Effect of exercise on (23)Na MRI and relaxation characteristics of the human calf muscle.

The acute affect of voluntary muscle contractions performed by healthy volunteers was evaluated using (23)Na nuclear magnetic resonance (NMR). Three-dimensional gradient-echo (23)Na images, pulse-acquired spectra, and transverse relaxation times were obtained before and after ankle flexion-extension exercise. The muscle sodium concentration was calculated from (23)Na images using a 40 mM NaCl standard and the measured T(2) values. Before exercise the muscle sodium concentration was 26+/-4 mmole/kg wet weight. This agrees closely with literature values, suggesting that muscle Na(+) is fully NMR visible. The (23)Na image intensity increased by 34%+/-7% in the exercised muscle and diminished with a half-life of 30+/-6 minutes. The pulse-acquired spectra, however, did not show any significant change in muscle signal intensity following exercise, but the relative contribution of the slow T(2) component increased. The calculated sodium concentration also did not change significantly after the exercise. We therefore infer that the changes in (23)Na magnetic resonance imaging (MRI) were due to a change in sodium-macromolecular interaction rather than a change in tissue sodium content. We believe that this report represents the first study of (23)Na MRI of skeletal muscle.