MRI contrast enhanced study of cartilage proteoglycan degradation in the rabbit knee.

Early degeneration of cartilage is accompanied by a loss of proteoglycans and consequent changes in the content of water. Conventional magnetic resonance imaging (MRI) cannot reliably detect this change, since the relaxation properties of the cartilage are dominated by its collagen content. The applicability of a positively charged nitroxide as an MRI contrast agent in detection of the content of the negatively charged proteoglycans within the cartilage was investigated. The results from both MRI and electron paramagnetic resonance (EPR) spectroscopy indicate that the accumulation of the contrast agent reflects the amount of proteoglycans within the cartilage, presumably due to the electrostatic interactions between the negatively charged proteoglycans and the positively charged nitroxide. Such a contrast agent could be useful in the detection and study of early stages of the degeneration of joints.