DKI (弥散峰度成像) 英文PPT简介.ppt

DiffusionalKurtosisImaging DKI Contents DWI diffusionweightedimaging DTI diffusiontensorimaging DKI diffusionkurtosisimaging DWI原理 MR图像的信号组织T1 T2驰豫时间 H1的密度 分子弥散运动DWI图像利用扩散敏感梯度脉冲将水分子弥散效应扩大 来研究不同组织中水分子扩散运动的差异 DWI评估弥散的参数 通过两个以上不同弥散敏感梯度值 b值 的弥散加权象 可计算出弥散敏感梯度方向上水分子的表观弥散系数 apparentdiffusioncoefficientADC ADC In S低 S高 b高 b低 弥散敏感系数 b 值 r2 2g2 3 b值的取值范围为0 10000s mm2 较大的b值具有较大的弥散权重 对水分子的弥散运动越敏感 并引起较大的信号下降 但b值越大 图像信噪比也相应下降 如果b值太小 易受T2加权的影像 产生所谓的T2透射效应 T2shinethrougheffect 一般来说用大b值差的图像测得的ADC值较准确 故侧ADC值时宜选较高b值和较大的b值差ADC反映了水分子的扩散运动的能力 指水分子单位时间内扩散运动的范围 越高代表水分子扩散能力越强 均质介质中可以水分子的自由运动为各向同性 即在各个方向上的弥散强度大小一致 弥散张量D描述为球形 沿磁共振的三个主坐标的特征值为 1 2 3在脑白质中由于髓鞘的阻挡 水分子的弥散被限制在与纤维走行一致的方向上 具有较高的各向异性 此时弥散张量可表示为椭球形 其特征值 1 2 3 最大特征值对应的方向与经过该体素的纤维束走行平行 defectsofDTI ConventionalDTIfailstofullyutilizetheMRdiffusionmeasurementsthatareinherenttotissuemicrostructure DTIcomputesapparentdiffusivitybasedontheassumptionthatdiffusionweighted DW MRsignalhasamonoexponentialdependenceonthediffusionfactor b value DTIimplicitlyassumesthatwatermoleculediffusionoccursinafreeandunrestrictedenvironmentwithaGaussiandistributionofdiffusiondisplacement defectsofDTI Inbiologicaltissue complexcellularmicrostructuresmakewaterdiffusionahighlyhinderedorrestrictedprocess Non monoexponentialdecaysareexperimentallyobservedinbothwhitematterandgraymatter Moreover thesimplifieddescriptionofthediffusionprocessinvivobya2nd order3DdiffusivitytensorpreventsDTIfrombeingtrulyeffectiveincharacterizingrelativelyisotropictissuesuchasGM EveninWM theDTImodelcanfailifthetissuecontainssubstantialcrossingordivergingfibers defectsofDTI Asaresult DTIquantitationisb valuedependentandDTIfailstofullyutilizethediffusionmeasurementsthatareinherenttotissuemicrostructure Kurtosis Kurtosisherereferstotheexcesskurtosisthatisthenormalizedandstandardizedfourthcentralmomentofthewaterdisplacementdistribution ItisadimensionlessmeasurethatquantifiesthedeviationofthewaterdiffusiondisplacementprofilefromtheGaussiandistributionofunrestricteddiffusion providingameasureofthedegreeofdiffusionhindranceorrestriction fourthcentralmoment 四阶中心距 主要用来衡量随机分布变量的分布在均值附近的陡峭程度SincethedeviationfromGaussianbehaviorisgovernedbythecomplexityofthetissuewithinwhichthewaterisdiffusing thisexcessdiffusionalkurtosiscanberegardedasameasureofatissue sdegreeofstructure OtheradvantagesofDKI Meankurtosis MK theaverageapparentkurtosisalongalldiffusiongradientencodingdirections hasbeenmeasuredanddemonstratedtoofferanimprovedsensitivityindetectingdevelopmentalandpathologicalchangesinneuraltissuesascomparedtoconventionalDTI Inaddition directionalkurtosisanalysishasbeenformulatedtorevealdirectionallyspecificinformation suchasthewaterdiffusionkurtosesalongthedirectionparallelorperpendiculartotheprinciplewaterdiffusiondirectionasdeterminedbythe2nd orderdiffusiontensor DKIprovidesahigher orderdescriptionofrestrictedwaterdiffusionprocessbya2nd order3Ddiffusivitytensor DTasinconventionalDTI togetherwitha4th order3Dkurtosistensor KT Conditions Themethodisbasedonthesametypeofpulsesequencesemployedforconventionaldiffusion weightedimaging DWI buttherequiredbvaluesaresomewhatlargerthanthoseusuallyusedtomeasurediffusioncoefficients Inthebrain bvaluesofabout2000s mm2aresufficient Atleast15non collinearandnon coplanardirectionsarerequiredtoconstructKT DKIvsq spaceimagingtechniques DKIhasacloserelationshiptoq spaceimagingtechniques q spaceimagingmethodshaveindeedrecentlybeenemployedtoestimatediffusionalkurtosis Theprincipaldifferencebetweenthemisthatq spaceimagingseekstoestimatethefulldiffusiondisplacementprobabilitydistributionratherthanjustthekurtosis Asaconsequence q spaceimagingismoredemandingintermsofimagingtimeandgradientstrengths MeasuringthediffusionalkurtosisrequiresonlymodestincreasesinbvaluesAndDKIislessdemandingintermsofhardwarerequirementsandpostprocessingeffort Kurtosistensor KT derivedparameters MK meankurtosis MKisameasureoftheoverallkurtosis Itdoesnothaveanydirectionalspecificity MK的大小取决于感兴趣区内组织的结构复杂程度 结构越复杂非正态分布水分子扩散受限越显著 MK也即越大K Axialkurtosis andK Radialkurtosis canbedefinedasthekurtosisparallelandperpendiculartotheprinciplediffusioneigenvector e1 K 越大表明在该方向非正态分布水分子扩散受限越明显 反之则表明扩散受限越弱FAK fractionalanisotropyofkurtosis SimilartoFAinDTI theanisotropyofdirectionalkurtosiscanbeconvenientlydefinedasFAKKA越小即表示越趋于各向同性扩散 若组织结构越紧密越规则 KA越大 DKIparametricmaps DKIparametricmaps TypicalDKI derivedparametricmapsfromasinglesliceofa invivo b formalin fixedadultratbrainsandc anormalhumansubject male 44yearsold Axialdiffusivity radialdiffusivity meandiffusivity MD axialkurtosis K radialkurtosis K meankurtosis MK fractionalanisotropy FA directionallyencodedcolourFA DEC FA andfractionalanisotropyofkurtosis FAK mapsarecomputedfromDKImodel DKIparametricmaps For a rawDWIswereacquiredbySEEPIwithTR TE 3000 30 3ms 5 17ms slicethickness 1mm FOV 30 30mm2 datamatrix 128 128 zerofilledto256 256 NEX 4 6b values 0 0 0 5 1 0 1 5 2 0and2 5ms m2 andalong30directionsusing7Tscanner DKIparametricmaps For b rawDWIswereacquiredwiththesameparametersasthoseforinvivoexceptTE 34 3ms 9msandb valuesof0 0 1 0 2 0 3 0 4 0and5 0ms m2 Alargerb valuerangewasusedinexvivoexperimentduetothegenerallylowerdiffusivities DKIparametricmaps For c rawDWIswereacquiredbySEEPIwithTR TE 2300 109ms slicethickness 2mm FOV 256 256mm2 datamatrix 128 128 NEX 2 6b values 0 0 0 5 1 0 1 5 2 0and2 5ms m2 andalong30directionsusinga3TSiemensscanner DKIparametricmaps HigherMKisfoundinWM indicatingagenerallyhigherdegreeofdiffusioncomplexityandrestrictionintheWMstructures ItcanbeseenfromthedirectionalkurtosismapsthatsuchhighMKinWMismainlycontributedbyK Thissuggeststheexistenceofheterogeneityandrestricteddiffusioninaxonalstructures BothMKandK exhibitstrongcontrastbetweenWMandGMstructures DKIparametricmaps BothMKandK exhibitstrongcontrastbetweenWMandGMstructures PositivemeananddirectionalkurtosesareobservedinbothWMandGM indicatingfasterDWsignaldecayatlowerb valuesandrestricteddiffusionenvironmentinbothWMandGMunderinvivoandformalin fixedconditions DKIshowsageneraldecreaseindiffusivityandincreaseinkurtosisinWMandGMofthefixedbrains Thebreakdownsofneurofilamentsandmicrotubulescausedbyfixativesarebelievedtoproducemorediffusionbarriersandhenceleadtothe decreaseandK increase Otherfixationeffectssuchastissueshrinkage decreaseinmembranepermeability increaseinaxonalpackingdensityandreductionofextracellularspaceinparenchymaalsolikelycontributetothesignificant decreaseandK increase Directionalkurtosisanalysisoffixedexperimentalautoimmuneencephalitis EAE spinalcord TheinflammatoryneurodegenerativediseaseEAEischaracterizedbybothaxonallossanddemyelinationInrecentDKIstudies therearepromisingresultsofusingMKtodetectchangesinnormalorpathologicalneuraltissueHowever asanaverageofkurtosesalongallthediffusiondirections MKcanlosesensitivityandspecificityinprobingdirectionalchangesofpathologicaltissue EAEspinalcord K isfoundtobesignificantlyincreasedand decreasedinthelesionarea reductionislikelyduetocytoskeletalperturbationordebrisformationwhentheaxonalstructuresbreakdownInaddition increaseswhereasK decreaseslikelybecauseofthedemyelinationandaxonallossthatalsoleadtolessdiffusionrestrictioninradialdirection EAEspinalcord ThedirectionallyaveragedMDandMKarefoundtobelesssensitivetoEAEpathologyduetotheoppositetrendsofdiffusivityandkurtosischangesinaxialandradialdirection MonitoringpostnatalbrainmaturationbyconventionalDTI CC corpuscallosum 胼胝体 EC externalcapsule 外囊 CP cerebralpeduncle 大脑脚 AC anteriorcommissure 前联合 CT cerebralcortex 脑皮质 HP hippocampus 海马 CPu caudateputamen 新纹状体 MonitoringpostnatalbrainmaturationbyconventionalDTI Thesensitivityof indetectingratbrainWMmaturationisgenerallyobservedtobethehighestatlowb valueAtrelativelylowb values theapparentdiffusivityisprimarilycontributedfromthefastwaterdiffusionactivitiesinextracellularspacethatdependonbothcellularmicrostructureandmembranepermeability Theuseoflowb valuecanbestdetectthesechanges Thehigh sensitivityatlowb valueobservedinthecurrentstudysuggeststhealterationsofthesefastwaterdiffusionactivitiesalongaxonaldirectionduringbrainmaturation Suchalterationsmayresultfromtheincreaseinpackingdensityoffiberbundlesandaxons axonaldiameterincrease changesinneurofibrils andincreasedcomplexityofextracellularmatrix MonitoringpostnatalbrainmaturationbyconventionalDTI Whereasthatof isthehighestathighb valueThediffusionchangesprobedinWMusinghighb valuesareascribedmoretotheslowwatermoleculediffusionparticularlyalongtheradialdirectionwhentraversingthemembranesandmyelinsheathsThehighsensitivityof athighb valueindetectingbrainmaturationshowninthefigurelikelyreflectstheseWMmicrostructuralchanges includingmyelinationandaxonaldensityanddiameterchangesduringpostnatalbraindevelopment MonitoringpostnatalbrainmaturationbyconventionalDTI FAquantitationisalsoaffectedbytheb valueanditsabilityindetectingbrainmaturationalchangesvariesamongdifferentstructures MonitoringpostnatalbrainmaturationbyDKI Figure7ashowsthatthesensitivityoffittingallthemulti b valueDWIstoDTImodelisgenerallysimilartothatofemployingamediumb value b 1 5ms m2 showninFigure6InFigure7b thegeneralandcontinualkurtosisincreasewithageisobserved indicatingthatmorediffusionrestrictionoccursduringbrainmaturationinbothWMandGMstructures TheDKI deriveddiffusivityandkurtosisindicesarehighlysensitivetobraindevelopmentalchanges MonitoringpostnatalbrainmaturationbyDKI Both andK ofWMarefoundtoincreaseswithage whichmayarisefromvariousbiologicaleventsduringearlypostnatalbrainmaturation Theincreaseofdiffusivitycanbecausedbyaxoplasmicflowduringthemyelinationperiodneuronallossandaxonalpruningthatshortenstheaxonlengthcanleadtoanincreaseofrestriction MonitoringpostnatalbrainmaturationbyDKI TheincreaseofK inWMislikelyascribedtothemyelinationandmodificationofaxonalstructuresthatincreasesrestrictionintheradialdirection DKIanalysisalsorevealsthatdiffusionrestrictionintherelativelyisotropicGMincreaseswithage ThismayreflectthemoredenselypackedstructuresandthedendriticarchitecturalmodificationinGM DTIVSDKIinmonitoringpostnatalbrainmaturation WhenthereisalargeK theestimateddiffusivityinconventionalDTIshowsalargediscrepancywiththediffusivityestimatedinDKIa