CT-Physics--CT研发者必备

CHAPTER19I.BASICCONCEPTSCTPHYSICSMahadevappaMahesh,MS,PhD1.WhatisCT?CTisamethodforacquiringandreconstructinganimageofathincrosssectionofanobject.Itisbasedonmeasurementsofx-rayattenuationthroughthesectionusingmanydifferentprojec-tions.Thisisachievedbyrotatingbothx-raytubeanddetectorsaroundthepatient.2.HowisCTdifferentfromconventionalradiographs?CTdiffersfromconventionalradiographyintwosignificantways:■CTformsacross-sectionalimage,eliminatingthesuperimpositionofstructuresthatoccursinplainfilmimagingbecauseofcompressionofthree-dimensional(3D)bodystructuresontothetwo-dimensionalrecordingsystem.■ThesensitivityofCTtosubtledifferencesinx-rayattenuationisatleastafactorof10higherthannormallyachievedbyfilmscreenrecordingsystemsbecauseofthevirtualeliminationofscatter.3.WhatarethebasicprinciplesofCT?FundamentallyaCTscannermakesmanymeasurementsofattenuationthroughtheplaneofafinitethicknesscrosssectionofthebody.Thesystemusesthesedatatoreconstructadigitalimageofthecrosssectioninwhicheachpixelintheimagerepresentsameasurementofthemeanattenuationofabox-likeelement(voxel)extendingthroughthethicknessofthesection.Anattenuationmeasurementquantifiesthefractionofradiationremovedinpassingthroughagivenamountofaspecificmaterialofthickness∆x,asshowninFig.1-1.Attenuationisexpressedas:It=Ioe−µ∆xwhere,ItandIoarethex-rayintensitiesmeasuredwithandwithoutthematerialinthex-raybeampath,respectively,andµisthelinearattenuationcoefficientofthespecificmaterial(seeFig.1-1).Theimagereconstructionprocess,suchasthefilteredback-projectionmethodandmanyothermethods,areappliedtoderivetheaverageattenuationcoefficient(µ)valuesforeachvoxelinthecrosssection,usingmanyraysfrommanydifferentrotationalanglesaroundthecrosssection.4.HowisCTsignalproduced?ACTsignalresultsfromtissuediscriminationbasedonthevariationsinattenuationbetween“voxels,”whichdependsondifferencesinvoxeldensityandatomicnumberofelementspresentandisinfluencedbythedetectedmeanphotonenergy.5.WhatisaCTimage?ACTimageasshownlateriscomposedofpixels(pictureelements).Eachpixelontheimagerepresentstheaveragex-rayattenuationinasmallvolume(voxel)thatextendsthroughthetis-suesection.InFig.1-2,thepixelsizeisexaggerated.Inaddition,inarealCTimage,alltissueswithinasinglepixelwouldbethesameshadeofgray.A034047-Ch01.qxd5/18/0612:56AMPage96.Whatispitch?Theconceptof“pitch”wasintroductionwiththearrivalofhelicalorspiralCTscanners.Pitchisdefinedastheratiooftabletravelpergantryrotationtothex-raybeamwidth.Pitch=I/WwhereIistablefeedpergantryrotation(mm/rotation)andWisx-raybeamwidth(mm).Accordingly,■Pitch=1:Impliescontiguousslicesimilartoconventionalstep-and-shootscan,forexample:10-mmslicethick-nesswith10-mmsliceinterval■Pitch1:Impliesextendedimagingandreducedpatientdosewithloweraxialspatialresolution■Pitch1:ImpliesoverlappingandhigherpatientdosewithhigheraxialspatialresolutionWithsingle-rowdetectorCT(SDCT)scanners,theconceptofpitchisstraightforward.Withmultiple-rowdetectorCT(MDCT),theconceptofpitchwasmuddledwiththeintroductionoftwodifferentdefinitionsforpitchwithfollowingvariations:■Beampitchorcollimatorpitchsimilartothepreviouslydefinedpitch:Typicalvaluesare0.75,1,1.25,....■Detectorpitch:RatiooftablefeedpergantryrotationtowidthofsingleDASchannelwidth.Typicalvaluesare4,6,12,18,....However,withrecentinternationalagreement,pitchisagaindefinedassimplyratiooftabletraveltototalx-raybeamwidth.AdoptingtheusageofcommondefinitionofpitchwouldbeapplicableequallytobothSDCTandMDCTandeliminatesconfusionexistingbetweentherelationshipofradiationdoseandvariousdefinitionsofpitch(Fig.1-3).CTPHYSICS10Figure1-2.ACTimageiscomposedofpixels.Eachpixelontheimagerepresentsthex-rayattenuationinasmallvolume(voxel)thatextendsthroughthetissuesection.w=sectionwidth.Figure1-1.Anattenuationmeasurementquantifiesthefractionofradiationremovedinpassingthroughagivenamountofaspecificmaterialofthickness∆x.A034047-Ch01.qxd5/18/0612:56AMPage107.Whatis“fieldofview”(FOV)inCT?TheFOVinCTistheareaofscanregionthatisincludedintheimagereconstruction.TherearetwotypesofFOV:scanFOV(SFOV)anddisplayFOV(DFOV).SFOVistheregionwithinthegantryopening,theanatomythatisincludedinthereconstruction.SFOVislessthanthephysicalopeningoftheCTgantry,whichisthereasonwhypartoftheanatomyiscutoffinscanninglargerpatients.Ontheotherhand,DFOVisareaofreconstructedimagethatcanbedisplayed.SmallerDFOVresultsinlargerimagesize.TheSFOVinfluencesthephysicaldimensionsofimagepixel.A10-cmFOVina512×512matrixresultsinpixeldimensionsofapproximately0.2mm,anda35-cmFOVproducespixelwidthsofabout0.7mm(Fig.1-4).CTPHYSICS11Figure1-3.Withrecentinternationalagreement,pitchisagaindefinedassimplyratiooftabletraveltototalx-raybeamwidth.I=tabletravel(mm)/rotation,W=beamwidth(mm),T=singledataacquisitionchannelwidth(mm),N=numberofactivedataacquisitionchannels.Figure1-4.FieldofviewinCTscan.A034047-Ch01.qxd5/18/0612:56AMPage118.Whatdoesgantryrotationspeedmean?ItisthespeedatwhichtheCTgantryrotatesoncearoundthepatient.Itisoftencalled“scantime.”Scantimeisconsistentlydecreasingwithincreasingdemandforhighertemporalresolu-tion.Scantimeof