Schyboll Felix, Jaekel Uwe, Weber Bernd, Neeb Heiko
Department of Mathematics and Technology, University of Applied Science Koblenz, Remagen, Germany.
Department of Epileptology, University Hospital Bonn and Center for Economics and Neuroscience, University of Bonn, Bonn, Germany.
MAGMA. 2018 Aug;31(4):501-510. doi: 10.1007/s10334-018-0678-8. Epub 2018 Feb 20.
Recent MRI studies have shown that the orientation of nerve fibres relative to the main magnetic field affects the R*(= 1/T*) relaxation rate in white matter (WM) structures. The underlying physical causes have been discussed in several studies but are still not completely understood. However, understanding these effects in detail is of great importance since this might serve as a basis for the development of new diagnostic tools and/or improve quantitative susceptibility mapping techniques. Therefore, in addition to the known angular dependence of R*, the current study investigates the relationship between fibre orientation and the longitudinal relaxation rate, R (= 1/T), as well as the apparent water content.
For a group of 16 healthy subjects, a series of gradient echo, echo-planar and diffusion weighted images were acquired at 3T from which the decay rates, the apparent water content and the diffusion direction were reconstructed. The diffusion weighted data were used to determine the angle between the principle fibre direction and the main magnetic field to examine the angular dependence of R and apparent water content.
The obtained results demonstrate that both parameters depend on the fibre orientation and exhibit a positive correlation with the angle between fibre direction and main magnetic field.
These observations could be helpful to improve and/or constrain existing biophysical models of brain microstructure by imposing additional constraints resulting from the observed angular dependence R and apparent water content in white matter.
近期的磁共振成像（MRI）研究表明，神经纤维相对于主磁场的方向会影响白质（WM）结构中的R*（=1/T*）弛豫率。一些研究已探讨了其潜在的物理原因，但仍未完全理解。然而，详细了解这些效应非常重要，因为这可能为开发新的诊断工具和/或改进定量磁化率映射技术奠定基础。因此，除了已知的R*角度依赖性外，本研究还探究了纤维方向与纵向弛豫率R（=1/T）以及表观含水量之间的关系。
对16名健康受试者，在3T条件下采集了一系列梯度回波、回波平面和扩散加权图像，从中重建出衰减率、表观含水量和扩散方向。利用扩散加权数据确定主纤维方向与主磁场之间的夹角，以研究R和表观含水量的角度依赖性。
所得结果表明，这两个参数均取决于纤维方向，且与纤维方向和主磁场之间的夹角呈正相关。
通过施加由观察到的白质中R和表观含水量的角度依赖性所产生的额外约束条件，这些观察结果可能有助于改进和/或限制现有的脑微观结构生物物理模型。
