Anisotropy of T₂ and T₁ relaxation time in articular cartilage at 3 T

Purpose: The anisotropy of R-2 and R-1 rho relaxation rates in articular cartilage contains information about the collagenous structure of the tissue. Here we determine and study the anisotropic and isotropic components of T-2 and T-1 rho relaxation parameters in articular cartilage with a clinical 3T MRI device. Furthermore, a visual representation of the topographical variation in anisotropy is given via anisotropy mapping. Methods: Eight bovine stifle joints were imaged at 22 orientations with respect to the main magnetic field using T-2, continuous-wave (CW) T-1 rho, and adiabatic T-1 rho mapping sequences. Relaxation rates were separated into isotropic and anisotropic relaxation components using a previously established relaxation anisotropy model. Pixel-wise anisotropy values were determined from the relaxation-time maps using Michelson contrast. Results: The relaxation rates obtained from the samples displayed notable variation depending on the sample orientation, magnetization preparation, and cartilage layer. R-2 demonstrated significant anisotropy, whereas CW-R-1 rho (300 Hz) and CW-R1 rho (500 Hz) displayed a low degree of anisotropy. Adiabatic R-1 rho was largely isotropic. In the deep cartilage regions, relaxation rates were generally faster and more anisotropic than in the cartilage closer to the tissue surface. The isotropic relaxation rate components were found to have similar values regardless of measurement sequence. Conclusions: The fitted relaxation model for T-2 and T-1 rho demonstrated varying amounts anisotropy, depending on magnetization preparation, and studied the articular cartilage layer. Anisotropy mapping of full joints showed varying amounts of anisotropy depending on the quantitative MRI parameter and topographical location, and in the case of T-2, showed systematic changes in anisotropy across cartilage depth.