Translational Neuroradiology Unit, Neuroimmunology Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA.
Neuroimage. 2012 Jan 16;59(2):979-85. doi: 10.1016/j.neuroimage.2011.08.064. Epub 2011 Aug 27.
T₂-weighted MRI at high field is a promising approach for studying noninvasively the tissue structure and composition of the brain. However, the biophysical origin of T₂ contrast, especially in white matter, remains poorly understood. Recent work has shown that R₂ (=1/T₂) may depend on the tissue's orientation relative to the static magnetic field (B(0)) and suggested that this dependence could be attributed to local anisotropy in the magnetic properties of brain tissue. In the present work, we analyzed high-resolution, multi-gradient-echo images of in vivo marmoset brains at 7T, and compared them with ex vivo diffusion tensor images, to show that R₂ relaxation in white matter is highly sensitive to the fiber orientation relative to the main field. We directly demonstrate this orientation dependence by performing in vivo multi-gradient-echo experiments in two orthogonal brain positions, uncovering a nearly 50% change in the R₂ relaxation rate constant of the optic radiations. We attribute this substantial R₂ anisotropy to local subvoxel susceptibility effects arising from the highly ordered and anisotropic structure of the myelin sheath.
T₂ 加权 MRI 在高场中是一种很有前途的方法，可以非侵入性地研究大脑的组织结构和组成。然而，T₂ 对比的生物物理起源，特别是在白质中，仍然知之甚少。最近的研究表明，R₂（=1/T₂）可能取决于组织相对于静磁场（B（0））的方向，并表明这种依赖性可能归因于脑组织磁性质的局部各向异性。在本工作中，我们分析了在 7T 下活体狨猴大脑的高分辨率、多梯度回波图像，并将其与离体扩散张量图像进行比较，以表明白质中的 R₂ 弛豫对相对于主磁场的纤维方向非常敏感。我们通过在两个正交的脑位进行体内多梯度回波实验直接证明了这种方向依赖性，揭示了视神经辐射的 R₂ 弛豫率常数几乎有 50%的变化。我们将这种显著的 R₂ 各向异性归因于来自髓鞘高度有序和各向异性结构的局部亚体素磁化率效应。
