Absence of a significant extravascular contribution to the skeletal muscle BOLD effect at 3 T

Blood oxygenation level dependent (BOLD) contrast in skeletal may reflect the contributions of both intravascular and extravascular relaxation effects. The purpose of this study was to determine the significance of the extravascular BOLD effect in skeletal muscle at 3 T. In experiments, R₂* was measured before and during arterial occlusion under the following conditions: (1) the leg extended and rotated (to vary the capillary orientation with respect to the amplitude of static field) and (2) with the blood's signal nulled using a multiecho vascular space occupancy experiment. In the leg rotation protocol, 3 min of arterial occlusion decreased oxyhemoglobin saturation from 67% to 45% and increased R₂* from 34.2 to 36.6 sec^-1, but there was no difference in the R₂* response to occlusion between the extended and rotated positions. Numerical simulations of intra- and extravascular BOLD effects corresponding to these conditions predicted that the intravascular BOLD contribution to the R₂* change was always > 50 times larger than the extravascular BOLD contribution. Blood signal nulling eliminated the change in R₂* caused by arterial occlusion. These data indicate that under these experimental conditions, the contribution of the extravascular BOLD effect to skeletal muscle R ₂* was too small to be practically important.