Relationship between muscle T2* relaxation properties and metabolic state:: a combined localized 31P-spectroscopy and 1H-imaging study

A multi-volume P-31-magnetic resonance spectroscopy localization procedure was implemented to compare directly muscle metabolism and proton T-2* relaxation properties in the human plantar flexor muscles during exercise. Localized P-31-spectra were collected simultaneously from the medial gastrocnemius, lateral gastrocnemius and soleus muscles during exercise using beta(1)-insensitive Hadamard Spectroscopic Imaging (HSI). T-2*-weighted gradient-echo images were acquired at rest and immediately following high-intensity plantar flexion exercise. T-2* mapping of the individual calf muscles showed that plantar flexion with the knee extended produces significant increases (P < 0.0001) in the mean (SEM) T-2* Of the medial [35.6 (1.2) ms vs 28.5 (0.5) ms at rest] and lateral gastrocnemius [35.6 (0.9) ms vs 26.2 (0.9) ms at rest], but not in the soleus [26.7 (0.6) ms vs 27.3 (0.8) ms at rest]. In accordance with the changes in T-2*, the ratio of inorganic phosphate to phosphocreatine (P-i:PCr) and the intracellular muscle pH shifted significantly in the gastrocnemii, while the soleus showed no change in muscle pH and only a moderate increase in P-i-to-Ph. Comparison of spectroscopic and relaxation parameters in both gastrocnemius revealed a significant relationship between post-exercise T-2* and intracellular pH (r = 0.72-0.76) and Pi-to-Ph ratios (r = 0.81-0.88) during exercise. Using an improved method of localization, this study confirms the Existence of a strong relationship between transverse relaxation properties and the metabolic state in skeletal muscles engaged in heavy exercise.