扩散时间依赖性、幂律标度和灰质交换
Diffusion time dependence, power-law scaling, and exchange in gray matter
摘要：
扩散MRI（dMRI）提供反映扩散自旋与生物系统的微结构特征的相互作用的对比度，但由于其与底层微结构的关系的模糊性，其特异性仍然有限。为了提高特异性，白质（WM）的生物物理模型通常根据标准模型（SM）表达dMRI信号，并且最近在灰质（GM）中尝试并入细胞体（SANDI模型）。然而，这些模型所依据的假设的有效性在很大程度上仍然没有确定，特别是在通用汽车公司。观察模型的独特功能特性，如与一维扩散相关的$b^{-1/2}$幂律，已成为实验验证的一种富有成效的策略。在GM中没有这种签名已经被解释为神经突水交换，非线性形态，和/或模糊体细胞信号的贡献。在这里，我们提出了在现实的神经元模拟表明，曲率和分支不破坏坚持幂律在不可渗透的神经突，但他们的信号被淹没的胞体在典型的实验条件下：然而，我们确定了一个可实现的实验制度，其中神经突信号占主导地位。此外，我们发现，交换驱动的时间依赖性产生的行为相反，预期从限制扩散，从而提供了一个功能签名消除歧义的两种效果。我们目前的数据在离体大鼠大脑dMRI实验在1000场，并观察到一个时间依赖性与大量的交流和GM棒幂律一致。第一个发现表明有显著的水交换，而第二个发现表明有一小群不可渗透的神经突。为了量化我们的观测结果，我们利用Kärger交换模型，并将相应的信号时间依赖性纳入SM和SANDI。
英文摘要：
Diffusion MRI (dMRI) provides contrast that reflect diffusing spins' interactions with microstructural features of biological systems, but its specificity remains limited due to the ambiguity of its relation to the underlying microstructure. To improve specificity, biophysical models of white matter (WM) typically express dMRI signals according to the Standard Model (SM) and have more recently in gray matter (GM) attempted to incorporate cell soma (the SANDI model). The validity of the assumptions underlying these models, however, remains largely undetermined, especially in GM. Observing the models' unique, functional properties, such as the $b^{-1/2}$ power-law associated with 1d diffusion, has emerged as a fruitful strategy for experimental validation. The absence of this signature in GM has been explained by neurite water exchange, non-linear morphology, and/or obscuring soma signal contributions. Here, we present simulations in realistic neurons demonstrating that curvature and branching does not destroy the stick power-law in impermeable neurites, but that their signal is drowned by the soma under typical experimental conditions: Nevertheless, we identify an attainable experimental regime in which the neurite signal dominates. Furthermore, we find that exchange-driven time dependence produces a behavior opposite to that expected from restricted diffusion, thereby providing a functional signature disambiguating the two effects. We present data from dMRI experiments in ex vivo rat brain at ultrahigh field and observe a time dependence consistent with substantial exchange and a GM stick power-law. The first finding suggests significant water exchange while the second suggests a small sub-population of impermeable neurites. To quantify our observations, we harness the Kärger exchange model and incorporate the corresponding signal time dependence in SM and SANDI.
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