DKI (弥散峰度成像) 英文简介资料.ppt

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1、DKI(弥散峰度成像)英文PPT简介Contents DWI(diffusion weighted imaging)DTI(diffusion tensor imaging)DKI(diffusion kurtosis imaging)defects of DTIConventional DTI fails to fully utilize the MR diffusion measurements that are inherent to tissue microstructure.DTI computes apparent diffusivity based on the assumpti

2、on that diffusion weighted(DW)MR signal has a monoexponential dependence on the diffusion factor(b-value).DTI implicitly assumes that water molecule diffusion occurs in a free and unrestricted environment with a Gaussian distribution of diffusion displacement.defects of DTI In biological tissue,comp

3、lex cellular microstructures make water diffusion a highly hindered or restricted process.Non-monoexponential decays are experimentally observed in both white matter and gray matter.Moreover,the simplified description of the diffusion process in vivo by a 2nd-order 3D diffusivity tensor prevents DTI

4、 from being truly effective in characterizing relatively isotropic tissue such as GM.Even in WM,the DTI model can fail if the tissue contains substantial crossing or diverging fibers.defects of DTIAs a result,DTI quantitation is b-value dependent and DTI fails to fully utilize the diffusion measurem

5、ents that are inherent to tissue microstructure.KurtosisKurtosis here refers to the excess kurtosis that is the normalized and standardized fourth central moment of the water displacement distribution.It is a dimensionless measure that quantifies the deviation of the water diffusion displacement pro

6、file from the Gaussian distribution of unrestricted diffusion,providing a measure of the degree of diffusion hindrance or restriction.fourth central moment: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Since the deviation from Gaussian behavior is governed by the complexity of the tissue within which the water is

7、diffusing,this excess diffusional kurtosis can be regarded as a measure of a tissues degree of structure.Other advantages of DKIMean kurtosis(MK),the average apparent kurtosis along all diffusion gradient encoding directions,has been measured and demonstrated to offer an improved sensitivity in dete

8、cting developmental and pathological changes in neural tissues as compared to conventional DTI.In addition,directional kurtosis analysis has been formulated to reveal directionally specific information,such as the water diffusion kurtoses along the direction parallel or perpendicular to the principl

9、e water diffusion direction as determined by the 2nd-order diffusion tensorDKI provides a higher-order description of restricted water diffusion process by a 2nd-order 3D diffusivity tensor(DT as in conventional DTI)together with a 4th-order 3D kurtosis tensor(KT).ConditionsThe method is based on th

10、e same type of pulse sequences employed for conventional diffusion-weighted imaging(DWI),but the required b values are somewhat larger than those usually used to measure diffusion coefficients.In the brain,b values of about 2000 s/mm2 are sufficient.At least 15 non-collinear and non-coplanar directi

11、ons are required to construct KT.DKI vs q-space imaging techniquesDKI has a close relationship to q-space imaging techniques.q-space imaging methods have indeed recently been employed to estimate diffusional kurtosis.The principal difference between them is that q-space imaging seeks to estimate the

12、 full diffusion displacement probability distribution rather than just the kurtosis.As a consequence,q-space imaging is more demanding in terms of imaging time and gradient strengths.Measuring the diffusional kurtosis requires only modest increases in b valuesAnd DKI is less demanding in terms of ha

13、rdware requirements and postprocessing effort.Kurtosis tensor(KT)derived parametersMK(mean kurtosis):MK is a measure of the overall kurtosis.It does not have any directional specificity.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(Axial kurtosis)and K(Radial kurtosis):can be defined as the

14、kurtosis parallel and perpendicular to the principle diffusion eigenvector(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 (fractional anisotropy of kurtosis)Similar to FA in DTI,the anisotropy of directional kurtosis can be conveniently defined as FAK KA 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DKI parametric ma

15、psDKI parametric mapsTypical DKI-derived parametric maps from a single slice of a)in vivo,b)formalin-fixed adult rat brains and c)a normal human subject(male,44 years old).Axial diffusivity(/),radial diffusivity(),mean diffusivity(MD),axial kurtosis(K/),radial kurtosis(K),mean kurtosis(MK),fractiona

16、l anisotropy(FA),directionally encoded colour FA(DEC-FA)and fractional anisotropy of kurtosis(FAK)maps are computed from DKI model.DKI parametric mapsFor(a),raw DWIs were acquired by SE EPI with TR/TE=3000/30.3ms,/=5/17ms,slice thickness=1mm,FOV=3030mm2,data matrix=128128(zero filled to 256256),NEX=

17、4,6 b-values(0.0,0.5,1.0,1.5,2.0 and 2.5ms/m2)and along 30 directions using 7T scannerDKI parametric mapsFor(b),raw DWIs were acquired with the same parameters as those for in vivo except TE=34.3ms,=9ms and b-values of 0.0,1.0,2.0,3.0,4.0 and 5.0ms/m2.A larger b-value range was used in ex vivo exper

18、iment due to the generally lower diffusivities.DKI parametric maps For(c),raw DWIs were acquired by SE EPI with TR/TE=2300/109ms,slice thickness=2mm,FOV=256256mm2,data matrix=128128,NEX=2,6 b-values(0.0,0.5,1.0,1.5,2.0 and 2.5ms/m2)and along 30 directions using a 3T Siemens scanner DKI parametric ma

19、ps Higher MK is found in WM,indicating a generally higher degree of diffusion complexity and restriction in the WM structures.It can be seen from the directional kurtosis maps that such high MK in WM is mainly contributed by K.This suggests the existence of heterogeneity and restricted diffusion in

20、axonal structuresBoth MK and K exhibit strong contrast between WM and GM structures.DKI parametric mapsBoth MK and K exhibit strong contrast between WM and GM structures.Positive mean and directional kurtoses are observed in both WM and GM,indicating faster DW signal decay at lower b-values and rest

21、ricted diffusion environment in both WM and GM under in vivo and formalin-fixed conditions.DKI shows a general decrease in diffusivity and increase in kurtosis in WM and GM of the fixed brainsThe breakdowns of neurofilaments and microtubules caused by fixatives are believed to produce more diffusion

22、 barriers and hence lead to the/decrease and K/increase.Other fixation effects such as tissue shrinkage,decrease in membrane permeability,increase in axonal packing density and reduction of extracellular space in parenchyma also likely contribute to the significant decrease and K increase.Directiona

23、l kurtosis analysis of fixed experimental autoimmune encephalitis(EAE)spinal cordThe inflammatory neurodegenerative disease EAE is characterized by both axonal loss and demyelinationIn recent DKI studies,there are promising results of using MK to detect changes in normal or pathological neural tissu

24、e However,as an average of kurtoses along all the diffusion directions,MK can lose sensitivity and specificity in probing directional changes of pathological tissueEAE spinal cordK/is found to be significantly increased and/decreased in the lesion area/reduction is likely due to cytoskeletal perturb

25、ation or debris formation when the axonal structures break downIn addition,increases whereas K decreases likely because of the demyelination and axonal loss that also lead to less diffusion restriction in radial direction.EAE spinal cordThe directionally averaged MD and MK are found to be less sensi

26、tive to EAE pathology due to the opposite trends of diffusivity and kurtosis changes in axial and radial direction.Monitoring postnatal brain maturation by conventional DTICC:corpus callosum（胼胝体）;EC:external capsule（外囊）;CP:cerebral peduncle（大脑脚）;AC:anterior commissure;（前联合）CT:cerebral cortex（脑皮质）;HP

27、:hippocampus（海马）;CPu:caudate putamen（新纹状体）Monitoring postnatal brain maturation by conventional DTIThe sensitivity of/in detecting rat brain WM maturation is generally observed to be the highest at low b-value At relatively low b-values,the apparent diffusivity is primarily contributed from the fast

28、 water diffusion activities in extracellular space that depend on both cellular microstructure and membrane permeability.The use of low b-value can best detect these changes.The high/sensitivity at low b-value observed in the current study suggests the alterations of these fast water diffusion activ

29、ities along axonal direction during brain maturation.Such alterations may result from the increase in packing density of fiber bundles and axons,axonal diameter increase,changes in neurofibrils,and increased complexity of extracellular matrix.Monitoring postnatal brain maturation by conventional DTI

30、Whereas that of is the highest at high b-valueThe diffusion changes probed in WM using high b-values are ascribed more to the slow water molecule diffusion particularly along the radial direction when traversing the membranes and myelin sheathsThe high sensitivity of at high b-value in detecting bra

31、in maturation shown in the figure likely reflects these WM microstructural changes,including myelination and axonal density and diameter changes during postnatal brain development.Monitoring postnatal brain maturation by conventional DTIFA quantitation is also affected by the b-value and its ability

32、 in detecting brain maturational changes varies among different structures.Monitoring postnatal brain maturation by DKIFigure 7a shows that the sensitivity of fitting all the multi-b-value DWIs to DTI model is generally similar to that of employing a medium b-value(b=1.5ms/m2)shown in Figure 6In Fig

33、ure 7b,the general and continual kurtosis increase with age is observed,indicating that more diffusion restriction occurs during brain maturation in both WM and GM structures.The DKI-derived diffusivity and kurtosis indices are highly sensitive to brain developmental changes.Monitoring postnatal bra

34、in maturation by DKI Both/and K/of WM are found to increases with age,which may arise from various biological events during early postnatal brain maturation.The increase of diffusivity can be caused by axoplasmic flow during the myelination periodneuronal loss and axonal pruning that shortens the ax

35、on length can lead to an increase of restrictionMonitoring postnatal brain maturation by DKIThe increase of K in WM is likely ascribed to the myelination and modification of axonal structures that increases restriction in the radial direction.DKI analysis also reveals that diffusion restriction in t

36、he relatively isotropic GM increases with age.This may reflect the more densely packed structures and the dendritic architectural modification in GM DTI VS DKI in monitoring postnatal brain maturation When there is a large K,the estimated diffusivity in conventional DTI shows a large discrepancy wit

37、h the diffusivity estimated in DKI approach.As K in all the structures is positive,DTI-derived diffusivities are generally lower than those by DKI.The relatively high sensitivity of the in monoexponential DTI model is mainly a result of increasing K with age(while the changes of in DKI are moderate)

38、.DTI VS DKI in monitoring postnatal brain maturationDTI-derived/is related to the increase of both K/and/derived in DKI that manifests opposite and competing effects.herefore diminished sensitivity in detecting maturational changes of/in conventional DTI are observed.The separation of/and K/can impr

39、ove the characterization of neural tissue along the axial direction.Because the complex biological modification of WM along axonal direction affects both diffusivity and kurtosis,information obtained in conventional DTI is inadequate to fully infer the microstructural changes during brain maturation

40、.Other applicationsDKI may serve as a more sensitive tool to detect and characterize such subtle changes in both WM and GM.DKI has also been applied in various pathological states,including Alzheimers disease,schizophrenia and attention deficit and hyperactivity disorderDKI has also been sought to resolve the crossing of WM fibers and possibly lead to more accurate tracking and characterization.From:WU Menglin