Absolute and relative contributions of BOLD effects to the muscle functional MRI signal intensity time course: Effect of exercise intensity

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 90‐s isometric dorsiflexion contractions at 30% and 60% of maximum voluntary contraction (MVC) while T2‐weighted (TR/TE = 4000 ms/35 ms) 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% (standard error [SEM] = 2.6%) to 32.4% (SEM = 6.4%). At t = 88 s, 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% (SEM = 2.3%) to 40.4% (SEM = 5.4%). SI increased by ∼17% and at t = 56 and 88 s was greater than at t = 8 and 20 s. 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. Magn Reson Med 58:335–345, 2007. © 2007 Wiley‐Liss, Inc.