Investigating the Field Strength Dependence of BOLD Onset Time

Ian D. Driver¹, Susan T. Francis¹, Penelope A. Gowland¹

This work assesses the origin of a shift to earlier BOLD onset times with increasing field strength, observed in response to a visual stimulus. Potential causes due to inflowing blood and different relative contributions of arteries and veins as a result of T2* changes across field strength are ruled out. Rather, a delayed venous blood oxygenation response with respect to blood flow increases, accompanied by a reduced capillary extravascular signal contribution at 1.5 T, consistent with previous Monte Carlo simulations, can be shown to cause the later onset time at 1.5 T compared to 7T.