基于多通道Si制光電倍增管前端模擬電路的PET數據采集

BaseonAFront-endAnalogCircuitforMulti-channelSiPMReadoutofPET

楊英健1270954質量湮沒現象電子對湮沒缺中子核素衰變產生e+介質中e-γ光子,511KeVγ光子,511KeV

keV:1000電子伏特，是為使電子加速通過1000V電壓差所需要的能量。正電子核素標記的放射性藥物常用的正電子核素11C、13N、15O、18F電子回旋加速器產生短半衰期正電子核素標記的放射性藥物腫瘤代謝示蹤劑：葡萄糖及氨基酸類似物血流灌注顯像劑腫瘤特異性顯像劑：單抗及受體顯像其他：核酸顯像、缺氧顯像

放射性核素衰變，質子轉變為中子發射正電子，正電子很快與周圍的電子相結合，發生湮沒現象。符合檢測電路利用兩個γ射線光子向相反方向傳播的這一特性，通過“符合檢測”技術來實現投影數據的采集，并由此實現斷層成像。重建算法，形成某個斷面上放射性同位素的分布圖。符合檢測電路原理框圖檢測器濾波

濾除高頻噪音脈沖高度分析投影數據成像裝置數據采集A、B、C、D四個光電倍增管的輸出決定光子的入射點，也決定接下來要進行的脈沖高度分析。A、B、C、D四個光電倍增管的輸出分別是IA、

IB、IC、ID。脈沖高度分析脈沖過高可能是由于多個γ射線光子同時到達。脈沖過低可能是由于入射光子同時到達檢測器前經過了散射。PET系統中，有效的入射光子的能量是511keV為中心，大致在350~650keV的范圍內。數據存儲湮沒事件發生的位置、響應線的位置都具有不確定性。PET系統中數據的存儲只能一個一個事件進行。檢測器發生符合事件的位置湮沒時間發生的位置我們知道的只是符合檢測器的位置，并由此判斷出在這兩個檢測器連線的位置上發生了湮沒時間，不能判斷湮沒事件發生的準確位置。采樣數據的正弦圖的表示方法有關在特定響應曲線上發生的湮沒事件可以在正弦圖上記錄下來。只要把每次發生的湮沒事件都以累加的方式在正弦圖上記錄下來，待數據相對完整后，就可以現實斷層圖像的重建。衰減校正在圖像重建之前要對數據進行衰減校正衰減校正是為了準確地確定放射性核素在人體內的密度分布。人體組織的傳播距離PET衰減校正示意圖μ1和μ2為傳播路徑上人體組織的平均衰減系數。在上圖中，射線在人體組織中衰減分別為。湮沒時間發生后相對的兩個光子到達檢測器B和C在等效圖中，射線在人體組織中衰減等效為。為了準確進行衰減校正，可用體外輻射源繞人體旋轉一周，用透射的方法測出響應路徑上的衰減，然后在圖像重建之前對數據進行衰減校正。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.

成像裝置DataAcquisitionA、B、C、D四個光電倍增管的輸出(fourpositionencodingsignals)決定光子的入射點，也決定接下來要進行的脈沖高度分析。A、B、C、D四個光電倍增管的輸出分別是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