DKI彌散峰度成像英文簡介培訓課件

DKI彌散峰度成像英文簡介Contents

DWI(diffusionweightedimaging)DTI(diffusiontensorimaging)DKI(diffusionkurtosisimaging)2DKI彌散峰度成像英文簡介DWI原理MR圖像的信號組織T1、T2馳豫時間、H1的密度、分子彌散運動DWI圖像利用擴散敏感梯度脈沖將水分子彌散效應擴大，來研究不同組織中水分子擴散運動的差異

3DKI彌散峰度成像英文簡介DWI評估彌散的參數(shù)

通過兩個以上不同彌散敏感梯度值（b值）的彌散加權象，可計算出彌散敏感梯度方向上水分子的表觀彌散系數(shù)（apparentdiffusioncoefficient

ADC）

ADC=In(S低/S高)/(b高-b低)彌散敏感系數(shù)(b)值==r2σ2g2(△-σ/3)b值的取值范圍為0~10000s/mm2，較大的b值具有較大的彌散權重，對水分子的彌散運動越敏感，并引起較大的信號下降，但b值越大，圖像信噪比也相應下降，如果b值太小，易受T2加權的影像，產(chǎn)生所謂的T2透射效應(T2shinethrougheffect)，一般來說用大b值差的圖像測得的ADC值較準確，故側ADC值時宜選較高b值和較大的b值差ADC反映了水分子的擴散運動的能力,指水分子單位時間內擴散運動的范圍，越高代表水分子擴散能力越強。4DKI彌散峰度成像英文簡介

均質介質中可以水分子的自由運動為各向同性，即在各個方向上的彌散強度大小一致，彌散張量D描述為球形，沿磁共振的三個主坐標的特征值為λ1=λ2=λ3在腦白質中由于髓鞘的阻擋，水分子的彌散被限制在與纖維走行一致的方向上，具有較高的各向異性，此時彌散張量可表示為橢球形，其特征值λ1>λ2>λ3，最大特征值對應的方向與經(jīng)過該體素的纖維束走行平行5DKI彌散峰度成像英文簡介defectsofDTIConventionalDTIfailstofullyutilizetheMRdiffusionmeasurementsthatareinherenttotissuemicrostructure.

DTIcomputesapparentdiffusivitybasedontheassumptionthatdiffusionweighted(DW)MRsignalhasamonoexponentialdependenceonthediffusionfactor(b-value).DTIimplicitlyassumesthatwatermoleculediffusionoccursinafreeandunrestrictedenvironmentwithaGaussiandistributionofdiffusiondisplacement.6DKI彌散峰度成像英文簡介defectsofDTI

Inbiologicaltissue,complexcellularmicrostructuresmakewaterdiffusionahighlyhinderedorrestrictedprocess.

Non-monoexponentialdecaysareexperimentallyobservedinbothwhitematterandgraymatter.Moreover,thesimplifieddescriptionofthediffusionprocessinvivobya2nd-order3DdiffusivitytensorpreventsDTIfrombeingtrulyeffectiveincharacterizingrelativelyisotropictissuesuchasGM.EveninWM,theDTImodelcanfailifthetissuecontainssubstantialcrossingordivergingfibers.

7DKI彌散峰度成像英文簡介defectsofDTIAsaresult,DTIquantitationisb-valuedependentandDTIfailstofullyutilizethediffusionmeasurementsthatareinherenttotissuemicrostructure.8DKI彌散峰度成像英文簡介KurtosisKurtosisherereferstotheexcesskurtosisthatisthenormalizedandstandardizedfourthcentralmomentofthewaterdisplacementdistribution.ItisadimensionlessmeasurethatquantifiesthedeviationofthewaterdiffusiondisplacementprofilefromtheGaussiandistributionofunrestricteddiffusion,providingameasureofthedegreeofdiffusionhindranceorrestriction.fourthcentralmoment:四階中心距，主要用來衡量隨機分布變量的分布在均值附近的陡峭程度SincethedeviationfromGaussianbehaviorisgovernedbythecomplexityofthetissuewithinwhichthewaterisdiffusing,thisexcessdiffusionalkurtosiscanberegardedasameasureofatissue’sdegreeofstructure.9DKI彌散峰度成像英文簡介OtheradvantagesofDKIMeankurtosis(MK),theaverageapparentkurtosisalongalldiffusiongradientencodingdirections,hasbeenmeasuredanddemonstratedtoofferanimprovedsensitivityindetectingdevelopmentalandpathologicalchangesinneuraltissuesascomparedtoconventionalDTI.Inaddition,directionalkurtosisanalysishasbeenformulatedtorevealdirectionallyspecificinformation,suchasthewaterdiffusionkurtosesalongthedirectionparallelorperpendiculartotheprinciplewaterdiffusiondirectionasdeterminedbythe2nd-orderdiffusiontensor10DKI彌散峰度成像英文簡介DKIprovidesahigher-orderdescriptionofrestrictedwaterdiffusionprocessbya2nd-order3Ddiffusivitytensor(DTasinconventionalDTI)togetherwitha4th-order3Dkurtosistensor(KT).11DKI彌散峰度成像英文簡介ConditionsThemethodisbasedonthesametypeofpulsesequencesemployedforconventionaldiffusion-weightedimaging(DWI),buttherequiredbvaluesaresomewhatlargerthanthoseusuallyusedtomeasurediffusioncoefficients.Inthebrain,bvaluesofabout2000s/mm2aresufficient.Atleast15non-collinearandnon-coplanardirectionsarerequiredtoconstructKT.12DKI彌散峰度成像英文簡介DKIvsq-spaceimagingtechniquesDKIhasacloserelationshiptoq-spaceimagingtechniques.q-spaceimagingmethodshaveindeedrecentlybeenemployedtoestimatediffusionalkurtosis.Theprincipaldifferencebetweenthemisthatq-spaceimagingseekstoestimatethefulldiffusiondisplacementprobabilitydistributionratherthanjustthekurtosis.Asaconsequence,q-spaceimagingismoredemandingintermsofimagingtimeandgradientstrengths.MeasuringthediffusionalkurtosisrequiresonlymodestincreasesinbvaluesAndDKIislessdemandingintermsofhardwarerequirementsandpostprocessingeffort.13DKI彌散峰度成像英文簡介Kurtosistensor(KT)derivedparametersMK(meankurtosis):MKisameasureoftheoverallkurtosis.Itdoesnothaveanydirectionalspecificity.MK的大小取決于感興趣區(qū)內組織的結構復雜程度，結構越復雜非正態(tài)分布水分子擴散受限越顯著，MK也即越大K∥(Axialkurtosis)andK⊥(Radialkurtosis)

:canbedefinedasthekurtosisparallelandperpendiculartotheprinciplediffusioneigenvector(e1)K⊥越大表明在該方向非正態(tài)分布水分子擴散受限越明顯，反之則表明擴散受限越弱FAK(fractionalanisotropyofkurtosis)SimilartoFAinDTI,theanisotropyofdirectionalkurtosiscanbeconvenientlydefinedasFAK

KA越小即表示越趨于各向同性擴散;若組織結構越緊密越規(guī)則，KA越大14DKI彌散峰度成像英文簡介DKIparametricmaps15DKI彌散峰度成像英文簡介DKIparametricmapsTypicalDKI-derivedparametricmapsfromasinglesliceofa)invivo,b)formalin-fixedadultratbrainsandc)anormalhumansubject(male,44yearsold).Axialdiffusivity(λ//),radialdiffusivity(λ⊥),meandiffusivity(MD),axialkurtosis(K//),radialkurtosis(K⊥),meankurtosis(MK),fractionalanisotropy(FA),directionallyencodedcolourFA(DEC-FA)andfractionalanisotropyofkurtosis(FAK)mapsarecomputedfromDKImodel.16DKI彌散峰度成像英文簡介DKIparametricmapsFor(a),rawDWIswereacquiredbySEEPIwithTR/TE?=?3000/30.3?ms,δ/Δ?=?5/17?ms,slicethickness?=?1?mm,FOV?=?30?×?30?mm2,datamatrix?=?128?×?128(zerofilledto256?×?256),NEX?=?4,6b-values(0.0,0.5,1.0,1.5,2.0and2.5?ms/μm2)andalong30directionsusing7Tscanner17DKI彌散峰度成像英文簡介DKIparametricmapsFor(b),rawDWIswereacquiredwiththesameparametersasthoseforinvivoexceptTE?=?34.3?ms,δ?=?9?msandb-valuesof0.0,1.0,2.0,3.0,4.0and5.0?ms/μm2.Alargerb-valuerangewasusedinexvivoexperimentduetothegenerallylowerdiffusivities.18DKI彌散峰度成像英文簡介DKIparametricmaps

HigherMKisfoundinWM,indicatingagenerallyhigherdegreeofdiffusioncomplexityandrestrictionintheWMstructures.ItcanbeseenfromthedirectionalkurtosismapsthatsuchhighMKinWMismainlycontributedbyK⊥.ThissuggeststheexistenceofheterogeneityandrestricteddiffusioninaxonalstructuresBothMKandK⊥

exhibitstrongcontrastbetweenWMandGMstructures.20DKI彌散峰度成像英文簡介DKIparametricmapsBothMKandK⊥

exhibitstrongcontrastbetweenWMandGMstructures.PositivemeananddirectionalkurtosesareobservedinbothWMandGM,indicatingfasterDWsignaldecayatlowerb-valuesandrestricteddiffusionenvironmentinbothWMandGMunderinvivoandformalin-fixedconditions.21DKI彌散峰度成像英文簡介DKIshowsageneraldecreaseindiffusivityandincreaseinkurtosisinWMandGMofthefixedbrainsThebreakdownsofneurofilamentsandmicrotubulescausedbyfixativesarebelievedtoproducemorediffusionbarriersandhenceleadtotheλ//

decreaseandK//

increase.Otherfixationeffectssuchastissueshrinkage,decreaseinmembranepermeability,increaseinaxonalpackingdensityandreductionofextracellularspaceinparenchymaalsolikelycontributetothesignificantλ⊥decreaseandK⊥

increase.22DKI彌散峰度成像英文簡介Directionalkurtosisanalysisoffixedexperimentalautoimmuneencephalitis(EAE)spinalcordTheinflammatoryneurodegenerativediseaseEAE

ischaracterizedbybothaxonallossanddemyelinationInrecentDKIstudies,therearepromisingresultsofusingMKtodetectchangesinnormalorpathologicalneuraltissue

However,asanaverageofkurtosesalongallthediffusiondirections,MKcanlosesensitivityandspecificityinprobingdirectionalchangesofpathologicaltissue23DKI彌散峰度成像英文簡介EAEspinalcordK//

isfoundtobesignificantlyincreasedandλ//

decreasedinthelesionareaλ//

reductionislikelyduetocytoskeletalperturbationordebrisformationwhentheaxonalstructuresbreakdownInaddition,λ⊥

increaseswhereasK⊥

decreaseslikelybecauseofthedemyelinationandaxonallossthatalsoleadtolessdiffusionrestrictioninradialdirection.24DKI彌散峰度成像英文簡介EAEspinalcordThedirectionallyaveragedMDandMKarefoundtobelesssensitivetoEAEpathologyduetotheoppositetrendsofdiffusivityandkurtosischangesinaxialandradialdirection.25DKI彌散峰度成像英文簡介MonitoringpostnatalbrainmaturationbyconventionalDTI

CC:corpuscallosum（胼胝體）;EC:externalcapsule（外囊）;CP:cerebralpeduncle（大腦腳）;AC:anteriorcommissure;（前聯(lián)合）CT:cerebralcortex（腦皮質）;HP:hippocampus（海馬）;CPu:caudateputamen（新紋狀體）26DKI彌散峰度成像英文簡介MonitoringpostnatalbrainmaturationbyconventionalDTI

Thesensitivityofλ//

indetectingratbrainWMmaturationisgenerallyobservedtobethehighestatlowb-value

Atrelativelylowb-values,theapparentdiffusivityisprimarilycontributedfromthefastwaterdiffusionactivitiesinextracellularspacethatdependonbothcellularmicrostructureandmembranepermeability.

Theuseoflowb-valuecanbestdetectthesechanges.Thehighλ//

sensitivityatlowb-valueobservedinthecurrentstudysuggeststhealterationsofthesefastwaterdiffusionactivitiesalongaxonaldirectionduringbrainmaturation.

Suchalterationsmayresultfromtheincreaseinpackingdensityoffiberbundlesandaxons,axonaldiameterincrease,changesinneurofibrils,andincreasedcomplexityofextracellularmatrix.27DKI彌散峰度成像英文簡介MonitoringpostnatalbrainmaturationbyconventionalDTI

Whereasthatofλ?

isthehighestathighb-valueThediffusionchangesprobedinWMusinghighb-valuesareascribedmoretotheslowwatermoleculediffusionparticularlyalongtheradialdirectionwhentraversingthemembranesandmyelinsheathsThehighsensitivityofλ?

athighb-valueindetectingbrainmaturationshowninthefigurelikelyreflectstheseWMmicrostructuralchanges,includingmyelinationandaxonaldensityanddiameterchangesduringpostnatalbraindevelopment.28DKI彌散峰度成像英文簡介MonitoringpostnatalbrainmaturationbyconventionalDTIFAquantitationisalsoaffectedbytheb-valueanditsabilityindetectingbrainmaturationalchangesvariesamongdifferentstructures.29DKI彌散峰度成像英文簡介MonitoringpostnatalbrainmaturationbyDKIFigure7ashowsthatthesensitivityoffittingallthemulti-b-valueDWIstoDTImodelisgenerallysimilartothatofemployingamediumb-value(b?=?1.5?ms/μm2)showninFigure6InFigure7b,thegeneralandcontinualkurtosisincreasewithageisobserved,indicatingthatmorediffusionrestrictionoccursduringbrainmaturationinbothWMandGMstructures.TheDKI-deriveddiffusivityandkurtosisindicesarehighlysensitivetobraindevelopmentalchanges.30DKI彌散峰度成像英文簡介MonitoringpostnatalbrainmaturationbyDKI

Bothλ//

andK//

ofWMarefoundtoincreaseswithage,whichmayarisefromvariousbiologicaleventsduringearlypostnatalbrainmaturation.Theincreaseofdiffusivitycanbecausedbyaxoplasmicflowduringthemyelinationperiodneuronallossandaxonalpruningthatshortenstheaxonlengthcanleadtoanincreaseofrestriction31DKI彌散峰度成像英文簡介MonitoringpostnatalbrainmaturationbyDKITheincreaseofK⊥

inWMislikelyascribedtothemyelinationandmodificationofaxonalstructuresthatincreasesrestrictionintheradialdirection.

DKIanalysisalsorevealsthatdiffusionrestrictionintherelativelyisotropicGMincreaseswithage.ThismayreflectthemoredenselypackedstructuresandthedendriticarchitecturalmodificationinGM32DKI彌散峰度成像英文簡介DTIVSDKIinmonitoringpostnatalbrainmaturation

WhenthereisalargeK,theestimateddiffusivityinconventionalDTIshowsalargediscrepancywiththediffusivityestimatedinDKIapproach.AsKinallthestructuresispositive,DTI-derived