Relationship between muscle T-2* relaxation properties and metabolic state: a combined localized P-31-spectroscopy and H-1-imaging study

A multi-volume P-31-magnetic resonance spectroscopy localization procedure was implemented to compare directly muscle metabolism and proton T-2* relax ation properties in the human plantar flexor muscles during exercise. Local ized P-31-spectra were collected simultaneously from the medial gastrocnemi us, lateral gastrocnemius and soleus muscles during exercise using beta(1)- insensitive Hadamard Spectroscopic Imaging (HSI). T-2*-weighted gradient-ec ho images were acquired at rest and immediately following high-intensity pl antar 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 a t 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 p hosphocreatine (P-i:PCr) and the intracellular muscle pH shifted significan tly 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 rel axation parameters in both gastrocnemius revealed a significant relationshi p 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 loc alization, this study confirms the Existence of a strong relationship betwe en transverse relaxation properties and the metabolic state in skeletal mus cles engaged in heavy exercise.