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

The acute affect of voluntary muscle contractions performed by healthy volu nteers was evaluated using Na-23 nuclear magnetic resonance (NMR). Three-di mensional gradient-echo Na-23 images, pulse-acquired spectra, and transvers e relaxation times were obtained before and after ankle flexion-extension e xercise. The muscle sodium concentration was calculated from Na-23 images u sing 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 vis ible. The Na-23 image intensity increased by 34% +/- 7% in the exercised mu scle and diminished with a half-life of 30 +/- 6 minutes, The pulse-acquire d spectra, however, did not show any significant change in muscle signal in tensity following exercise, but the relative contribution of the slow T-2 c omponent increased. The calculated sodium concentration also did not change significantly after the exercise. We therefore infer that the changes in N a-23 magnetic resonance imaging (MRI) were due to a change in sodium-macrom olecular interaction rather than a change in tissue sodium content. We beli eve that this report represents the first study of Na-23 MRI of skeletal mu scle. (C) 2000 Wiley-Liss, Inc.