Origin of cartilage laminae in MRI

To understand the origin of the laminated appearance of cartilage in MRI (the magic angle effect), microscopic MRI (μMRI) experiments were performed at 14‐μm pixel resolution on normal canine articular cartilage from the shoulder joints. Two‐dimensional images of the spin‐spin relaxation time (T2) of the cartilage‐bone plug at two angles (0° and 55°) were calculated quantitatively. A distinct T2 anisotropy was observed as a function of the cartilage tissue depth. The surface and the deep regions exhibit strong orientational dependence of T2, whereas the upper‐middle region exhibits little orientational dependence of T2. These three μMRI regions correspond approximately to the three histologic zones in cartilage tissue. The results from the bulk T2 measurements agreed with these μMRI results. Our studies show that the laminated appearance of cartilage in MRI is caused by T2 anisotropy of the tissue. We further suggest that the molecular origin of the T2 anisotropy is the nuclear dipolar interaction. The structure of the cartilage tissue indicates that the collagen meshwork defines this T2 anisotropy. The results show that the T2 anisotropy provides an indirect but sensitive indicator for the orientation of macromolecular structures in cartilage. The clinical implications of this anisotropy are discussed.