Correlations of T1ρ With Properties of Articular Cartilage Depend on the Spin‐Lock Amplitude and Orientation of the Sample

Various findings on the correlations of the continuous wave (CW-) T1ρ relaxation with properties of articular cartilage have been reported, suggesting its potential for cartilage diagnostics. The aim of the study was to combine all the previously reported aspects and investigate the association of CW-T1ρ relaxation time in healthy bovine cartilage to different properties of cartilage with a range of spin-lock amplitudes and cartilage orientations. Bovine cartilage-bone plugs (n = 11) were imaged in four orientations at 9.4T to measure T1, T2, and CW-T1ρ with spin-lock amplitudes varying between 100 and 5000 Hz. For reference, biomechanical moduli, polarized light microcopy anisotropy and optical density were measured. Correlation between the relaxation times and the reference parameters were determined for each spin-lock amplitude and sample orientation. Significant increase in both CW-T1ρ and T2 relaxation times was observed, especially in the deep layers of cartilage, when the sample was oriented to 55° orientation. The CW-T1ρ relaxation anisotropy was reduced with increasing spin-lock amplitude and reached a stable level of about 10% at a spin-lock amplitude of 1500 Hz or higher. Young's modulus, instantaneous modulus and optical density correlated negatively with the T1ρ relaxation times measured with spin-lock amplitudes higher than 500 Hz. The results demonstrate that CW-T1ρ relaxation time is highly dependent on the orientation of cartilage with respect to the main magnetic field (B₀), and the level of orientation dependence is related to the spin-lock amplitude. Correlation of CW-T1ρ with cartilage properties is dependent both on the orientation and spin-lock amplitude.