Cardiff University Brain Research Imaging Centre (CUBRIC), School of Physics and Astronomy, Cardiff University, Cardiff, UK; University Medical Center Utrecht, Utrecht University, Utrecht, the Netherlands.
Cardiff University Brain Research Imaging Centre (CUBRIC), School of Psychology, Cardiff University, Cardiff, UK.
Neuroimage. 2021 Aug 1;236:117967. doi: 10.1016/j.neuroimage.2021.117967. Epub 2021 Apr 29.
The anisotropy of brain white matter microstructure manifests itself in orientational-dependence of various MRI contrasts, and can result in significant quantification biases if ignored. Understanding the origins of this orientation-dependence could enhance the interpretation of MRI signal changes in development, ageing and disease and ultimately improve clinical diagnosis. Using a novel experimental setup, this work studies the contributions of the intra- and extra-axonal water to the orientation-dependence of one of the most clinically-studied parameters, apparent transverse relaxation T. Specifically, a tiltable receive coil is interfaced with an ultra-strong gradient MRI scanner to acquire multidimensional MRI data with an unprecedented range of acquisition parameters. Using this setup, compartmental T can be disentangled based on differences in diffusional-anisotropy, and its orientation-dependence further elucidated by re-orienting the head with respect to the main magnetic field B→. A dependence of (compartmental) T on the fibre orientation w.r.t. B→ was observed, and further quantified using characteristic representations for susceptibility- and magic angle effects. Across white matter, anisotropy effects were dominated by the extra-axonal water signal, while the intra-axonal water signal decay varied less with fibre-orientation. Moreover, the results suggest that the stronger extra-axonal T orientation-dependence is dominated by magnetic susceptibility effects (presumably from the myelin sheath) while the weaker intra-axonal T orientation-dependence may be driven by a combination of microstructural effects. Even though the current design of the tiltable coil only offers a modest range of angles, the results demonstrate an overall effect of tilt and serve as a proof-of-concept motivating further hardware development to facilitate experiments that explore orientational anisotropy. These observations have the potential to lead to white matter microstructural models with increased compartmental sensitivity to disease, and can have direct consequences for longitudinal and group-wise T- and diffusion-MRI data analysis, where the effect of head-orientation in the scanner is commonly ignored.
脑白质微观结构的各向异性表现在各种 MRI 对比的各向异性依赖性上，如果忽略这种各向异性依赖性，将会导致显著的定量偏倚。了解这种各向异性依赖性的起源可以增强对 MRI 信号变化在发育、衰老和疾病中的解释，并最终改善临床诊断。本工作使用一种新颖的实验装置，研究了内轴索和外轴索水对最受临床研究的参数之一——表观横向弛豫 T 的各向异性依赖性的贡献。具体来说，可倾斜接收线圈与超强梯度 MRI 扫描仪接口，以获取具有前所未有的采集参数范围的多维 MRI 数据。使用该装置，可以根据扩散各向异性的差异来分离隔室 T，并通过相对于主磁场 B→重新定向头部来进一步阐明其各向异性依赖性。观察到（隔室）T 与纤维相对于 B→的方向有关，并且使用磁化率和魔角效应的特征表示来进一步量化这种依赖性。在整个白质中，各向异性效应主要由外轴索水信号主导，而内轴索水信号的衰减随纤维方向的变化较小。此外，结果表明，较强的外轴索 T 各向异性依赖性主要由磁化率效应主导（可能来自髓鞘），而较弱的内轴索 T 各向异性依赖性可能由多种微观结构效应共同驱动。尽管可倾斜线圈的当前设计仅提供了适度的角度范围，但结果证明了倾斜的总体效果，并为进一步的硬件开发提供了概念验证，以促进探索各向异性的实验。这些观察结果有可能导致具有增加的隔室对疾病敏感性的白质微观结构模型，并可能对纵向和组间 T 和扩散 MRI 数据分析产生直接影响，在这些分析中，通常忽略了头部在扫描仪中的方向的影响。
