Anisotropy of T₂ and T_1ρ relaxation time in articular cartilage at 3 T

The anisotropy of R₂ and R_1ρ 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 T₂ and T_1ρ 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.

Eight bovine stifle joints were imaged at 22 orientations with respect to the main magnetic field using T₂, continuous-wave (CW) T_1ρ, and adiabatic T_1ρ 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.

The relaxation rates obtained from the samples displayed notable variation depending on the sample orientation, magnetization preparation, and cartilage layer. R₂ demonstrated significant anisotropy, whereas CW-R_1ρ (300 Hz) and CW-R_1ρ (500 Hz) displayed a low degree of anisotropy. Adiabatic R_1ρ 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.

The fitted relaxation model for T₂ and T_1ρ 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 T₂, showed systematic changes in anisotropy across cartilage depth.