Increase in Fast-relaxing Sodium After a Brief Plantarflexion Exercise Detected by Multi-Echo UTE MRI and Biexponential Regression

We measured the exercise-induced change in fast and slow T2-relaxing sodium in calf muscles separately using a UTE sequence with multiple echoes. Three minutes of plantarflexion exercise resulted in an increase in the fast-relaxing sodium and no change in the slow-relaxing component. Based on the known physiology that muscle intracellular sodium increases during muscle contraction, the measured fast-relaxing sodium might represent muscle intracellular sodium. This finding is valuable in studying intra- and extracellular sodium separately.