Orientation-Independent T2 Mapping Enhances MRI-Based Cartilage Characterization

Quantitative T2 mapping is an important MRI method for assessing degenerative changes in articular cartilage. Recently, in a measurement setup with automated sample re-orientation, it was demonstrated that T2 can be split into its orientation-independent components. This quantitative MRI study aims to assess the diagnostic significance of the automated approach with ex vivo human cartilage.

T2 maps of 30 human osteochondral samples harvested from 5 cadaveric individuals were acquired at 9.4T in 13 orientations, allowing calculation of the T2 components. Additionally, T1, adiabatic T1ρ, and continuous wave T1ρ with two spin-lock frequencies were scanned in a single orientation. For reference, the collagen network anisotropy, proteoglycan content and biomechanical indentation properties were measured. The relationships between quantitative MRI and reference parameters were studied using Mann-Whitney U-test and Spearman's rank correlation. All parameters were compared between healthy and degenerated groups based on OARSI grading.

The anisotropic relaxation rate component of T2 (R2a), and all T1 and T1ρ parameters differed (p < 0.05) between the groups in superficial cartilage. R2a correlated moderately with PLM anisotropy (r = 0.44) and optical density (r = - 0.37) in the deep zone. Isotropic T2 component (R2i) correlated with instantaneous modulus (r = 0.48), and R2a with phase shift between stress and strain during indentation testing (r = - 0.44). T1 and T1ρ parameters correlated with both, instantaneous and dynamic modulus in several zones of cartilage.

The elevation of T2 in degenerated cartilage is primarily driven by the R2a component, whereas the R2i component showed no significant difference between healthy and degenerated human articular cartilage.