(40%zincbrassand1%tin),redbrass(gunmetal),oryellowbrass(33%zinc).Useofleadedbrass,aluminumbrass,whitebrass,orgildingmetalisnotrecommended.ForthefirsteditionofIEC61267,kVPwastobeadjustedtoachieveadesiredbeamHVL.[11]Forthesecondedition,morestringentconstraintswereplacedonthebeamqualitybeforeaddedfiltrationratherthanallowingkVPadjustments.Asaconsequence,theconditionsrecommendedinthesecondeditionareapplicableonlytolaboratoryfacilities.IthasalsobeenreportedthatCRsystemresponsevariesbyapproximately10%overalimitedarangeofkV.[14]Measurementsmadebytaskgroupmem-bersonavarietyofclinicalsystems(bothCRandDR)indicatethatsystemresponseisveryconsistentwithintherecommendedrangesofHVL,kV,andaddedfiltration(seeappendixB).Systemresponseintermsoffor-processingsignalperunitexposurevariedbylessthanorequalto6%withkVP/filtercom-binationsaslowas67kVPwith0mmAl(5.93mmAlHVL)to73kVPwith8mmAl(8.62mmAlHVL).Similarresultshavebeenfoundbyothersandreportedintheliterature(seeFigure1).[4]ForthislimitedrangeofkV,5%toleranceseemsreasonable.WhileuseoftheRQA5beamisacceptableforcalibration,TG-116recommendsstandardbeamconditionsusingcopperfoilandhighlyavailabletype1100aluminumwithaspecifiedkVPrangewhoseaccuracyhasbeenindependentlyverifiedtobewithin3%oftheindicatedvalue(seeTable1).ThetargetHVLisintendedtobereasonablyclosetoRQA5(6.80.25mmAl).MinoradjustmentsinindicatedkVPandaddedfiltrationarepermittedtoachievethetargetbeamquality.Whilenotrequired,itisacceptabletovarythekVPbyupto5%andtheamountofaddedaluminumwithinthelistedrangetoachieveabeamqualitythatisascloseaspossibletothelistedtargetHVL.ThegeneratorusedforthestandardbeammustbecapableofmaintainingaconstantkVthroughouttheentireexposure.

B.STANDARDBEAMGEOMETRYANDCALIBRATIONVERIFICATIONPROCEDURE

ThissectiondescribesthemeasurementgeometrytobeusedtodetermineKSTDunderthestandardradiationexposureconditionsshowninFigure3.Thestepstousewhenmakingthesemeasurementsaresummarizedbelow.

1.Priortoanymeasurementsverifythatthex-raysourcehasacceptableexposurerepro-ducibility(coefficientofvariation

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