Centre d'Imagerie Biomédicale, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
Centre d'Imagerie Biomédicale, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
Neuroimage. 2018 Nov 1;181:314-322. doi: 10.1016/j.neuroimage.2018.07.020. Epub 2018 Jul 11.
A two-compartment model of diffusion in white matter, which accounts for intra- and extra-axonal spaces, is associated with two plausible mathematical scenarios: either the intra-axonal axial diffusivity D is higher than the extra-axonal D (Branch 1), or the opposite, i.e. D < D (Branch 2). This duality calls for an independent validation of compartment axial diffusivities, to determine which of the two cases holds. The aim of the present study was to use an intracerebroventricular injection of a gadolinium-based contrast agent to selectively reduce the extracellular water signal in the rat brain, and compare diffusion metrics in the genu of the corpus callosum before and after gadolinium infusion. The diffusion metrics considered were diffusion and kurtosis tensor metrics, as well as compartment-specific estimates of the WMTI-Watson two-compartment model. A strong decrease in genu T and T relaxation times post-Gd was observed (p < 0.001), as well as an increase of 48% in radial kurtosis (p < 0.05), which implies that the relative fraction of extracellular water signal was selectively decreased. This was further supported by a significant increase in intra-axonal water fraction as estimated from the two-compartment model, for both branches (p < 0.01 for Branch 1, p < 0.05 for Branch 2). However, pre-Gd estimates of axon dispersion in Branch 1 agreed better with literature than those of Branch 2. Furthermore, comparison of post-Gd changes in diffusivity and dispersion between data and simulations further supported Branch 1 as the biologically plausible solution, i.e. D > D. This result is fully consistent with other recent measurements of compartment axial diffusivities that used entirely different approaches, such as diffusion tensor encoding.
一种双室模型的扩散在白质，这考虑到内和外轴空间，与两个合理的数学情景相关联：或者内轴突轴向扩散率 D 高于外轴突 D（分支 1），或者相反，即 D<D（分支 2）。这种二元性要求对隔室轴向扩散率进行独立验证，以确定哪种情况成立。本研究的目的是使用脑室内注射一种基于钆的对比剂来选择性地降低大鼠脑的细胞外水信号，并比较注射前后胼胝体膝部的扩散度量。所考虑的扩散度量包括扩散和峰度张量度量，以及 WMTI-Watson 双室模型的隔室特异性估计。在 Gd 后观察到膝部 T 和 T 弛豫时间的强烈下降（p<0.001），以及径向峰度增加 48%（p<0.05），这意味着细胞外水信号的相对分数被选择性地降低。这进一步得到了从双室模型估计的内轴突水分数增加的支持，对于两个分支都是如此（分支 1 为 p<0.01，分支 2 为 p<0.05）。然而，分支 1 的 Gd 前轴突弥散估计与文献更一致，而分支 2 的则不然。此外，对数据和模拟的 Gd 后扩散率和弥散变化的比较进一步支持分支 1 是生物学上合理的解决方案，即 D>D。这一结果与其他最近使用完全不同方法（如扩散张量编码）测量隔室轴向扩散率的测量结果完全一致。
