In vivo quantification of T2⁎ anisotropy in white matter fibers in marmoset monkeys

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 (B0) 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 7 T, 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.

► Nature of T2⁎ contrast in white matter remains poorly understood. ► R2⁎ (=1/T2⁎) may depend on tissue orientation relative to static magnetic field. ► Orientation dependence of R2⁎ is demonstrated in the marmoset brain in vivo. ► Orthogonal head orientations induce 50% change in R2⁎ values in white matter. ► R2⁎ anisotropy may be caused by the anisotropic structure of the myelin sheath.