23Na MRI accurately measures fixed charge density in articular cartilage

One of the initiating steps of osteoarthritis is the loss of proteoglycan (PG) molecules from the cartilage matrix. One method for assessing cartilage integrity, therefore, is to measure the PG content or fixed charge density (FCD) of cartilage. This report shows the feasibility of calculating FCD by (23)Na MRI and introduces MRI protocols for human studies, in vivo. (23)Na MRI was used to measure the sodium concentration inside bovine patellar cartilage. The sodium concentration was then converted to FCD (mM) by considering ideal Donnan equilibrium. These FCD measurements were compared to FCD measurements obtained through standard dimethylmethylene blue PG assays. There was a high correlation (slope = 0.89, r(2) = 0.81) between the FCD measurements obtained by (23)Na MRI and those obtained by the PG assays. These methods were then employed in quantifying the FCD of articular cartilage of human volunteers in vivo. Two imaging protocols were compared: one using a birdcage coil, the other using a transmit/receive surface coil. Both methodologies gave similar results, with the average sodium concentration of normal human patellar cartilage ranging from approximately 240 to 260 mM. This corresponds to FCDs of -158 mM to -182 mM.