用代谢物和水的双扩散编码磁共振波谱研究人脑灰质和白质的房室扩散和显微结构特性
Compartmental diffusion and microstructural properties of human brain gray and white matter studied with double diffusion encoding magnetic resonance spectroscopy of metabolites and water
摘要：
水信号的双扩散编码（DDE）磁共振测量提供了一种独特的能力，从细胞的整体宏观取向分布的组织结构单元中的微观各向异性扩散的影响分开。然而，检测到的微观各向异性的特异性是有限的，因为信号在不同的细胞类型和跨组织区室上被平均。并行执行代谢物DDE光谱（DDES）和水DDES，其中使用广泛的b值逐渐消除细胞外贡献，提供了补充措施，从中可以估计细胞内和细胞外微观分数各向异性（$μ$ FA）和扩散率。代谢物主要局限于细胞内空间，因此为特定细胞类型的细胞内扩散率提供了基准。在这里，我们的目的是通过结合水和代谢物DDES实验来估计组织和隔室特异性人脑微观结构。我们在7 T MRI扫描仪上对人类受试者的两个脑区进行了DDES，这两个脑区包含大量不同的白质（WM）和灰质（GM）：顶叶白质（PWM）和枕叶灰质（OGM）。PWM和OGM中代谢物DDES实验的结果表明神经元和神经胶质内存在高度各向异性的细胞内空间，灰质神经胶质可能除外。水和tNAA在细胞质中的曲折度值，通过显微平行扩散率相对于感兴趣体积中的GM/WM组织分数的相关性分析获得，对于两种分子来说是非常相似的，同时在灰质和白质之间表现出明显的差异，提示GM中的神经元细胞体和树突比WM中的长程轴突中发现的那些更拥挤的细胞质和更复杂的细胞形态学。
英文摘要：
Double diffusion encoding (DDE) magnetic resonance measurements of the water signal offers a unique ability to separate the effect of microscopic anisotropic diffusion in structural units of tissue from the overall macroscopic orientational distribution of cells. However, the specificity in detected microscopic anisotropy is limited as the signal is averaged over different cell types and across tissue compartments. Performing side-by-side metabolite DDE spectroscopy (DDES) and water DDES in which a wide range of b-values is used to gradually eliminate the extracellular contribution provides complementary measures from which intracellular and extracellular microscopic fractional anisotropies ($μ$FA) and diffusivities can be estimated. Metabolites are largely confined to the intracellular space and therefore provide a benchmark for intracellular diffusivity of specific cell types. Here, we aimed to estimate tissue- and compartment-specific human brain microstructure by combining water and metabolites DDES experiments. We performed DDES in human subjects in two brain regions that contain widely different amounts of white matter (WM) and gray matter (GM): parietal white matter (PWM) and occipital gray matter (OGM) on a 7 T MRI scanner. Results of the metabolite DDES experiments in both PWM and OGM suggest a highly anisotropic intracellular space within neurons and glia, with the possible exception of gray matter glia. Tortuosity values in the cytoplasm for water and tNAA, obtained with correlation analysis of microscopic parallel diffusivity with respect to GM/WM tissue fraction in the volume of interest, are remarkably similar for both molecules, while exhibiting a clear difference between gray and white matter, suggesting a more crowded cytoplasm and more complex cytomorphology of neuronal cell bodies and dendrites in GM than those found in long-range axons in WM.
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