Diffusion tensor imaging of articular cartilage as a measure of tissue microstructure.

OBJECTIVE: To use diffusion tensor MR micro-imaging to observe differences in magnitude and anisotropy of water diffusion between 'healthy' cartilage and cartilage enzymatically degraded to simulate arthritic damage.
METHODS: Diffusion tensor images (156 x 156 microm in-plane resolution, 2mm slice thickness) of bovine cartilage were obtained at either 4.7 or 7.0 T using pulsed field gradient spin echo sequences. The parameters determined were: maximum and mean diffusivity, direction of the maximum diffusion eigenvector with respect to the normal to the articular surface and fractional anisotropy (FA) of diffusion.
RESULTS: Both maximum and mean diffusion eigenvalues were found to decrease, respectively, from approximately 1.95 x 10(-9) and 1.80 x 10(-9) m2 s(-1) at the articular surface to approximately 1.08 x 10(-9) and 0.79 x 10(-9) m2 s(-1) in the deep zone. A systematic change was observed in the direction of the eigenvector corresponding to maximum diffusivity, reflecting the expected change in orientation of the collagen macrofibrillar bundles. Degradation with trypsin to remove proteoglycans resulted in a 10-15% increase in apparent diffusion coefficient of water in the cartilage, with no apparent change in FA.
CONCLUSIONS: These methods have the potential to be used to probe local changes in tissue microstructure and the hydrodynamic status of cartilage during development of osteoarthritis.