基于FPGA前端电子正电子发射(英文版)

FPGA-BasedFront-EndElectronicsforPositronEmissionTomographyMichaelHaselman1,DonDeWitt1,WendyMcDougald2,ThomasK.Lewellen2,RobertMiyaoka2,andScottHauck1MichaelHaselman:[e-mail];DonDeWitt:[e-mail];WendyMcDougald:[e-mail];ThomasK.Lewellen:[e-mail];RobertMiyaoka:[e-mail];ScottHauck:[e-mail]1DepartmentofElectricalEngineering,UniversityofWashington,Seattle,WA2DepartmentofRadiology,UniversityofWashington,Seattle,WAAbstractModernFieldProgrammableGateArrays(FPGAs)arecapableofperformingcomplexdiscretesignalprocessingalgorithmswithclockratesabove100MHz.ThiscombinedwithFPGA’slowexpense,easeofuse,andselecteddedicatedhardwaremakethemanidealtechnologyforadataacquisitionsystemforpositronemissiontomography(PET)scanners.Ourlaboratoryisproducingahigh-resolution,small-animalPETscannerthatutilizesFPGAsasthecoreofthefront-endelectronics.Forthisnextgenerationscanner,functionsthataretypicallyperformedindedicatedcircuits,oroffline,arebeingmigratedtotheFPGA.Thiswillnotonlysimplifytheelectronics,butthefeaturesofmodernFPGAscanbeutilizestoaddsignificantsignalprocessingpowertoproducehigherresolutionimages.Inthispapertwosuchprocesses,sub-clockratepulsetimingandeventlocalization,willbediscussedindetail.WeshowthattimingperformedintheFPGAcanachievearesolutionthatissuitableforsmall-animalscanners,andwilloutperformtheanalogversiongivenalowenoughsamplingperiodfortheADC.Wewillalsoshowthatthepositionofeventsinthescannercanbedeterminedinrealtimeusingastatisticalpositioningbasedalgorithm.KeywordsPET;FPGA;DigitalSignalProcessing;PulseTiming;EventLocalization1.IntroductionTheabilitytoproduceimagesoftheinsideofalivingorganismwithoutinvasivesurgeryhasbeenamajoradvancementinmedicineoverthelast100years.ImagingtechniquessuchasX-raycomputertomography(CT)andmagneticresonanceimaging(MRI)havegivendoctorsandscientiststheabilitytoviewhigh-resolutionimagesoftheanatomicalstructuresinsidethebody.Whilethishasledtoadvancementsindiseasediagnosisandtreatment,thereisalargesetofdiseasesthatonlymanifestaschangesinanatomicalstructureinthelatestagesofthedisease,orneveratall.Thishasgivenrisetoanotherbranchofmedicalimaging,calledfunctionalimaging,whichisabletocapturecertainmetabolicactivitiesinsidealivingbody.Positronemissiontomography(PET)isthemostadvancedformofthisimagingmodality.Traditionally,thefront-endelectronicsofPETscannershaveconsistedofmanydiscretepartsthatperformthenecessarydataacquisitionandpulseprocessingfunctions.TherearecurrenteffortstoutilizeFPGAsforallormostofthedataacquisition[3,4]becausetheyNIHPublicAccessAuthorManuscriptFPGA.Authormanuscript;availableinPMC2011September27.Publishedinfinaleditedformas:FPGA.2009February22;2009(7):93–102.doi:10.1145/1508128.1508143.NIH-PAAuthorManuscriptNIH-PAAuthorManuscriptNIH-PAAuthorManuscriptprovidethenecessaryspeedtoprocesstheincomingdatawithoutthecomplexityofanapplication-specificcircuit.TechnologyadvanceshavealsoyieldedFPGAswiththelogiccapacitytofitallofthenecessarytaskaswellasaddingtasksthatwherenotpossiblebefore.MostcurrentscannersusetheFPGAstocollectdataanddosomesimplefiltering.ThispaperdiscussesouruseofFPGAsinaPETscannerandhowweutilizethecapabilitiesofmodernFPGAstohelpproducehigherresolutionimages.2.PositronEmissionTomographyPETisamedicalimagingmodalitythatusesradioactivedecaystomeasurecertainmetabolicactivitiesinsidelivingorganisms.Itdoesthisthroughthreemaincomponents(seeFigure1).Thefirststepistogenerateandadministertheradioactivetracer.Atracerismadeupofaradioactiveisotopeattachedtoametabolicallyactivemolecule.Thetraceristheninjectedintothebodytobescanned.Afterenoughtimehaslapsedforthetracertodistributeandconcentrateincertaintissues,thesubjectisplacedinsidethescanner.TheradioactivedecayeventfortracersusedinPETstudiesproducespositrons.Thepositronswilltravelashortdistanceintissue(asillustratedinFigure2)andeventuallyannihilatewithanelectrontoproducetwo511KeVantiparallelphotons.Thesephotonsarewhatthescannercaptures.Thescanner,thesecondcomponentofPET,consistsofaringofsensorsthatdetectthephotonsandelectronicsthatprocessthesignalsarisingfromthesensors.Thesensorsaremadeupofscintillatorcrystalsandphotodetectors.Thescintillatorconvertsthe511KeVphotonintomanyvisiblelightphotons,andthephotodetectorconvertsthevisiblelightintoanelectricalpulse.Thesepulsesareprocessedbythefront-endelectronicstodeterminetheparametersofeachpulse(i.e.energy,timing).Finally,thedataissenttoahostcomputerthatperformstomographicimagereconstructiontoturnthedataintoa3-Dimage.2.1RadiopharmaceuticalThefirststeptoproduceaPETscanistogeneratearadiopharmaceutical(oftencalledatracer).Tosynthesizethetracer,ashort-livedradioactiveisotopeisattachedtoametabolicallyactivemolecule.Theshorthalf-lifereducestheexposuretoionizingradiation,butitalsomeansthatthetracerhastobeproducedclosetothescanners,asitdoesnotstoreforverylong.Themostcommonlyusedtracerisfluorine-18flourodeoxyglucose([F-18]FDG).[F-18]FDGisananalogofglucosethathasahalf-lifeof110minutes.[F-18]FDGissimilarenoughtogl