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

The acute affect of voluntary muscle contractions performed by healthy volunteers was evaluated using 23Na nuclear magnetic resonance (NMR). Three‐dimensional gradient‐echo 23Na images, pulse‐acquired spectra, and transverse relaxation times were obtained before and after ankle flexion‐extension exercise. The muscle sodium concentration was calculated from 23Na images using a 40 mM NaCl standard and the measured T2 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 23Na 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 T2 component increased. The calculated sodium concentration also did not change significantly after the exercise. We therefore infer that the changes in 23Na 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 23Na MRI of skeletal muscle. J. Magn. Reson. Imaging 2000;11:532–538. © 2000 Wiley‐Liss, Inc.