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外文资料-- Simulation Study of Electrode System Optimization in Electrical Impedance Tomography.PDF

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外文资料-- Simulation Study of Electrode System Optimization in Electrical Impedance Tomography.PDF

CorrespondingauthorShaHong.emailsha_hong_2000yahoo.comSimulationStudyofElectrodeSystemOptimizationinElectricalImpedanceTomographyWangYan,RenChaoshi,ZhaoShu,WangLei,ShaHongInstituteofBiomedicalEngineeringChineseAcademyofMedicalSciencesPekingUnionMedicalCollegeTianjin,China.E-mailsarahyky163.cmAbstractElectrodeliesinthefrontoftheElectricalimpedancetomographyEITsystemandcontactswithhumanbodydirectly.ItisoneofthemostsensitiveandthepivotalpartsofEITsystem.Thetypeandtheconfigurationofelectrodesysteminfluencethereconstructionimagegreatly,especiallyforthecentralareaofbody,wherethemeasurementsensitivityisverylow.Asimulationplatformisdevelopedtooptimizetheelectrodestructureparameterbasedonthecoerciveequipotentialnodesmodelfortheimagingobjectofthedeepareaofbody.Theinfluencesonthemeasurementsensitivityandtheimagereconstructionqualityhavebeenstudiedusingdifferentelectrodetypesandconfigurations.Thesimulationresultssuggestthatoptimalselectionofelectrodetypesandconfigurationscanhelpfullyincreasetheimagequalityandmeasurementsensitivityoftheimageespeciallyincentralarea.SeveraltypeandconfigurationparameterofEITelectrodethatfitforcentralobjectimagingarealsopresented.Keywords-EIT;electrodesystem;reconstructionquality;measurementsensitivity;simulationevaluationI.0BINTRODUCTIONEITisoneoftheimportantresearchtopicsintoday’sbiomedicalengineering[1]-[3].EITelectrodearrayisoneofthemostsensitiveandthepivotalpartsofEITsystem.UsuallytenormoreelectrodesareusedinEITsystem.TheelectrodearrayliesinthefrontoftheEITsystemandcontactswithhumanbodydirectly.Thestructureandperformanceoftheelectrodesystemgreatlyinfluencetheoriginalinformationextraction,realtimepropertyofthesystem,imageresolvingpower,andespeciallytheinformationextractionfromcentralarea,wheretheEITmeasurementsensitivityispoor.Intheearlyoptimumstudyofelectrodeconfiguration,itiswidelyacceptedthatcurrentexcitationelectrodesshouldbeaslargeaspossiblewhilethevoltagemeasuringelectrodesarerequiredassmallaspossible[4].PingHua[5]studiedtheinfluenceofcontactimpedancebetweenelectrodeandskin,andshowedthatthebesteffectcanbeobtainedwhenthetotalwidthofelectrodearrayisabout80-90oftheimagedarea’sperimeter.E.J.Woo[6]usedannularcurrentandvoltageelectrodescovering90ofthesurface.P.A.T.Pinheiro[7]reportedthattheoptimumareaunderneaththeelectrodesis80oftheoverallboundaryareafortheadjacentpatternsand60fortheoppositeones.WangChaoetal[8][9]carriedonthesimulationresearchontheinfluenceofelectrodewidthusingthecoerciveequipotentialnodefiniteelementmethodFEMmodel,andshowedthatthebesteffectwasobtainedwhentheoverlayrateofelectrodesis57.1.AlthoughcontinuousprogressesonthechoiceofelectrodestructureparameterinEIThavebeenmade,thecompleteandsatisfactoryresulthasnotbeenseensofar.Inpractice,thedesignofEITelectrodestructureandparameterisstillbasedonexperience.AmongEITelectrodes,thecompoundelectrodehasmorecomplexstructure.FourparametersofthecompoundelectroderestricteachotherandtheirinfluenceonEITsystemaremuchcomplex.Theinfluencesofallparametersshouldbeconsideredcomprehensively.Usingonlytheoverlayrateofelectrodesisnotagoodwaytodealwiththecompoundelectrodeoptimizingdesign.InEITmeasurement,thecentralarea,correspondingtodeepareaofbody,isoftentheregionofinterest.Butthemeasurementsensitivityofthecentralareaisrelativelylowerandtheimagequalityisrelativelypoor.HowtoraisethemeasurementsensitivityandimprovetheimagequalityofthecentralareaisakeyprobleminEITmeasurement.TheexcitationpatternisalsoanimportfactorinEITmeasurement.Adjacentexcitationhasmoreindependentmeasurenumber.Oppositeexcitationpatternhaslessindependentmeasure,butthefielddistributionismorereasonablethanthatinadjacentpattern.Inthispaper,asimulationstudyofEITelectrodestructureandparameterdesignaimingattheimagingobjectofcentralareaispresented.Usingthecoerciveequipotentialnodemodelforcompoundelectrode[10],asimulationplatformisdevelopedtocarryouttheoptimizingdesignoftheelectrodesystem.Basedonthemodelandtheplatform,theinfluencesoftheelectrodenumber,type,configuration,andexcitationpatternoftheelectrodesystemonthemeasurementsensitivityandtheimagereconstructionqualityhavebeenstudied.ThesimulationstudyofEITimagingwithdifferentdepthsofimagingobjecthasbeenmadewithequipotentialbackprojectionalgorithm.978-1-4244-4713-8/10/25.002010IEEEII.1BMETHODA.4BFiniteElementModelforComplexElectrodesElectrodeplatesinroundorrectangleshapeareusuallyusedinEIT.Theprojectionsofbothelectrodesonthetomogramplanearealineandthelengthofthelineequalstothewidthoftheelectrodes,calledlineelectrode,asshownin1and2ofFig.1.Forthecompoundelectrodewithacomplexstructure,theprojectiononthetomogramplaneconsistsof3lines,asshownin3and4ofFig.1.Theexcitationelectrodeconsistsoftwolines,jinFig.1,ontheleftandtherightrespectively,andthemiddleline,cinFig.1,representsthemeasurementelectrode.Thereare4structureparametersforthecompoundelectrodeFig.1,theexcitationelectrodewidthj,themeasurementelectrodewidthc,thespacebetweentheexcitationelectrodeandthemeasurementelectrodeb,andthedistancebetweentwoadjacentcompoundelectrodesa.EachofthemispresentedbythecountofFEMnodes.ForanEITelectrodesystemwithcertainboundarysuchas256nodesinthisstudy,whentheelectrodenumbers32hereandelementsdividedinFEM1210herearedetermined,thesumofelectrodewideandthespacebetweenadjacentelectrodesisfixedtoo8here.Figure1.ThelineelectrodeandcompoundelectrodeinEITAfiniteelementmethodFEMofthecompoundelectrodeusingcoerciveequipotentialnodemodelhasusedtostudytheEITelectrodestructureparameteroptimizing[10].B.5BEstimateFunctionsInordertoestimatetheinfluencesofelectrodestructureandparametersonthemeasurementsensitivityandtheimagereconstructionqualityinEIT,themeasurementsensitivityfunctionSandtheimagereconstructionqualityfunctionDaredefinedrespectivelyasfollows.110BThemeasurementsensitivityfunctionS10011010∑∑−−NiMjijSNMSHereMisthemeasurementnumbercorrespondingofonecurrentexcitation.Nisthenumberofcurrentexcitations.Sijdenotesthemeasurementsensitivityforjthmeasurementinithcurrentexcitation0101σσ−−VVSijSijshowsthatthevoltageofthemeasurementelectrodechangesfromV0toV1whiletheconductanceofacertainregioninthemeasuredfieldchangesfromσ0toσ1.SistheaveragesensitivityofallNtimesofexcitations.Itonlydependsontheelectricfielddistributionduetotheconductancechangeofthemeasuredregion,andisindependentofimagereconstructionalgorithmandhardwareerrorssupposethatsignaltonoiseratioofeachelectrodeisfixed.ThebiggertheSis,thehigherthemeasurementsensitivityis.211BTheImageReconstructionQualityFunctionD10025511−∑MpSpGpGiMD(p1,2,M)HereMisthenumberofalltheelementsdividedintheimagingregion.Gspisthegrayvalueofpthelementintheimagingmodel.Gipisthegrayvalueofpthelementinthereconstructedimage.Dshowsthenormalizedaveragevalueofthegreyscaledifferencesbetweenthereconstructedimageandtheimagingmodelofeachelementintheimagingregion.Dsensitivelyreflectsthereconstructedimagequality.UsingD,thedifferencebetweenthereconstructedimageandtheimagingmodelcanbeevaluatedwithoutanyimageprocessing.ThesmallerDis,thelessthedifferencebetweenthereconstructedimageandtheimagingmodelis,thehigherthereconstructedimagequalityis.C.6BImagingModelsInthestudyofthispaperthetotalnumberoffiniteelementsis1210,including734nodes.Thereare256nodesonthecircumferenceofthemeasuredfield.6imagingmodelscorrespondingtodifferentmeasurementdepthhavebeenusedFig.2.Theimagedobjectmovesfromtheboundarytothecentralareafrommodel1to6,andthemodel6istheimagedobjectofcentralarea,correspondingtodeepareaofbody.III.2BSIMULATIONEXPERIMENTANDRESULTA.7BEffectsofElectrodeNumberEITsystemwith8,16,32,64,128and256pointelectrodesareusedrespectivelyforstudy.TheestimateresultsusingfunctionsDfor6imagingmodelsinFig.1isshowninFig.3.Theimagingresultsofmodel3andmodel6areshowedinFig.4.B.EffectsofElectrodeTypeandStructureParameterThereconstructionimageshavebeenmadeusingthecoerciveequipotentialnodemethodforlineelectrodeandcompoundelectrode.Somecomparisonstudiesoftheparameterinfluencesonmeasurementsensitivityandreconstructionqualitybetweenlineelectrodeandcompoundelectrodehavebeencarriedout.Figure2.ImagingmodelsofdifferentdepthsFigure3.TheeffectsofelectrodenumberonimagequalityFigure4.Imagingresultswithdifferentelectrodenumberformodel3and6Thetotalwidthofbothlineelectrodeandcompoundelectrodearesettothesame.Asanexample,thewidthoflineelectrodeis5FEMnodes,whilecompoundelectrodehas3cases,inwhichthetotalwidthsofthemarealso5FEMnodes.Thefirsta3,j1,c3andb0,theseconda3,j1,c1andb1,thethirda3,j2,c1andb0.Theothercasesofdifferentwidthhavesimilarresult.ThefunctionsSandDofthereconstructionimagescorrespondingtoimagemodel6andmodel3respectivelyhavebeencalculatedandshownintableIandtableII.C.9BEffectofExcitationPatternTheeffectofexcitationpatternofEITonthereconstructedqualityandthemeasurementsensitivityhasinvestigated.Theexcitationpattern,whichdetermineshowthecurrentflowsintotheinterestingarea,influencesthedistributionofelectromagneticfielddirectly.Inthisstudy,theadjacent,intervalandoppositeexcitationmodeareused.Taken32electrodesasanexample,theexcitationcurrentisappliedbytwoelectrodes,andtheintervaloftheelectrodesischangedfrom1to16.Whentheintervalis1,itisadjacentexcitation.Andwhentheintervalis16,itisoppositeexcitation.Othercasesarecalledintervalexcitation,Fig.5.Thereconstructionresultsof32electrodesatintervalsof1,4,8,12,16electrodesrespectivelyhavebeenmadewiththeimagemodelsshowninFig.2.TheeffectsofexcitationpatternonimagequalityandmeasurementsensitivityhavebeenshowninFig.6andFig.7.TABLEI.COMPARISONOFLINEELECTRODEANDCOMPOUNDELECTRODEFORMODEL6TABLEII.COMPARISONOFLINEELECTRODEANDCOMPOUNDELECTRODEFORMODEL3Figure5.Adjacentexcitation,intervalexcitationandoppositeexcitationpatternFigure6.EffectsofexcitationpatternonimagequalityElectrodeTypeandStructureParameterDSLineelectrode5nodeswide18.7060.003979a3j1c3b018.5840.003897a3j1c1b118.5760.003889a3j2c1b018.5890.003977ElectrodetypeandstructureparameterDSLineelectrode5nodeswide14.3560.001871a3j1c3b014.6100.001836a3j1c1b114.4750.001831a3j2c1b014.4510.001875510152025308163264128256D[[[[[[electrodecountmodel6model5model4model3model2model100.0050.010.0150.020.0251481216S[[[Intervalofcurrentelectrodesmodel6model5model4model3model2model1Figure7.EffectsofexcitationpatternonmeasurementsensitivityIV.3BRESULTSANDDISCUSSIONSFromFig3andFig4,itisshowedthatfornoncentralimagingobject,smallerelectrodenumberwillincreasethereconstructionerror.Butwhentheelectrodenumberturneduptoacertainextent,theimprovementbecamelesssignificant.Theimagequalityisalmostthesamewhenelectrodenumberismorethan64inthestudycondition.Fortheimagedobjectofcentralarea,especiallyformodel6,reconstructionqualityisverysimilar.Whentheelectrodenumberenlargedto128and256,theimagingqualityturnedabitworse.Thissuggeststhatforcentralobject,biggerelectrodenumbercannotbringbetterimagingquality.Inthispaper,the32and64electrodesystemisbetter.IntableI,fortheimagingobjectinthecentralarea,compoundelectrodehashigherimagequality,thanlineelectrode.AndintableII,fortheimagingobjectatsemi-radius,theeffectoflineelectrompoundelectrodeishelpedtoimproveimagequalityofthecentralarea.ThemeasurementsensitivitySoflineelectrodearealmostthesamewiththatofthethirdcaseofcompoundelectrode,andbetterthantheothertwocases,inbothtableIandII.Itmeansthat,ifthetotalelectrodewidthisfixed,themeasurementsensitivityisbetterwhenlargeexcitationelectrodehasbeenused.FromFig.6and7,itcanbeseenthatforeverydepthsofimagingobject,imagequalityturnsworseatintervalsofbiggerdistance,andthemeasurementsensitivityisincreasedsignificantly.Itisthesuggestionthatoptimizingofelectrodesystemaccordingtotheapplicationrequirement,forexampletheimagingobjectofdifferentdepthsandthedifferentexcitationpattern,isveryimportant.ThesimulationstudyofEITelectrodestructureandparameterdesignishelpfultogethighermeasurementsensitivityandgoodimagequality.12BREFERENCES[1]BarberD.C.andBrownB.H.,“Appliedpotentialtomography”,J.Phsy.ESci.instrum.,1984,17,pp.723-733.[2]BrownBH,“Medicalimpedancetomographyandprocessimpedancetomographyabriefreview”,Meas.Sci.Technol.2001,12,pp.991-996.[3]RenChaoshi,“Bioimpedancemeasurementtechnology.ChinaMedicalDeviceInformation”,2004,10,1,pp.21-25.[4]Paulson.K.,“Parallelalgorithmsforthree-dimensionalelectricalimpedancetomography”,PhD.Thesis,SchoolofComputingandMathematicalSciences,OxfordBrooksUniversity,1992.[5]PingHua,EungJeWoo,JohnGWebster,andWillisJ.Tompkins.“Usingcompoundelectrodesinelectricalimpedancetomography”,IEEETrans.Biome.Engn.,1993,40,1,pp.29-34[6]E.J.Woo,p.Hua,J.G.Websteretal.,“Finite-elementmethodinelectricalimpedancetomography”.Med.Bio.Eng.Comput.1994,32,pp.530-536.[7]P.A.T.Pinheiro,W.W.LohandF.J.Dickin.,“Optimalsizedelectrodesforelectricalresistancetomography”.ElectronicsLetters1998,34,1,pp.69-70[8]WangChao,WangHuaxiang,“Optimizingdesignofelectrodeconstructioninmedicalelectricalimpedancetomographysystem”.Journalofthefourthmilitarymedicaluniversity,2001,22,1,pp.78-80.[9]WangHuaxiang,WangChao,YinWuliang,“OptimumdesignofthestructureoftheelectrodeforamedicalEITsystem”,Meas.Sci.Technol.2001,12,pp.1020-1023.[10]WangYan,ShaHong,RenChaoshi,“Optimumdesignofelectrodestructureandparametersinelectricalimpedancetomography”,Physiol.Meas.2006,27,pp.291–306.51015202530351481216D[[[[Intervalofcurrentelectrodesmodel6model5model4model3model2model1

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