Human skeletal muscle: Sodium MR imaging and quantification - Potential applications in exercise and disease

PURPOSE: To use sodium 23 magnetic resonance (MR) imaging to quatify non-invasively total sodium in human muscle and to apply the technique in exercise and musculoskeletal disease. MATERIALS AND METHODS: Total [Na] sodium was determined from the ratio of the relaxation-corrected ²³Na signal intensities measured from short echo-time (0.4 msec) ²³Na images to those from an external saline solution reference. The method was validated with the blinded use of saline solutions of varying sodium concentrations. [Na] was measured in the calf muscles in 10 healthy volunteers. ²³Na MR imaging also was performed in two healthy subjects after exercise, two patients with myotonic dystrophy, and two patients with osteoarthritis. RESULTS: ²³Na MR imaging yielded a total [Na] value of 28.4 mmol/kg of wet weight ± 3.6 (SD) in normal muscle, consistent with prior biopsy data. Spatial resolution was 0.22 mL, with signal-to-noise ratio of 10-15. Mean signal intensity elevations were 16% and 22% after exercise and 47% and 70% in dystrophic muscles compared with those at normal resting levels. In osteoarthritis, mean signal intensity reductions were 36% and 15% compared with those in unaffected knee joints. CONCLUSION: ²³Na MR imaging can be used to quantify total [Na] in human muscle. The technique may facilitate understanding of the role of the sodium-potassium pump and perfusion in normal and diseased muscle.