DKI (弥散峰度成像)ppt课件

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
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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）
• DTI implicitly assumes that water molecule diffusion occurs in a free and unrestricted environment with a Gaussian distribution of diffusion displacement.
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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.
DKI
Diffusional Kurtosis Imaging
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Contents
DWI(diffusion weighted imaging) DTI(diffusion tensor imaging) DKI(diffusion kurtosis imaging)
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DWI原理
MR图像的信号
组织T1、T2驰豫
• 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 .
• 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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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.
• 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.
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。
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Kurtosis
• Kurtosis here refΒιβλιοθήκη Baidurs to the excess kurtosis that is the normalized and standardized fourth central moment of the water displacement distribution .
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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
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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).