Exercise Intensity Effects on the Muscle Functional MRI Signal Intensity Time Course: Absolute and Relative Contributions of BOLD Effects

摘要

The time course of exercise-induced T2-weighted signal intensity (SI) changes contains an initial rise, early dip, and secondary rise. The purposes of this study were to test the hypothesis that the secondary rise occurs earlier during more intense contractions and to determine the contribution of BOLD contrast to the SI changes. Eight subjects performed 90s isometric dorsiflexion contractions at 30 and 60% of maximum voluntary contraction (MVC) while T2-weighted (TR/TE=4000/35ms) images were acquired and total hemoglobin ([THb]) and oxy-Hb saturation (%HbO2) were measured. At 30% MVC, [THb] remained constant and %HbO2 decreased from 66.3% (SE=2.6) to 32.4% (SE=6.4). At t=88s, SI increased by ~8% and was greater than at t=8 and 56s. At 60% MVC, [THb] remained constant and %HbO2 decreased from 70.2% (SE=2.3) to 40.4% (SE=5.4). SI increased by ~17% and at t=56 and 88s was greater than at t=8 and 20s. The absolute contribution of calculated BOLD effects was –1% at 30 and 60% MVC. The relative contribution was greater at 30% than at 60% MVC (up to –26% and –10%, respectively). We conclude that the secondary rise occurs earlier at 60% MVC and that the relative contribution of BOLD effects is greater during less intense contractions.