Localized Semi-LASER Dynamic 31P Magnetic Resonance Spectroscopy of the Soleus During and Following Exercise at 7 T

Abstract

Object This study demonstrates the applicability of semi-LASER localized dynamic $^{31}$P 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 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 9 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%). pH in SOL increased rapidly and, in contrast to GM, remained elevated until the end of exercise. Conclusion $^{31}$P 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 Pi peak shape.