Orientation anisotropy of quantitative MRI relaxation parameters in ordered tissue

In highly organized tissues, such as cartilage, tendons and white matter, several quantitative MRI parameters exhibit dependence on the orientation of the tissue constituents with respect to the main imaging magnetic field (B₀). In this study, we investigated the dependence of multiple relaxation parameters on the orientation of articular cartilage specimens in the B₀. Bovine patellar cartilage-bone samples (n = 4) were investigated ex vivo at 9.4 Tesla at seven different orientations, and the MRI results were compared with polarized light microscopy findings on specimen structure. Dependences of T₂ and continuous wave (CW)-T_1ρ relaxation times on cartilage orientation were confirmed. T₂ (and T₂*) had the highest sensitivity to orientation, followed by T_RAFF2 and adiabatic T_2ρ. The highest dependence was seen in the highly organized deep cartilage and the smallest in the least organized transitional layer. Increasing spin-lock amplitude decreased the orientation dependence of CW-T_1ρ. T₁ was found practically orientation-independent and was closely followed by adiabatic T_1ρ. The results suggest that T₁ and adiabatic T_1ρ should be preferred for orientation-independent quantitative assessment of organized tissues such as articular cartilage. On the other hand, based on the literature, parameters with higher orientation anisotropy appear to be more sensitive to degenerative changes in cartilage.