DKI (弥散峰度成像) 英文PPT简介

DKI provides a higherorder 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).
defects of DTI • In biological tissue, complex 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 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 .
Kurtosis tensor (KT) derived parameters
• MK(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 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
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Kurtosis
• Kurtosis 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 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 diffusing, this excess diffusional kurtosis can be regarded as a measure of a tissue’s degree of structure.
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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 λ1=λ2=λ3
• 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 λ1>λ2>λ3，最大特征值对应的 方向与经过该体素的纤维束走 行平行
DKI parametric maps
DKI parametric maps
DKI
Diffusional Kurtosis Imaging
Contents
DWI(diffusion weighted imaging) DTI(diffusion tensor imaging) DKI(diffusion kurtosis imaging)
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
Other advantages of DKI
• Mean 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 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 principle water diffusion direction as determined by the 2nd-order diffusion tensor
defects 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.
defects 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 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.
Conditions
• The method is based on the 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 directions are required to construct KT.
DKI vs q-space imaging techniques
• DKI 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 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 values • And DKI is less demanding in terms of hardware requirements and postprocessing effort.
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 （apparent diffusion coefficient ADC） ADC=In(S低/S高)/(b高-b低) •弥散敏感系数(b)值= =r2σ2g2(△-σ/3) b 值的取值范围为0~10 000s/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(T2 shine through effect)，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。