Dynamic PCr and pH imaging of human calf muscles during exercise and recovery using 31 P gradient‐Echo MRI at 7 Tesla

Purpose Simultaneous acquisition of spatially resolved 31 P‐MRI data for evaluation of muscle specific energy metabolism, i.e., PCr and pH kinetics. Methods A three‐dimensional (3D) gradient‐echo sequence for multiple frequency‐selective excitations of the PCr and P i signals in an interleaved sampling scheme was developed and tested at 7 Tesla (T). The pH values were derived from the chemical shift‐induced phase difference between the resonances. The achieved spatial resolution was ∼2 mL with image acquisition time below 6 s. Ten healthy volunteers were studied performing plantar flexions during the delay between 31 P‐MRI acquisitions, yielding a temporal resolution of 9–10 s. Results Signal from anatomically matched regions of interest had sufficient signal‐to‐noise ratio to allow single‐acquisition PCr and pH quantification. The P i signal was clearly detected in voxels of actively exercising muscles. The PCr depletions were in gastrocnemius 42 ± 14% (medialis), 48 ± 17% (lateralis) and in soleus 20 ± 11%. The end exercise pH values were 6.74 ± 0.18 and 6.65 ± 0.27 for gastrocnemius medialis and lateralis, respectively, and 6.96 ± 0.12 for soleus muscle. Conclusion Simultaneous acquisition of PCr and P i images with high temporal resolution, suitable for measuring PCr and pH kinetics in exercise‐recovery experiments, was demonstrated at 7T. This study presents a fast alternative to MRS for quantifying energy metabolism of posterior muscle groups of the lower leg. Magn Reson Med 75:2324–2331, 2016. © 2015 Wiley Periodicals, Inc.