Dynamic echo planar imaging of exercised muscle

Abstract

Echo planar imaging was used to record dynamic changes in tissue transverse relaxation rates (delta R2) in the anterior tibialis muscle during dorsi-flexion exercise and recovery. Using a single spin-echo technique to calculate the change in relaxation rate produced by the exercise a time resolution of 4 s was achieved for each measurement of delta R2. For a fixed workload of 70% of maximum voluntary contraction (MVC), the duration of dorsi-flexion exercise was varied and measurements of delta R2 were obtained throughout exercise and for at lease 5 min of recovery. Comparisons were made between the single echo results and those obtained using multiple echo measurements of T2 with much lower time resolution, to verify that the two techniques gave the same results. We found on average that delta R2 decreased by an average of 8.7 s-1 within the tibialis with an average rate of decrease during exercise of delta R2/ delta t(ex) = -0.061 s-2. For the high time resolution studies we consistently observed that there was a continued decrease in the measured value of delta R2 after the exercise, reaching a minimum value about a minute after the exercise ceased. This average rate of undershoot during the postexercise period was given by delta R2/ delta t(us) = -0.035 s-2. This effect has not been noted previously in MR imaging studies and may be attributed to increased flow within the tissue as contracting muscle fibers relax following exercise. The results can be interpreted using simple fast exchange or slow exchange models for tissue water relaxation. For the case of rapid exchange the changes in delta R2 may be indicative of an increase in the net water volume within the muscles, whereas in the case of slow exchange delta R2 is primarily a measure of intracellular volume increases.