Sodium relaxation times in the knee joint in vivo at 7T.

The sodium concentration correlates directly with the concentration of proteoglycans (PG) in cartilage, the loss of which is an early signature of osteoarthritis (OA). As a result, quantitative sodium MRI is a promising technique for assessing the degradation of articular cartilage in patients with OA. Sodium relaxation times can also provide information on the degradation of cartilage: it has already been shown on bovine cartilage that T(1) and T2long are longer and T2short shorter when the PG concentration decreases. In this study, sodium T(1), T2short and T2long relaxation maps were measured in vivo at 7 T on 8 healthy volunteers and in 4 different regions of the cartilage in the knee joint. The patellar, femoro-tibial medial, lateral, and femoral condyle cartilage have an average T(1)~20 ms, but different T2short (from 0.5 ms to 1.4 ms) and T2long (from 11.4 ms to 14.8 ms). Statistically significant differences in T(1), T2short and T2long were observed between the different regions in cartilage (p << 10(- 5)). Statistical differences in T(1) were also observed between male and female data (p << 10(- 5)). These relaxation times measurements can further be applied as correction factors for sodium concentration maps in vivo and can also be useful as complementary information to quantitative sodium MRI in the quest for detecting early OA. These measurements were done on low resolution sodium images in order to acquire sufficient quality data for fitting (5 images for T(1) and 9 images for T2*) while keeping the total time of acquisition of the data reasonable for the volunteer's comfort (1 h 15 min).