PET的多通道Si制光电倍增管-1270954杨英健

基于多通道Si制光电倍增管前端模拟电路的PET数据采集

BaseonAFront-endAnalogCircuitforMulti-channelSiPMReadoutofPET

杨英健1270954质量湮没现象电子对湮没缺中子核素衰变产生e+介质中e-γ光子,511KeVγ光子,511KeV

keV:1000电子伏特，是为使电子加速通过1000V电压差所需要的能量。正电子核素标记的放射性药物常用的正电子核素11C、13N、15O、18F电子回旋加速器产生短半衰期正电子核素标记的放射性药物肿瘤代谢示踪剂：葡萄糖及氨基酸类似物血流灌注显像剂肿瘤特异性显像剂：单抗及受体显像其他：核酸显像、缺氧显像

放射性核素衰变，质子转变为中子发射正电子，正电子很快与周围的电子相结合，发生湮没现象。符合检测电路利用两个γ射线光子向相反方向传播的这一特性，通过“符合检测”技术来实现投影数据的采集，并由此实现断层成像。重建算法，形成某个断面上放射性同位素的分布图。符合检测电路原理框图检测器滤波

滤除高频噪音脉冲高度分析投影数据成像装置数据采集A、B、C、D四个光电倍增管的输出决定光子的入射点，也决定接下来要进行的脉冲高度分析。A、B、C、D四个光电倍增管的输出分别是IA、

IB、IC、ID。脉冲高度分析脉冲过高可能是由于多个γ射线光子同时到达。脉冲过低可能是由于入射光子同时到达检测器前经过了散射。PET系统中，有效的入射光子的能量是511keV为中心，大致在350~650keV的范围内。数据存储湮没事件发生的位置、响应线的位置都具有不确定性。PET系统中数据的存储只能一个一个事件进行。检测器发生符合事件的位置湮没时间发生的位置我们知道的只是符合检测器的位置，并由此判断出在这两个检测器连线的位置上发生了湮没时间，不能判断湮没事件发生的准确位置。采样数据的正弦图的表示方法有关在特定响应曲线上发生的湮没事件可以在正弦图上记录下来。只要把每次发生的湮没事件都以累加的方式在正弦图上记录下来，待数据相对完整后，就可以现实断层图像的重建。衰减校正在图像重建之前要对数据进行衰减校正衰减校正是为了准确地确定放射性核素在人体内的密度分布。人体组织的传播距离PET衰减校正示意图μ1和μ2为传播路径上人体组织的平均衰减系数。在上图中，射线在人体组织中衰减分别为。湮没时间发生后相对的两个光子到达检测器B和C在等效图中，射线在人体组织中衰减等效为。为了准确进行衰减校正，可用体外辐射源绕人体旋转一周，用透射的方法测出响应路径上的衰减，然后在图像重建之前对数据进行衰减校正。Photomultiplier:transferslighttoelektricalsignal(photoemissivesensor)Siliconphotomultipliers(SiPM)isregardedasapromisingdeviceinmanyphotoncountingapplicationsincludinghigh-energyphysics,astroparticlephysics,andmedicalimaging.

SiPMsaresuitableforsignalreadoutschemesusingblockdetectorsinPET.Toconstructblockdetectors,anumberofsingle-channelSiPMswerearrangedinarectangulararraywithrelativelylargedeadspace.Specialframesormaterialsrequiredforfixingthemaretroublesomeintheconstructionofarraysandextensionofthedetectionarea.Onthecontrary,therecentdevelopmentoftileablemulti-channelSiPMsenablestheeasyconstructionofblockdetectors.

Moreover,themulti-channelSiPMshavearelativelysmalldeadspacebetweeneachchannel.Weemployedthesemulti-channelSiPMsforthedevelopmentofMR-compatiblePETblockdetectorswithandwithouttheuseofshortopticalfibersbetweenscintillationcrystalsandSiPMs.Weappliedthesamefront-endreadoutmodulesformulti-channelSiPMsalthoughthenumbersofSiPMchannelsweredifferentineachdetectorconfiguration.wethereforepresentthedetaileddesignschemeofthisfront-endreadoutmodulebasedonthechargedivisionnetworkthatwasemployedfortheeasyextensionofthemoduletoawiderdetectionarea.ThedetectorconfigurationandphysicalperformanceofvariousdetectorsdevelopedforsmallanimalPET/MR,opticalfiberPET/MR,anddoublelayerdepthofinteraction(DOI)PETusingthisfront-endreadoutmodulewillbepresented.Inthissituation,averysophisticatedandcomplicatedsystemisnecessarytoprocessthislargenumberofsignalsindividually.Furthermore,thelargenumberofsignallineshassomepotentialriskinsimultaneousPET/MRapplicationsbecauseitwouldrequireverycarefulshieldingforthelargenumberofsignalcablestoreduceRFinterferencebetweenPETandMRI.Electronicsforsignalmultiplexinganddataacquisition

WedesignedtheSiPMfront-endreadoutmodulewithwhichthefourtileable4X4channelSiPMscanbecombined(Fig.1).Thismeansthatthedetectormoduleneedsatleast64signallinesforasinglesignalreadoutschemeor128signallinesforadifferentialsignalreadoutschemeifnosignalmultiplexingisinvolved.

Inthissituation,averysophisticatedandcomplicatedsystem(i.e.ASIC)isnecessarytoprocessthislargenumberofsignalsindividually.Furthermore,thelargenumberofsignallineshassomepotentialriskinsimultaneousPET/MRapplicationsbecauseitwouldrequireverycarefulshieldingforthelargenumberofsignalcablestoreduceRFinterferencebetweenPETandMRI.Photomultiplier:transferslighttoelektricalsignal(photoemissivesensor)Fig.1.Schematicofthefront-endanalogcircuitfortheSiPMreadoutmodule.(a)SiPMbiasingcircuit.(b)Resistivechargedivisionnetwork(RCN).(c)Differentialamplifiercircuit.

成像装置DataAcquisitionA、B、C、D四个光电倍增管的输出(fourpositionencodingsignals)决定光子的入射点，也决定接下来要进行的脉冲高度分析。A、B、C、D四个光电倍增管的输出分别是IA、IB、IC、ID。Toverifywhetherthisfront-endreadoutmodulewassuitableforSiPMs,electriccircuitsimulationwasperformed.Wemultiplexedthesignalsfromthe64SiPMchannelstofourpositionencodingsignals(A,B,C,andD)usingaresistivechargedivisionnetwork(RCN)(Fig.1(b)).Thisfront-endreadoutmodulebasedonRCNmakesthePETblockdetectoreasilyextendable.Fig.2(a)showstheoutputpulsesmeasuredatpositionsA–DwhenonlytheSiPMcellnearesttoAwasfired.TheSiPMcellpositionsweresuccessfullyseparatedusingthesefourmultiplexedpositionsignalsasillustratedinFig.2(b).Fig.2.ElectricalstimulationresultsoftheSiPMblockdetector.(a)OutputpulsesmeasuredatpositionsA–DwhenonlytheSiPMcellnearesttoAwasfired.(b)Positionmapof64channelsobtainedusingthedecodingschemeshowninEqs.(1)and(2).

ThissimulationwasonlyforSiPMcellsandanalogfront-endcircuitswithoutconsiderationofscintillation,lightlossandothereffectsbecausethelinearityofXandYposi