Mesoscale diffusion magnetic resonance imaging (MRI) endeavors to bridge the gap between macroscopic white matter tractography and microscopic studies investigating the cytoarchitecture of human brain tissue. To ensure a robust measurement of diffusion at the mesoscale, acquisition parameters were arrayed to investigate their effects on scalar indices (mean, radial, axial diffusivity, and fractional anisotropy) and streamlines (i.e., graphical representation of axonal tracts) in hippocampal layers. A mesoscale resolution afforded segementation of the pyramidal cell layer (CA1‐4), the dentate gyrus, as well as stratum moleculare, radiatum, and oriens. Using ex vivo samples, surgically excised from patients with intractable epilepsy (n = 3), we found that shorter diffusion times (23.7 ms) with a b‐value of 4,000 s/mm² were advantageous at the mesoscale, providing a compromise between mean diffusivity and fractional anisotropy measurements. Spatial resolution and sample orientation exerted a major effect on tractography, whereas the number of diffusion gradient encoding directions minimally affected scalar indices and streamline density. A sample temperature of 15°C provided a compromise between increasing signal‐to‐noise ratio and increasing the diffusion properties of the tissue. Optimization of the acquisition afforded a system's view of intra‐ and extra‐hippocampal connections. Tractography reflected histological boundaries of hippocampal layers. Individual layer connectivity was visualized, as well as streamlines emanating from individual sub‐fields. The perforant path, subiculum and angular bundle demonstrated extra‐hippocampal connections. Histology of the samples confirmed individual cell layers corresponding to ROIs defined on MR images. We anticipate that this ex vivo mesoscale imaging will yield novel insights into human hippocampal connectivity.

中文翻译：

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离体人海马的中尺度扩散磁共振成像。

中尺度扩散磁共振成像 (MRI) 致力于弥合宏观白质纤维束成像和研究人脑组织细胞结构的微观研究之间的差距。为了确保对中尺度扩散的稳健测量，排列采集参数以研究它们对海马层中标量指数（平均、径向、轴向扩散率和分数各向异性）和流线（即轴突束的图形表示）的影响。中尺度分辨率提供了锥体细胞层 (CA1-4)、齿状回以及分子层、辐射层和东方的分割。使用从难治性癫痫患者（n= 3)，我们发现更短的扩散时间 (23.7 ms)，ab 值为 4,000 s/mm ²在中尺度上是有利的，提供了平均扩散率和分数各向异性测量之间的折衷。空间分辨率和样本方向对纤维束成像产生重大影响，而扩散梯度编码方向的数量对标量指数和流线密度的影响最小。15°C 的样品温度在提高信噪比和提高组织的扩散特性之间提供了一种折衷方案。采集的优化为系统提供了海马内和海马外连接的视图。纤维束成像反映了海马层的组织学边界。各个层的连通性以及来自各个子领域的流线都被可视化了。穿孔路径、下托和角束显示出海马外连接。样本的组织学证实了与 MR 图像上定义的 ROI 相对应的单个细胞层。我们预计这种离体中尺度成像将对人类海马连接产生新的见解。