Pang Yuxi
Department of Radiology, University of Michigan, Ann Arbor, MI, USA.
Magn Reson Imaging. 2023 Jul;100:73-83. doi: 10.1016/j.mri.2023.03.010. Epub 2023 Mar 23.
To overcome some limitations of previous proton orientation-dependent transverse relaxation formalisms in the human brain white matter (WM) by a generalized magic angle effect function.
A cylindrical helix model was developed embracing anisotropic rotational and translational diffusion of restricted molecules in WM, with the former characterized by an axially symmetric system. Transverse relaxation rates R and R were divided into isotropic R and anisotropic parts, R ∗ f(α,Φ - ε), with α denoting an open angle and ε an orientation (Φ) offset from DTI-derived primary diffusivity direction. The proposed framework (Fit A) was compared to prior models without ε on previously published water and methylene proton transverse relaxation rates from developing, healthy, and pathological WM at 3 T. Goodness of fit was represented by root-mean-square error (RMSE). F-test and linear correlation were used with statistical significance set to P ≤ 0.05.
Fit A significantly (P < 0.01) outperformed prior models as demonstrated by reduced RMSEs, e.g., 0.349 vs. 0.724 in myelin water. Fitted ε was in good agreement with calculated ε from directional diffusivities. Compared with those from healthy adult, the fitted R, R, and α from neonates were substantially reduced but ε increased, consistent with developing myelination. Significant positive (R) and negative (α and R) correlations were found with aging (demyelination) in elderly.
The developed framework can better characterize orientation dependences from a wide range of proton transverse relaxation measurements in the human brain WM, thus shedding new light on myelin microstructural alterations at the molecular level.
通过广义魔角效应函数克服前人在人脑白质（WM）质子取向依赖性横向弛豫形式体系中的一些局限性。
建立了一个圆柱螺旋模型，该模型包含了WM中受限分子的各向异性旋转和平移扩散，前者由轴对称系统表征。横向弛豫率R和R被分为各向同性部分R和各向异性部分R ∗ f(α,Φ - ε)，其中α表示张角，ε表示相对于扩散张量成像（DTI）导出的主扩散率方向的取向（Φ）偏移。将所提出的框架（拟合A）与之前不含ε的模型进行比较，这些模型基于之前发表的3T下发育中、健康和病理性WM的水和亚甲基质子横向弛豫率。拟合优度用均方根误差（RMSE）表示。使用F检验和线性相关性，将统计学显著性设定为P ≤ 0.05。
拟合A表现显著优于之前的模型（P < 0.01），RMSE降低，例如髓鞘水的RMSE为0.349对0.724。拟合得到的ε与根据方向扩散率计算得到的ε高度吻合。与健康成年人相比，新生儿拟合得到的R、R和α显著降低，但ε增加，这与髓鞘形成的发育过程一致。在老年人中发现R与衰老（脱髓鞘）呈显著正相关，α和R与衰老呈显著负相关。
所开发的框架能够更好地表征人脑WM中广泛的质子横向弛豫测量的取向依赖性，从而在分子水平上为髓鞘微观结构改变提供了新的认识。
