Dynamic echo planar imaging of exercised muscle

Echo planar imaging was used to record dynamic changes in tissue transverse relaxation rates (ΔR2) 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 ΔR2. For a fixed workload of 70% of maximum voluntary contraction (MVC), the duration of dorsi-flexion exercise was varied and measurements of ΔR2 were obtained throughout exercise and for at least 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 ΔR2 decreased by an average of 8.7 s−1 within the tibialis with an average rate of decrease during exercise of ΔR2Δtex = −0.061 s−2. For the high time resolution studies we consistently observed that there was a continued decrease in the measured value of ΔR2 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 ΔR2Δtus = −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 ΔR2 may be indicative of an increase in the net water volume within the muscle, whereas in the case of slow exchange ΔR2 is primarily a measure of intracellular volume increases.