Cytosolic diffusivity and microscopic anisotropy of N‐acetyl aspartate in human white matter with diffusion‐weighted MRS at 7 T,NMR in Biomedicine

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Cytosolic diffusivity and microscopic anisotropy of N‐acetyl aspartate in human white matter with diffusion‐weighted MRS at 7 T
NMR in Biomedicine ( IF 2.7 ) Pub Date : 2020-03-31 , DOI: 10.1002/nbm.4304
Henrik Lundell ₁ , Carson Ingo _{2,

3} , Tim B Dyrby _{1,

4} , Itamar Ronen ₅

Affiliation

Metabolite diffusion measurable in humans in vivo with diffusion‐weighted spectroscopy (DW‐MRS) provides a window into the intracellular morphology and state of specific cell types. Anisotropic diffusion in white matter is governed by the microscopic properties of the individual cell types and their structural units (axons, soma, dendrites). However, anisotropy is also markedly affected by the macroscopic orientational distribution over the imaging voxel, particularly in DW‐MRS, where the dimensions of the volume of interest (VOI) are much larger than those typically used in diffusion‐weighted imaging. One way to address the confound of macroscopic structural features is to average the measurements acquired with uniformly distributed gradient directions to mimic a situation where fibers present in the VOI are orientationally uniformly distributed. This situation allows the extraction of relevant microstructural features such as transverse and longitudinal diffusivities within axons and the related microscopic fractional anisotropy. We present human DW‐MRS data acquired at 7 T in two different white matter regions, processed and analyzed as described above, and find that intra‐axonal diffusion of the neuronal metabolite N‐acetyl aspartate is in good correspondence to simple model interpretations, such as multi‐Gaussian diffusion from disperse fibers where the transverse diffusivity can be neglected. We also discuss the implications of our approach for current and future applications of DW‐MRS for cell‐specific measurements.

中文翻译：

N-乙酰天冬氨酸在人白质中的细胞溶质扩散率和微观各向异性，在 7 T 下具有扩散加权 MRS

使用扩散加权光谱 (DW-MRS) 在人体中可测量的代谢物扩散为了解特定细胞类型的细胞内形态和状态提供了一个窗口。白质中的各向异性扩散受单个细胞类型及其结构单元（轴突、胞体、树突）的微观特性控制。然而，各向异性也受到成像体素上宏观方向分布的显着影响，特别是在 DW-MRS 中，其中感兴趣体积 (VOI) 的尺寸远大于扩散加权成像中通常使用的尺寸。解决宏观结构特征混淆的一种方法是平均使用均匀分布的梯度方向获得的测量值，以模拟 VOI 中存在的纤维定向均匀分布的情况。这种情况允许提取相关的微观结构特征，例如轴突内的横向和纵向扩散率以及相关的微观分数各向异性。我们展示了在 7 T 下在两个不同的白质区域采集的人类 DW-MRS 数据，如上所述进行处理和分析，并发现神经元代谢物的轴突内扩散N-乙酰天冬氨酸与简单的模型解释非常吻合，例如来自分散纤维的多高斯扩散，其中横向扩散率可以忽略不计。我们还讨论了我们的方法对 DW-MRS 当前和未来在细胞特异性测量中的应用的影响。

更新日期：2020-03-31

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