The dual-gain mode a way to enhance the dynamic ra

arXiv:physics/0607021v1[physics.med-ph]4Jul2006Thedual-gainmode:awaytoenhancethedynamicrangeofX-raydetectorsEvangelosMatsinosandWolfgangKaisslVarianMedicalSystemsImagingLaboratoryGmbH,T¨afernstrasse7,CH-5405Baden-D¨attwil,SwitzerlandE-mail:[e-mail]@varian.comAbstract.VarianMedicalSystemshasmanufacturedandrecentlyputintooperationaclinically-applicablesolutionforimage-guidedradiationtherapy.Cone-beamCTimaging,oneoftheoperationmodesoftheimagingunitofthisdevice,aimsathigh-qualityvolumetricreconstruction.Toboosttheimagequality,thedual-gainmode,asuccessfulmeansforenhancingthedynamicrangeoftheflat-paneldetectorsandobtainingbetterresultsinthecontrastofthereconstructedimage,wasdevelopedandsuccessfullytestedduringthelastfewyears.TheimportantstepsinthecalibrationofthismodeinvolveacorrectiontothepulsewidthsassociatedwiththeX-rayproduction,theassessmentofthedetectorsignalabovewhichnonlineareffectsbecomesignificantandthedeterminationofsomepropertiesofthedetectorpixels,namely,ofdarkfields,flatnesscorrections,etc.Finally,thedefect-pixelmapisobtained,containingdeadandflickeringpixels,aswellaspixelswithpropertieswhicharesufficiently‘outofrange’.Aneffectobservedintheoffsetcorrectionofrawimagesseemstooriginateinthemismatchbetweenthemeasuredandextrapolatedvaluesofthedarkfield;acorrectionschemeisproposedtotakeaccountofthiseffect.Inthedataprocessing,whichisachievedonthebasisofthefilesproducedinthecalibrationofthemode,thehigh-gainsignalisusedwhenevermeaningful;otherwise,itissubstitutedbythelow-gainsignal,properlyscaled.Defectpixelsareinterpolatedfromtheirgoodneighbours;aprocedureachievingacorrectionincaseofsmalldefect-pixelclustersisoutlined.PACSnumbers:87.57.Ce,85.57.Nk,87.59.FmKeywords:cone-beamCT,flat-paneldetector,dualgain,imagequality1.IntroductionThedetailedknowledgeconcerningthepositionofthetumourandofthevitalorgansduringthetreatmenthaslongbeenoneofthemostcrucialanddemandingproblemsinradiationtherapy.Disregardingpatient-localisationuncertainties,onemayidentifytwotimescalesinrelationtoorganmotion:onepertainingtodailyvariations(interfractionmotion),theothertotherespirationcycle(intrafractionmotion).Thesetwoscalesaffectdifferentlythevariousregionswithinthehumananatomy.Thedual-gainmodeinCTimaging2Toovercometheabsenceofamonitoringprocess,whichwouldbepracticalenoughforeverydayapplication,theoncologistsaddamarginaroundthetargetinordertomaximisetheprobabilityofdestroyingtheentiretumourbytheendofthetreatment.Theinevitableconsequenceofincreasingthevolumeoftheregionwhichreceivesthelargestradiationimpactisthedestructionofhealthytissue.Aimingatincreasingtheprecisioninradiationtherapy,VarianMedicalSystems,Inc.(VMS),PaloAlto,CA,hasrecentlydeployedanumberofnewproducts.Inautumn2004,theVMSClinacaccelerator,equippedwithimagingfunctionality‡,the‘On-BoardImager’(OBI),cameintooperation(figure1).Asthisdeviceiscapableofbothtrackingandtargetingtumours,itrepresentsaclinically-applicablesolutionforimage-guidedradiationtherapy.ThetwomaincomponentsoftheOBIsystemarethelow-energy(kV)X-raytube(simplycalled‘tube’hereupon)andtheflat-paneldetector(FPD);bothareattachedtothebodyofthedeviceviaasystemofroboticarmsenabling2D(forthetube)or3D(forthedetector)movement.Thedetectorwhichiscurrentlyinusemaybeoperatedinfluoroscopic(real-timemovingimages,tobeusedforpositionandverification)orradiographicmode(high-resolutionimages,tobeusedfordiagnosisandplanning).Theavailabilityofreliablelarge-areaFPDsatthecommerciallevelhasbeenacrucialfactorinthedevelopmentofthehigh-resolutionvolumetricreconstruction.Thetransitionfromtraditionalcomputedtomography(CT)tomodernimagingmodalitiesiswellunderway.Incircularcone-beamCT(CBCT)imaging,thealgorithmofFeldkampetal(1984)isagoodstartingpointforfurtherinvestigation;variantsofthisalgorithmhaveappearedintheliterature,e.g.,seeGrassetal(2000)andTangetal(2005).Theproblemofthereconstructionincaseofahelicalsource-detectortrajectoryhasbeenstudiedforanumberofyears,e.g.,seeFuchsetal(2000)andthecorrespondingreferencesinGrassetal(2000).Newtechniques,aimingatimprovingtheimagequalitybysuppressingthenoise,havesurfacedwiththepapersofPan(1999),Kachelrießetal(2001),andPanandYu(2003).Finally,renewedinterestin‘1800plus’reconstructionswastriggeredwiththeworksofYuandPan(2003),andZouetal(2005).Toyieldreliableinformation,theimagingunitmustbeproperlycalibrated;thecalibrationsmaybeclassifiedasgeneralandmode-specific.Thegeneralcalibrationsincludethegeometriccalibration(yieldingthevaluesofimportantspatialparameters,e.g.,ofthesource-axisandsource-detectordistances,aswellasthecorrectionsduetothemechanicalimperfection),theI0calibration(yieldingtheopen-geometrysignal-theso-called‘aircounts’-asafunctionofthepositiononthedetector),thenorm-phantomcalibration(yieldingimagesofanormphantom,thatis,ofacylindricalobjectofknowndensity,whichmaysubsequentlybeusedasreferenceinthereconstructionofotherobjects),thecalibrationforthebeam-hardeningcorrection§,etc.Themode-‡Theideaofintegratingtheimaginganddeliveryunitshadbeenproposedalmostonedecadeagoasameanstofacilitatethede