Multi-parametric MRI characterization of enzymatically degraded articular cartilage

Several laboratory and rotating frame quantitative MRI parameters were evaluated and compared for detection of changes in articular cartilage following selective enzymatic digestion. Bovine osteochondral specimens were subjected to 44 h incubation in control medium or in collagenase or chondroitinase ABC to induce superficial collagen or proteoglycan (glycosaminoglycan) alterations. The samples were scanned at 9.4 T for T₁, T_{1 Gd} (dGEMRIC), T₂, adiabatic T_{1 ρ}, adiabatic T_{2 ρ}, continuous-wave T_{1 ρ}, T_RAFF2, and T_{1 sat} relaxation times and for magnetization transfer ratio (MTR). For reference, glycosaminoglycan content, collagen fibril orientation and biomechanical properties were determined. Changes primarily in the superficial cartilage were noted after enzymatic degradation. Most of the studied parameters were sensitive to the destruction of collagen network, whereas glycosaminoglycan depletion was detected only by native T₁ and T_{1 Gd} relaxation time constants throughout the tissue and by MTR superficially. T₁, adiabatic T_{1 ρ}, adiabatic T_{2 ρ}, continuous-wave T_{1 ρ}, and T_{1 sat} correlated significantly with the biomechanical properties while T_{1 Gd} correlated with glycosaminoglycan staining. The findings indicated that most of the studied MRI parameters were sensitive to both glycosaminoglycan content and collagen network integrity, with changes due to enzymatic treatment detected primarily in the superficial tissue. Strong correlation of T₁, adiabatic T_1ρ, adiabatic T_{2 ρ}, continuous-wave T_{1 ρ}, and T_{1 sat} with the altered biomechanical properties, reflects that these parameters were sensitive to critical functional properties of cartilage.