Bennett Kevin M, Schmainda Kathleen M, Bennett Raoqiong Tong, Rowe Daniel B, Lu Hanbing, Hyde James S
Department of Biophysics, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, USA.
Magn Reson Med. 2003 Oct;50(4):727-34. doi: 10.1002/mrm.10581.
Experience with diffusion-weighted imaging (DWI) shows that signal attenuation is consistent with a multicompartmental theory of water diffusion in the brain. The source of this so-called nonexponential behavior is a topic of debate, because the cerebral cortex contains considerable microscopic heterogeneity and is therefore difficult to model. To account for this heterogeneity and understand its implications for current models of diffusion, a stretched-exponential function was developed to describe diffusion-related signal decay as a continuous distribution of sources decaying at different rates, with no assumptions made about the number of participating sources. DWI experiments were performed using a spin-echo diffusion-weighted pulse sequence with b-values of 500-6500 s/mm(2) in six rats. Signal attenuation curves were fit to a stretched-exponential function, and 20% of the voxels were better fit to the stretched-exponential model than to a biexponential model, even though the latter model had one more adjustable parameter. Based on the calculated intravoxel heterogeneity measure, the cerebral cortex contains considerable heterogeneity in diffusion. The use of a distributed diffusion coefficient (DDC) is suggested to measure mean intravoxel diffusion rates in the presence of such heterogeneity.
扩散加权成像（DWI）的经验表明，信号衰减与大脑中水分子扩散的多隔室理论一致。这种所谓的非指数行为的来源是一个有争议的话题，因为大脑皮层包含相当大的微观异质性，因此难以建模。为了解释这种异质性并理解其对当前扩散模型的影响，开发了一种拉伸指数函数，将扩散相关的信号衰减描述为以不同速率衰减的源的连续分布，而不对参与源的数量做任何假设。使用自旋回波扩散加权脉冲序列对六只大鼠进行DWI实验，b值为500 - 6500 s/mm²。信号衰减曲线拟合为拉伸指数函数，20%的体素对拉伸指数模型的拟合优于双指数模型，尽管后者模型有一个更多的可调参数。基于计算出的体素内异质性测量值，大脑皮层在扩散方面存在相当大的异质性。建议使用分布式扩散系数（DDC）来测量存在这种异质性时的平均体素内扩散速率。
