The direction-dependence of apparent water exchange rate in human white matter
The direction-dependence of apparent water exchange rate in human white matter
复制标题
人体白质表观水交换率的方向依赖性
DOI：
10.1016/j.neuroimage.2021.118831
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发表时间：
2021-12-21
期刊：
影响因子：
5.7
通讯作者：
Bai，Ruiliang
中科院分区：
文献类型：
--
作者：
Li，Zhaoqing;Pang，Zhenfeng;Bai，Ruiliang
跨膜水交换是诊断和理解癌症、脑部疾病和其他疾病的潜在生物标志物。过滤交换成像（FEXI），扩散交换光谱适用于临床应用的特殊情况下，有可能揭示不同的生理水交换过程。然而，它仍然是有争议的，是否调制的扩散编码梯度方向可以影响的表观交换率（AXR）测量的FEXI在白色物质（WM），水扩散表现出强烈的各向异性。在这项研究中，我们探讨了扩散编码方向的FEXI在人脑白色物质的依赖性，通过执行FEXI与20个扩散编码方向上的临床3 T扫描仪在体内。结果表明，在具有一致取向纤维的WM体素中，当梯度垂直于纤维取向时测量的AXR值（0.77 ± 0.13s-1，所有受试者的平均值±标准差）显著大于当梯度平行于纤维取向时的AXR估计值（0.33 ± 0.14s-1，p< 0.001）。此外，在沿着这两个方向测量的AXR之间没有发现显著的相关性，表明它们测量的是不同的水分交换过程。此外，只有垂直AXR而不是平行AXR显示出对轴突直径的依赖性，表明垂直AXR可能反映了轴突内和细胞外空间之间的跨膜水交换。进一步的有限差分（FD）模拟有三个水隔室（轴突内，神经胶质内，和细胞外空间），以模仿WM微环境也表明，垂直AXR是更敏感的轴突水跨膜交换比平行AXR。综上所述，我们的研究结果表明，沿沿着不同方向测量的AXR可以用来探测WM中不同的水交换过程。
Transmembrane water exchange is a potential biomarker in the diagnosis and understanding of cancers, brain disorders, and other diseases. Filter-exchange imaging (FEXI), a special case of diffusion exchange spectroscopy adapted for clinical applications, has the potential to reveal different physiological water exchange processes. However, it is still controversial whether modulating the diffusion encoding gradient direction can affect the apparent exchange rate (AXR) measurements of FEXI in white matter (WM) where water diffusion shows strong anisotropy. In this study, we explored the diffusion-encoding direction dependence of FEXI in human brain white matter by performing FEXI with 20 diffusion-encoding directions on a clinical 3T scannerin-vivo. The results show that the AXR values measured when the gradients are perpendicular to the fiber orientation (0.77 ± 0.13s−1, mean ± standard deviation of all the subjects) are significantly larger than the AXR estimates when the gradients are parallel to the fiber orientation (0.33 ± 0.14s−1,p< 0.001) in WM voxels with coherently-orientated fibers. In addition, no significant correlation is found between AXRs measured along these two directions, indicating that they are measuring different water exchange processes. What's more, only the perpendicular AXR rather than the parallel AXR shows dependence on axonal diameter, indicating that the perpendicular AXR might reflect transmembrane water exchange between intra-axonal and extra-cellular spaces. Further finite difference (FD) simulations having three water compartments (intra-axonal, intra-glial, and extra-cellular spaces) to mimic WM micro-environments also suggest that the perpendicular AXR is more sensitive to the axonal water transmembrane exchange than parallel AXR. Taken together, our results show that AXR measured along different directions could be utilized to probe different water exchange processes in WM.