Relaxation anisotropy in cartilage by NMR microscopy (muMRI) at 14-microm resolution

To study the structural anisotropy and the magic-angle effect in articular cartilage, T1 and T2 images were constructed at a series of orientations of cartilage specimens in the magnetic field by using NMR microscopy (muMRI). An isotropic T1 and a strong anisotropic T2 were observed across the cartilage tissue thickness. Three distinct regions in the microscopic MR images corresponded approximately to the superficial, transitional, and radial histological zones in the cartilage. The percentage decrease of T2 follows the pattern of the curve of (3cos2theta - 1)2 at the radial zone, where the collagen fibrils are perpendicular to the articular surface. In contrast, little orientational dependence of T2 was observed at the transitional zone, where the collagen fibrils are more randomly oriented. The result suggests that the interactions between water molecules and proteoglycans have a directional nature, which is somehow influenced by collagen fibril orientation. Hence, T2 anisotropy could serve as a sensitive and noninvasive marker for molecular-level orientations in articular cartilage.