基于多通道Si制光電倍增管前端模擬電路的PET數(shù)據(jù)采集

BaseonAFront-endAnalogCircuitforMulti-channelSiPMReadoutofPET

楊英健1270954質(zhì)量湮沒現(xiàn)象電子對(duì)湮沒缺中子核素衰變產(chǎn)生e+介質(zhì)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(biāo)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(chǎn)生短半衰期正電子核素標(biāo)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(zhì)子轉(zhuǎn)變?yōu)橹凶影l(fā)射正電子，正電子很快與周圍的電子相結(jié)合，發(fā)生湮沒現(xiàn)象。符合檢測電路利用兩個(gè)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(shù)來實(shí)現(xiàn)投影數(shù)據(jù)的采集，并由此實(shí)現(xiàn)斷層成像。重建算法，形成某個(gè)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(shù)據(jù)成像裝置數(shù)據(jù)采集A、B、C、D四個(gè)光電倍增管的輸出決定光子的入射點(diǎn)，也決定接下來要進(jìn)行的脈沖高度分析。A、B、C、D四個(gè)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(gè)γ射線光子同時(shí)到達(dá)。脈沖過低可能是由于入射光子同時(shí)到達(dá)檢測器前經(jīng)過了散射。PET系統(tǒng)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(nèi)。數(shù)據(jù)存儲(chǔ)湮沒事件發(fā)生的位置、響應(yīng)線的位置都具有不確定性。PET系統(tǒng)中數(shù)據(jù)的存儲(chǔ)只能一個(gè)一個(gè)事件進(jìn)行。檢測器發(fā)生符合事件的位置湮沒時(shí)間發(fā)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(gè)檢測器連線的位置上發(fā)生了湮沒時(shí)間，不能判斷湮沒事件發(fā)生的準(zhǔn)確位置。采樣數(shù)據(jù)的正弦圖的表示方法有關(guān)在特定響應(yīng)曲線上發(fā)生的湮沒事件可以在正弦圖上記錄下來。只要把每次發(fā)生的湮沒事件都以累加的方式在正弦圖上記錄下來，待數(shù)據(jù)相對(duì)完整后，就可以現(xiàn)實(shí)斷層圖像的重建。衰減校正在圖像重建之前要對(duì)數(shù)據(jù)進(jìn)行衰減校正衰減校正是為了準(zhǔn)確地確定放射性核素在人體內(nèi)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(shù)。在上圖中，射線在人體組織中衰減分別為。湮沒時(shí)間發(fā)生后相對(duì)的兩個(gè)光子到達(dá)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(zhǔn)確進(jìn)行衰減校正，可用體外輻射源繞人體旋轉(zhuǎn)一周，用透射的方法測出響應(yīng)路徑上的衰減，然后在圖像重建之前對(duì)數(shù)據(jù)進(jìn)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(gè)光電倍增管的輸出(fourpositionencodingsignals)決定光子的入射點(diǎn)，也決定接下來要進(jìn)行的脈沖高度分析。A、B、C、D四個(gè)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