Multi -nuclear magnetic resonance methods for evaluating cartilage degeneration

Osteoarthritis (OA) is a metabolically active joint disorder of multiple etiologies affecting over 40 million Americans. Cartilage consists mainly of an extra-cellular matrix of type II collagen and proteoglycans (PG). Degradation of cartilage in an early osteoarthritic condition primarily involves the loss of the PG. As the PG are negatively charged macromolecules, a fixed charge density is created within the tissue. Sodium ions are attracted into the cartilage matrix to maintain electroneutrality. The loss of the PG in the early osteoarthritic state is accompanied by a loss of sodium, a change in the hydration of the tissue and alteration of the biomechanical properties. As there is no cure for OA, characterization of the disease in its early stages offers the best hope of halting the disease progression. To this end, we have developed several MRI based methodologies to non-invasively evaluate the integrity of the cartilage tissue, both in vitro and in vivo, by monitoring these three physiological processes. 23Na MRI was used to measure the sodium concentrations inside bovine articular cartilage in in vitro samples and in human articular cartilage, in vivo. Sodium concentrations in normal bovine cartilage averaged ∼313 mM, ranging as high as ∼390 mM in the center of the tissue. Studies performed on degraded cartilage showed that sodium concentrations could accurately be measured and correlated to FCD. Average sodium concentrations in vivo were found to range from ∼220 mM to ∼260 mM, with maximum values close to 350 mM. Water content in cartilage was also mapped using a multi-echo spin echo sequence. Water volume content was 81% in normal bovine cartilage and was not significantly altered in cartilage selectively PG degraded. The biomechanics of cartilage recovery following exercise induced compression were also measured using both 23Na and 1H MRI. A time constant for recovery was found to be between 8 and 10 minutes. 23Na data shows an increase in sodium signal intensity during the recovery period. Since these experiments were performed at high field, protocols were designed to measure the RF heating from typically used coil ensembles in both the 23Na and 1H imaging regimes