Post-contractile BOLD contrast in skeletal muscle at 7T reveals inter-individual heterogeneity in the physiological responses to muscle contraction

Muscle blood oxygenation-level dependent (BOLD) contrast is greater in magnitude and potentially more influenced by extravascular BOLD mechanisms at 7T than it is at lower field strengths. Muscle BOLD imaging of muscle contractions at 7T could, therefore, provide greater or different contrast than at 3T. The purpose of this study was to evaluate the feasibility of using BOLD imaging at 7T to assess the physiological responses to in vivo muscle contractions. Thirteen subjects (four females) performed a series of isometric contractions of the calf muscles while being scanned in a Philips Achieva 7T human imager. Following 2s maximal isometric plantarflexion contractions, BOLD signal transients ranging from 0.3 to 7.0% of the pre-contraction signal intensity were observed in the soleus muscle. We observed considerable inter-subject variability in both the magnitude and time course of the muscle BOLD signal. A subset of subjects (n = 7) repeated the contraction protocol at two different repetition times (TR: 1000 and 2500 ms) to determine the potential of T1-related inflow effects on the magnitude of the post-contractile BOLD response. Consistent with previous reports, there was no difference in the magnitude of the responses for the two TR values (3.8 ± 0.9 versus 4.0 ± 0.6% for TR = 1000 and 2500 ms, respectively; mean ± standard error). These results demonstrate that studies of the muscle BOLD responses to contractions are feasible at 7T. Compared with studies at lower field strengths, post-contractile 7T muscle BOLD contrast may afford greater insight into microvascular function and dysfunction.