DiffusionalKurtosisImaging

DKIContents

DWI(diffusionweightedimaging)DTI(diffusiontensorimaging)DKI(diffusionkurtosisimaging)DWI原理MR圖像的信號(hào)組織T1、T2馳豫時(shí)間、H1的密度、分子彌散運(yùn)動(dòng)DWI圖像利用擴(kuò)散敏感梯度脈沖將水分子彌散效應(yīng)擴(kuò)大，來(lái)研究不同組織中水分子擴(kuò)散運(yùn)動(dòng)的差異

均質(zhì)介質(zhì)中可以水分子的自由運(yùn)動(dòng)為各向同性，即在各個(gè)方向上的彌散強(qiáng)度大小一致，彌散張量D描述為球形，沿磁共振的三個(gè)主坐標(biāo)的特征值為

λ1=λ2=λ3在腦白質(zhì)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(shí)彌散張量可表示為橢球形，其特征值λ1>λ2>λ3，最大特征值對(duì)應(yīng)的方向與經(jīng)過(guò)該體素的纖維束走行平行defectsofDTIConventionalDTIfailstofullyutilizetheMRdiffusionmeasurementsthatareinherenttotissuemicrostructure.

DTIcomputesapparentdiffusivitybasedontheassumptionthatdiffusionweighted(DW)MRsignalhasamonoexponentialdependenceonthediffusionfactor(b-value).DTIimplicitlyassumesthatwatermoleculediffusionoccursinafreeandunrestrictedenvironmentwithaGaussiandistributionofdiffusiondisplacement.defectsofDTI

Inbiologicaltissue,complexcellularmicrostructuresmakewaterdiffusionahighlyhinderedorrestrictedprocess.

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

defectsofDTIAsaresult,DTIquantitationisb-valuedependentandDTIfailstofullyutilizethediffusionmeasurementsthatareinherenttotissuemicrostructure.OtheradvantagesofDKIMeankurtosis(MK),theaverageapparentkurtosisalongalldiffusiongradientencodingdirections,hasbeenmeasuredanddemonstratedtoofferanimprovedsensitivityindetectingdevelopmentalandpathologicalchangesinneuraltissuesascomparedtoconventionalDTI.Inaddition,directionalkurtosisanalysishasbeenformulatedtorevealdirectionallyspecificinformation,suchasthewaterdiffusionkurtosesalongthedirectionparallelorperpendiculartotheprinciplewaterdiffusiondirectionasdeterminedbythe2nd-orderdiffusiontensorConditionsThemethodisbasedonthesametypeofpulsesequencesemployedforconventionaldiffusion-weightedimaging(DWI),buttherequiredbvaluesaresomewhatlargerthanthoseusuallyusedtomeasurediffusioncoefficients.Inthebrain,bvaluesofabout2000s/mm2aresufficient.Atleast15non-collinearandnon-coplanardirectionsarerequiredtoconstructKT.Kurtosistensor(KT)derivedparametersMK(meankurtosis):MKisameasureoftheoverallkurtosis.Itdoesnothaveanydirectionalspecificity.MK的大小取決于感興趣區(qū)內(nèi)組織的結(jié)構(gòu)復(fù)雜程度，結(jié)構(gòu)越復(fù)雜非正態(tài)分布水分子擴(kuò)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(kuò)散受限越明顯，反之則表明擴(kuò)散受限越弱FAK(fractionalanisotropyofkurtosis)SimilartoFAinDTI,theanisotropyofdirectionalkurtosiscanbeconvenientlydefinedasFAK

KA越小即表示越趨于各向同性擴(kuò)散;若組織結(jié)構(gòu)越緊密越規(guī)則，KA越大DKIparametricmapsDKIparametricmapsTypicalDKI-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.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)andalong30directionsusing7TscannerDKIparametricmapsFor(b),rawDWIswereacquiredwiththesameparametersasthoseforinvivoexceptTE?=?34.3?ms,δ?=?9?msandb-valuesof0.0,1.0,2.0,3.0,4.0and5.0?ms/μm2.Alargerb-valuerangewasusedinexvivoexperimentduetothegenerallylowerdiffusivities.DKIparametricmaps

HigherMKisfoundinWM,indicatingagenerallyhigherdegreeofdiffusioncomplexityandrestrictionintheWMstructures.ItcanbeseenfromthedirectionalkurtosismapsthatsuchhighMKinWMismainlycontributedbyK⊥.ThissuggeststheexistenceofheterogeneityandrestricteddiffusioninaxonalstructuresBothMKandK⊥

exhibitstrongcontrastbetweenWMandGMstructures.DKIparametricmapsBothMKandK⊥

exhibitstrongcontrastbetweenWMandGMstructures.PositivemeananddirectionalkurtosesareobservedinbothWMandGM,indicatingfasterDWsignaldecayatlowerb-valuesandrestricteddiffusionenvironmentinbothWMandGMunderinvivoandformalin-fixedconditions.Directionalkurtosisanalysisoffixedexperimentalautoimmuneencephalitis(EAE)spinalcordTheinflammatoryneurodegenerativediseaseEAE

ischaracterizedbybothaxonallossanddemyelinationInrecentDKIstudies,therearepromisingresultsofusingMKtodetectchangesinnormalorpathologicalneuraltissue

However,asanaverageofkurtosesalongallthediffusiondirections,MKcanlosesensitivityandspecificityinprobingdirectionalchangesofpathologicaltissueEAEspinalcordK//

isfoundtobesignificantlyincreasedandλ//

decreasedinthelesionareaλ//

reductionislikelyduetocytoskeletalperturbationordebrisformationwhentheaxonalstructuresbreakdownInaddition,λ⊥

increaseswhereasK⊥

decreaseslikelybecauseofthedemyelinationandaxonallossthatalsoleadtolessdiffusionrestrictioninradialdirection.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(zhì)）;HP:hippocampus（海馬）;CPu:caudateputamen（新紋狀體）MonitoringpostnatalbrainmaturationbyconventionalDTI

Thesensitivityofλ//

indetectingratbrainWMmaturationisgenerallyobservedtobethehighestatlowb-value

Atrelativelylowb-values,theapparentdiffusivityisprimarilycontributedfromthefastwaterdiffusionactivitiesinextracellularspacethatdependonbothcellularmicrostructureandmembranepermeability.

Theuseoflowb-valuecanbestdetectthesechanges.Thehighλ//

sensitivityatlowb-valueobservedinthecurrentstudysuggeststhealterationsofthesefastwaterdiffusionactivitiesalongaxonaldirectionduringbrainmaturation.

Suchalterationsmayresultfromtheincreaseinpackingdensityoffiberbundlesandaxons,axonaldiameterincrease,changesinneurofibrils,andincreasedcomplexityofextracellularmatrix.MonitoringpostnatalbrainmaturationbyconventionalDTI

Whereasthatofλ?

isthehighestathighb-valueThediffusionchangesprobedinWMusinghighb-valuesareascribedmoretotheslowwatermoleculediffusionparticularlyalongtheradialdirectionwhentraversingthemembranesandmyelinsheathsThehighsensitivityofλ?

athighb-valueindetectingbrainmaturationshowninthefigurelikelyreflectstheseWMmicrostructuralchanges,includingmyelinationandaxonaldensityanddiameterchangesduringpostnatalbraindevelopment.MonitoringpostnatalbrainmaturationbyDKIFigure7ashowsthatthesensitivityoffittingallthemulti-b-valueDWIstoDTImodelisgenerallysimilartothatofemployingamediumb-value(b?=?1.5?ms/μm2)showninFigure6InFigure7b,thegeneralandcontinualkurtosisincreasewithageisobserved,indicatingthatmorediffusionrestrictionoccursduringbrainmaturationinbothWMandGMstructures.TheDKI-deriveddiffusivityandkurtosisindicesarehighlysensitivetobraindevelopmentalchanges.MonitoringpostnatalbrainmaturationbyDKI

Bothλ//

andK//

ofWMarefoundtoincreaseswithage,whichmayarisefromvariousbiologicaleventsduringearlypostnatalbrainmaturation.TheincreaseofdiffusivitycanbecausedbyaxoplasmicflowduringthemyelinationperiodneuronallossandaxonalpruningthatshortenstheaxonlengthcanleadtoanincreaseofrestrictionMonitoringpostnatalbrainmaturationbyDKITheincreaseofK⊥

inWMislikelyascribedtothemyelinationandmodificationofaxonalstructuresthatincreasesrestrictionintheradialdirection.

DKIanalysisalsorevealsthatdiffusionrestrictionintherelativelyisotropicGMincreaseswithage.ThismayreflectthemoredenselypackedstructuresandthedendriticarchitecturalmodificationinGMDTIVSDKIinmonitoringpostnatalbrainmaturation

WhenthereisalargeK,theestimateddiffusivityinconventionalDTIshowsalargediscrepancywiththediffusivityestimatedinDKIapproach.AsKinallthestructuresispositive,DTI-deriveddiffusivitiesaregenerallylowerthanthosebyDKI.Therelativelyhighsensitivityoftheλ⊥

inmonoexponentialDTImodelismainlyaresultofincreasingK⊥

withage(whilethechangesofλ⊥

inDKIaremoderate).DTIVSDKIinmonitoringpostnatalbrain