An order parameter without magic angle effect (OPTIMA) derived from R1ρ dispersion in ordered tissue

Abstract

Purpose: MR R₂ imaging of ordered tissue exhibits the magic angle effect, potentially masking subtle pathological changes in cartilage. This work aimed to develop an orientation-independent order parameter (S) exclusively sensitive to collagen degeneration.

Methods: A theory was developed based on $R_{1\rho }$ dispersion coupled with a simplified molecular motion model in which anisotropic $R_2^a\left(\theta \right)$ became directly proportional to correlation time ${\tau }_b\left(\theta \right)$ and S could be derived. This new parameter was validated with ex vivo $R_{1\rho }$ dispersion reported on orientated (n = 4), enzymatically depleted bovine cartilage (n = 6), and osteoarthritic human knee specimens (n = 14) at 9.4 Tesla, which was further demonstrated on 1 healthy human knee in vivo at 3 Tesla.

Results: ${\tau }_b\left(\theta \right)$ from orientation-dependent $R_{1\rho }$ dispersion revealed a significantly high average correlation (r = 0.89 ± 0.05, P < 0.05) with $R_2^a$ (θ) on cartilage samples and a moderate correlation (r = 0.48, P < 0.001) for the human knee in vivo. The derived S (10^-3 ) significantly decreased in advanced osteoarthritis (1.64 ± 0.03 vs. 2.30 ± 0.11, P < 0.001) and collagen-depleted samples (1.30 ± 0.11 vs. 2.12 ± 0.12, P < 0.001) when compared with early osteoarthritis and the control, respectively.

Conclusion: The proposed order parameter could be a potentially useful orientation-independent MR biomarker for collagen alterations in cartilage and other highly structured tissues.

Keywords: $R_{1\rho }$ dispersion; anisotropic R2; collagen; correlation time; magic angle effect; order parameter.