Effect of exercise on ²³Na MRI 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 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. (C) 2000 Wiley-Liss, Inc.