Change of diffusion tensor imaging parameters in articular cartilage with progressive proteoglycan extraction

Objective: To investigate changes of diffusion tensor imaging (DTI) parameters (mean apparent diffusion coefficient [ADC], fractional anisotropy [FA], and first eigenvector) with increasing proteoglycan (PG) extraction of articular cartilage. MATERIAL AND METHODS: Twelve cylindrical cartilage-on-bone samples were drilled from 4 human patellae (3 per patella). Each sample was divided into 2 pieces. One piece underwent histologic examination to assess the PG content of the native sample by safranin-O staining and its collagen architecture by polarized light microscopy. The other underwent magnetic resonance imaging (MRI) at 17.6 T for DTI measurement. After MRI, 2 of the 3 samples from each patella were immersed in a dilute trypsin solution (0.1 mg/mL), whereas the third sample was kept as a negative control in physiological saline. After incubation (6, 48, 72, and 96 hours), the samples were reimaged, stained for PG content and for the collagen orientation. Maps of ADC, FA, and the orientation of the first eigenvector as well as histology were available for each sample before and after incubation. Results: PG loss led to increased ADC and reduced safranin-O staining from the articular surface to the bone-cartilage interface. A significant correlation (r = 0.86, P < 0.01) was observed between the change in bulk ADC and PG loss. Regional analysis from the articular surface to the tide mark demonstrated depth dependent significant correlations of ADC and PG loss. FA and first eigenvector as well as polarized light microscopy showed only small changes in the order of magnitude of measurement errors, not correlating with PG loss. Conclusion: Mean diffusivity evidence by the ADC is linearly correlated to progressive PG extraction in articular cartilage. FA and the first eigenvector seem to be specific to the collagen architecture of cartilage. DTI has the potential to become a valuable biomarker for the workup of cartilage degeneration in osteoarthritis, since evaluation of the PG content and collagen architectural properties of cartilage can be performed with a single, non-contrast-enhanced proton-based MRI measurement.