Department of Radiology, University of California, San Diego, La Jolla, California, USA.
Hum Brain Mapp. 2013 Feb;34(2):327-46. doi: 10.1002/hbm.21454. Epub 2012 Jan 16.
Water diffusion magnetic resonance imaging (dMRI) is a powerful tool for studying biological tissue microarchitectures in vivo. Recently, there has been increased effort to develop quantitative dMRI methods to probe both length scale and orientation information in diffusion media. Diffusion spectrum imaging (DSI) is one such approach that aims to resolve such information based on the three-dimensional diffusion propagator at each voxel. However, in practice, only the orientation component of the propagator function is preserved when deriving the orientation distribution function. Here, we demonstrate how a straightforward extension of the linear spherical deconvolution (SD) model can be used to probe tissue orientation structures over a range (or "spectrum") of length scales with minimal assumptions on the underlying microarchitecture. Using high b-value Cartesian q-space data on a rat brain tissue sample, we demonstrate how this "restriction spectrum imaging" (RSI) model allows for separating the volume fraction and orientation distribution of hindered and restricted diffusion, which we argue stems primarily from diffusion in the extraneurite and intraneurite water compartment, respectively. Moreover, we demonstrate how empirical RSI estimates of the neurite orientation distribution and volume fraction capture important additional structure not afforded by traditional DSI or fixed-scale SD-like reconstructions, particularly in gray matter. We conclude that incorporating length scale information in geometric models of diffusion offers promise for advancing state-of-the-art dMRI methods beyond white matter into gray matter structures while allowing more detailed quantitative characterization of water compartmentalization and histoarchitecture of healthy and diseased tissue.
水磁共振弥散成像（dMRI）是一种强大的工具，可用于研究体内生物组织的微观结构。最近，人们越来越努力开发定量 dMRI 方法，以探测扩散介质中的长度尺度和方向信息。扩散谱成像（DSI）就是这样一种方法，它旨在根据每个体素的三维扩散传播子来解析这些信息。然而，在实践中，当推导出方向分布函数时，仅保留传播子函数的方向分量。在这里，我们展示了如何通过对线性球分解（SD）模型的简单扩展，在最小的微观结构假设下，在一系列（或“谱”）长度尺度上探测组织的方向结构。我们使用大鼠脑组织样本的高 b 值笛卡尔 q 空间数据，演示了这种“限制谱成像”（RSI）模型如何允许分离受阻和受限扩散的体积分数和方向分布，我们认为这主要源于细胞外和细胞内水隔间中的扩散。此外，我们展示了经验 RSI 对神经突方向分布和体积分数的估计如何捕获传统 DSI 或固定尺度 SD 样重建所无法提供的重要附加结构，尤其是在灰质中。我们得出的结论是，在扩散的几何模型中纳入长度尺度信息，有望将最先进的 dMRI 方法从白质推进到灰质结构，同时允许对水隔间和健康组织和病变组织的组织架构进行更详细的定量描述。
