Localized semi-LASER dynamic ³¹P magnetic resonance spectroscopy of the soleus during and following exercise at 7 T

Abstract

Objectives

This study demonstrates the applicability of semi-LASER localized dynamic ³¹P MRS to deeper lying areas of the exercising human soleus muscle (SOL). The effect of accurate localization and high temporal resolution on data specificity is investigated.

Materials and methods

To achieve high signal-to-noise ratio (SNR) at a temporal resolution of 6 s, a custom-built human calf coil array was used at 7T. The kinetics of phosphocreatine (PCr) and intracellular pH were quantified separately in SOL and gastrocnemius medialis (GM) muscle of nine volunteers, during rest, plantar flexion exercise, and recovery.

Results

The average SNR of PCr at rest was $64\pm 15$ in SOL ($83\pm 12$ in GM). End exercise PCr depletion in SOL ($19\pm 9$ %) was far lower than in GM ($74\pm 14$ %). The pH in SOL increased rapidly and, in contrast to GM, remained elevated until the end of exercise.

Conclusion

³¹P MRS in single-shots every 6 s localized in the deeper-lying SOL enabled quantification of PCr recovery times at low depletions and of fast pH changes, like the initial rise. Both high temporal resolution and accurate spatial localization improve specificity of Pi and, thus, pH quantification by avoiding multiple, and potentially indistinguishable sources for changing the Pi peak shape.