From diffusion-weighted MRI to anomalous diffusion imaging
From diffusion-weighted MRI to anomalous diffusion imaging
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DOI：
10.1002/mrm.21453
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发表时间：
2008-03-01
影响因子：
3.3
通讯作者：
Barrick, Thomas R.
中科院分区：
文献类型：
--
作者：
Hall, Matt G.;Barrick, Thomas R.
我们根据反常扩散理论，对具有 b 值的非单指数扩散加权信号衰减提出了一种新颖的解释。反常扩散是在局部不均匀的环境（例如脑组织）中扩散粒子的理论。在这种环境中，通常用于分析扩散MR数据的受限扩散模型是无效的，导致自旋均方位移的非线性时间依赖性，以及信号衰减的拉伸指数形式的预测。我们表明，这种预测直接导致了一个新参数，即反常指数，它可以通过扫描数据进行测量，并据此我们可以估计分形维数 d(w)，它对扩散自旋偏移的复杂性进行分类。我们从体内人脑数据构建异常指数和分形维数的图像。指数和维数的分布是在灰质、白质和脑脊液中构建的。我们观察到这些分布在生物学上合理的值处达到峰值，与之前的研究一致：灰质 d(w) = 2.366 +/- 0.31，白质 d(w) = 2.587 +/- 0.39，CSF d(w) = 1.970（众数）。与侧脑室脑脊液相比，灰质和白质之间观察到明显的对比。然后，我们考虑异常指数值的各向异性，并定义类似于通常从扩散张量图像生成的平均扩散率和分数各向异性的量。
We present a novel interpretation of non-monoexponential diffusion-weighted signal decay with b-value in terms of the theory of anomalous diffusion. Anomalous diffusion is the theory of diffusing particles in environments that are not locally homogeneous, such as brain tissue. In such environments the model of restricted diffusion commonly employed in the analysis of diffusion MR data is not valid, leading to a nonlinear time dependence for the mean-squared displacement of spins, and to a prediction of a stretched exponential form for the signal decay. We show that this prediction leads directly to a new parameter, the anomalous exponent, which may be measured from scan data and from this we can estimate a fractal dimension, d(w), which categorizes the complexity of the excursions of diffusing spins. We construct images of the anomalous exponent and fractal dimension from in vivo human brain data. Distributions of exponents and dimensions are constructed in grey and white matter and cerebrospinal fluid. We observe that these distributions peak at biologically plausible values consistent with previous studies: grey matter d(w) = 2.366 +/- 0.31, white matter d(w) = 2.587 +/- 0.39, CSF d(w) = 1.970 (mode). Marked contrast is observed between grey and white matter when compared with lateral ventricle CSF. We then consider the anisotropy of the value of the anomalous exponent and define quantities analogous to the mean diffusivity and fractional anisotropy that are commonly generated from diffusion tensor images.