T 2 *-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 T 2 * contrast, especially in white matter, remains poorly understood. Recent work has shown that R 2 * (=1/T 2 *) 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 R 2 * 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 R 2 * relaxation rate constant of the optic radiations. We attribute this substantial R 2 * anisotropy to local subvoxel susceptibility effects arising from the highly ordered and anisotropic structure of the myelin sheath.
- Anisotropic R relaxation rate
- High-field imaging
- Nonhuman primate brain
- White matter fiber orientation
ASJC Scopus subject areas
- Cognitive Neuroscience