DKI彌散峰度成像英文PPT簡介

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1、2021/8/61Diffusional Kurtosis Imaging DKI2021/8/62Contents DWI(diffusion weighted imaging)DTI(diffusion tensor imaging)DKI(diffusion kurtosis imaging)2021/8/63DWI原理組織T1、T2馳豫時間、H1的密度、分子彌散運動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 2021/8/64DWI評估彌散的參數 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（ap

2、parent diffusion coefficient ADC）ADC=In(S低/S高)/(b高-b低)彌散敏感系數(b)值=r22g2(-/3)b 值的取值范圍為010 000s/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，產生所謂的T2 透射效應(T2 shine through effect)，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2021/8/65 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

3、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 1=2=32021/8/66defects of DTI Conventional DTI fails to fully utilize the MR diffusion measurements that are inherent to tissue microstructure.DTI computes apparent diffusivity based on the assumption that diffusion weighted(DW)MR signal has a monoexponential depend

4、ence 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.2021/8/67defects of DTI In biological tissue,complex cellular microstructures make water diffusion a highly

5、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 from being truly effective in characterizing relatively is

6、otropic tissue such as GM.Even in WM,the DTI model can fail if the tissue contains substantial crossing or diverging fibers.2021/8/68defects of DTI As a result,DTI quantitation is b-value dependent and DTI fails to fully utilize the diffusion measurements that are inherent to tissue microstructure.2

7、021/8/69KurtosisKurtosis 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 profile from the Gaussian distribution of u

8、nrestricted diffusion,providing a measure of the degree of diffusion hindrance or restriction.fourth central moment: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Since the deviation from Gaussian behavior is governed by the complexity of the tissue within which the water is diffusing,this excess diffusional kurtos

9、is can be regarded as a measure of a tissues degree of structure.2021/8/610Other 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 detecting developmental and pathol

10、ogical 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 principle water diffusion direction as

11、 determined by the 2nd-order diffusion tensor2021/8/611DKI 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).2021/8/612ConditionsThe method is based on the same typ

12、e 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 directions are re

13、quired to construct KT.2021/8/613DKI 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

14、 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

15、rdware requirements and postprocessing effort.2021/8/614Kurtosis tensor(KT)derived parametersMK(mean kurtosis):MK is a measure of the overall kurtosis.It does not have any directional specificity.MK 的大小取決于感興趣區內組織的結構復雜程度，結構越復雜非正態分布水分子擴散受限越顯著，MK 也即越大K(Axial kurtosis)and K(Radial kurtosis):can be defin

16、ed as the kurtosis parallel and perpendicular to the principle diffusion eigenvector(e1)K越大表明在該方向非正態分布水分子擴散受限越明顯，反之則表明擴散受限越弱FAK (fractional anisotropy of kurtosis)Similar to FA in DTI,the anisotropy of directional kurtosis can be conveniently defined as FAK KA 越小即表示越趨于各向同性擴散;若組織結構越緊密越規則，KA 越大2021/8/

17、615DKI parametric maps2021/8/616DKI parametric maps Typical 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

18、(K),mean kurtosis(MK),fractional anisotropy(FA),directionally encoded colour FA(DEC-FA)and fractional anisotropy of kurtosis(FAK)maps are computed from DKI model.2021/8/617DKI parametric maps For(a),raw DWIs were acquired by SE EPI with TR/TE=3000/30.3ms,/=5/17ms,slice thickness=1mm,FOV=3030mm2,data

19、 matrix=128128(zero filled to 256256),NEX=4,6 b-values(0.0,0.5,1.0,1.5,2.0 and 2.5ms/m2)and along 30 directions using 7T scanner2021/8/618DKI parametric maps For(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 5.0m

20、s/m2.A larger b-value range was used in ex vivo experiment due to the generally lower diffusivities.2021/8/619DKI 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 a

21、long 30 directions using a 3T Siemens scanner 2021/8/620DKI parametric maps 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.

22、This suggests the existence of heterogeneity and restricted diffusion in axonal structuresBoth MK and K exhibit strong contrast between WM and GM structures.2021/8/621DKI parametric maps Both MK and K exhibit strong contrast between WM and GM structures.Positive mean and directional kurtoses are obs

23、erved in both WM and GM,indicating faster DW signal decay at lower b-values and restricted diffusion environment in both WM and GM under in vivo and formalin-fixed conditions.2021/8/622DKI shows a general decrease in diffusivity and increase in kurtosis in WM and GM of the fixed brains The breakdown

24、s of neurofilaments and microtubules caused by fixatives are believed to produce more diffusion 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 s

25、pace in parenchyma also likely contribute to the significant decrease and K increase.2021/8/623Directional kurtosis analysis of fixed experimental autoimmune encephalitis(EAE)spinal cord The inflammatory neurodegenerative disease EAE is characterized by both axonal loss and demyelination In recent D

26、KI studies,there are promising results of using MK to detect changes in normal or pathological neural tissue However,as an average of kurtoses along all the diffusion directions,MK can lose sensitivity and specificity in probing directional changes of pathological tissue2021/8/624EAE spinal cordK/is

27、 found to be significantly increased and/decreased in the lesion area/reduction is likely due to cytoskeletal perturbation 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 dif

28、fusion restriction in radial direction.2021/8/625EAE spinal cord The directionally averaged MD and MK are found to be less sensitive to EAE pathology due to the opposite trends of diffusivity and kurtosis changes in axial and radial direction.2021/8/626Monitoring postnatal brain maturation by conven

29、tional DTICC:corpus callosum（胼胝體）;EC:external capsule（外囊）;CP:cerebral peduncle（大腦腳）;AC:anterior commissure;（前聯合）CT:cerebral cortex（腦皮質）;HP:hippocampus（海馬）;CPu:caudate putamen（新紋狀體）2021/8/627Monitoring postnatal brain maturation by conventional DTIThe sensitivity of/in detecting rat brain WM maturati

30、on 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 water diffusion activities in extracellular space that depend on both cellular microstructure and membrane permeability.The use of low b-value can best

31、 detect these changes.The high/sensitivity at low b-value observed in the current study suggests the alterations of these fast water diffusion activities along axonal direction during brain maturation.Such alterations may result from the increase in packing density of fiber bundles and axons,axonal

32、diameter increase,changes in neurofibrils,and increased complexity of extracellular matrix.2021/8/628Monitoring postnatal brain maturation by conventional DTIWhereas 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 molec

33、ule diffusion particularly along the radial direction when traversing the membranes and myelin sheathsThe high sensitivity of at high b-value in detecting brain maturation shown in the figure likely reflects these WM microstructural changes,including myelination and axonal density and diameter chang

34、es during postnatal brain development.2021/8/629Monitoring postnatal brain maturation by conventional DTIFA quantitation is also affected by the b-value and its ability in detecting brain maturational changes varies among different structures.2021/8/630Monitoring postnatal brain maturation by DKIFig

35、ure 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 Figure 7b,the general and continual kurtosis increase with age is observed,indicating that more diffusion restriction occurs

36、during brain maturation in both WM and GM structures.The DKI-derived diffusivity and kurtosis indices are highly sensitive to brain developmental changes.2021/8/631Monitoring postnatal brain maturation by DKI Both/and K/of WM are found to increases with age,which may arise from various biological ev

37、ents 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 axon length can lead to an increase of restriction2021/8/632Monitoring postnatal brain maturation by DKI The incre

38、ase 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 the relatively isotropic GM increases with age.This may reflect the more densely packed structures and

39、 the dendritic architectural modification in GM 2021/8/633DTI VS DKI in monitoring postnatal brain maturation When there is a large K,the estimated diffusivity in conventional DTI shows a large discrepancy with the diffusivity estimated in DKI approach.As K in all the structures is positive,DTI-deri

40、ved 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).2021/8/634DTI VS DKI in monitoring postnatal brain maturationDTI-derived/is related to the

41、 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 improve the characterization of neural tissue along the axial direction.Because the c

42、omplex 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.2021/8/635Other applications DKI may serve as a more sensitive tool to detect an

43、d 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 disorder DKI has also been sought to resolve the crossing of WM fibers and possibly lead to more accurate tracking and characterization.2021/8/636From:WU Menglin部分資料從網絡收集整理而來，供大家參考，感謝您的關注！