Orientational dependence of T2 relaxation in articular cartilage: A microscopic MRI (microMRI) study

The experiments reported herein are the first MRI investigations of the orientational dependence of T(2) relaxation in articular cartilage at microscopic resolution over the 360 degrees angular space. For each of six canine cartilage specimens, 48 independent T(2)-weighted proton images were acquired for 12 different specimen orientations. Pixel-wise monoexponential fits of these proton images produced 12 T(2) relaxation images, each with an in-plane pixel resolution of 13.7 microm. Cartilage T(2) as a function of specimen orientation was shown to follow approximately the angular dependence of the nuclear dipole-dipole interaction, with local maxima at approximately 55 degrees, 125 degrees, 235 degrees, and 305 degrees. However, the relative amplitudes of the T(2) maxima deviated somewhat from those expected from the dipolar interaction. The amplitudes of these maxima also varied with tissue depth: the largest amplitudes were found in the radial zone, intermediate amplitudes were found in the superficial zone, and there was a continuous decrease in amplitude approaching the transitional zone from the superficial zone above and the radial zone below. We explain the discrepancy between the observed T(2) anisotropy and the angular dependence of the dipolar interaction by means of a simple model which considers the average of one isotropic and two anisotropic spin populations-the first being associated with "free" water, and the latter two arising from collagen-associated waters. We show that even for the "long" T(2) components, which arise in multiple-compartment studies of collagen-water systems, there appears to be two subpopulations. Each has the same peak value of T(2), but the angular dependence of one is shifted in phase by 90 degrees relative to the other by virtue of the fact that each is associated with groups of mutually perpendicular fibrils.