Gil Rita, Khabipova Diana, Zwiers Marcel, Hilbert Tom, Kober Tobias, Marques José P
Donders Centre for Cognitive Neuroimaging, Radboud University, Nijmegen, The Netherlands.
Champalimaud Center for the Unknown, Lisbon, Portugal.
NMR Biomed. 2016 Dec;29(12):1780-1790. doi: 10.1002/nbm.3616. Epub 2016 Nov 3.
Diffusion-weighted imaging (DWI) provides information that allows the estimation of white-matter (WM) fibre orientation and distribution, but it does not provide information about myelin density, fibre concentration or fibre size within each voxel. On the other hand, quantitative relaxation contrasts (like the apparent transverse relaxation, R2∗) offer iron and myelin-related contrast, but their dependence on the orientation of microstructure with respect to the applied magnetic field, B , is often neglected. The aim of this work was to combine the fibre orientation information retrieved from the DWI acquisition and the sensitivity to microstructural information from quantitative relaxation parameters. The in vivo measured quantitative transverse relaxation maps (R and R2∗) were decomposed into their orientation-dependent and independent components, using the DWI fibre orientation information as prior knowledge. The analysis focused on major WM fibre bundles such as the forceps major (FMj), forceps minor (FMn), cingulum (CG) and corticospinal tracts (CST). The orientation-dependent R parameters, despite their small size (0-1.5 Hz), showed higher variability across different fibre populations, while those derived from R2∗, although larger (3.1-4.5 Hz), were mostly bundle-independent. With this article, we have, for the first time, attempted the in vivo characterization of the orientation-(in)dependent components of the transverse relaxation rates and demonstrated that the orientation of WM fibres influences both R and R2∗ contrasts.
扩散加权成像（DWI）提供的信息可用于估计白质（WM）纤维的方向和分布，但它无法提供每个体素内的髓磷脂密度、纤维浓度或纤维大小的信息。另一方面，定量弛豫对比度（如表观横向弛豫，R2∗）可提供与铁和髓磷脂相关的对比度，但其对微观结构相对于所施加磁场B的方向的依赖性常常被忽视。这项工作的目的是将从DWI采集中获取的纤维方向信息与来自定量弛豫参数的微观结构信息敏感性相结合。利用DWI纤维方向信息作为先验知识，将体内测量的定量横向弛豫图（R和R2∗）分解为其方向依赖性和独立性成分。分析集中在主要的白质纤维束上，如主要钳状束（FMj）、次要钳状束（FMn）、扣带束（CG）和皮质脊髓束（CST）。尽管方向依赖性R参数的大小较小（0 - 1.5 Hz），但在不同纤维群体中显示出更高的变异性，而从R2∗导出的参数虽然较大（3.1 - 4.5 Hz），但大多与束无关。通过本文，我们首次尝试在体内表征横向弛豫率的方向依赖性（非依赖性）成分，并证明白质纤维的方向会影响R和R2∗对比度。
