Experimental determination of the BOLD field strength dependence in vessels and tissue

High resolution functional MRI (fMRI) experiments were performed in human visual cortex at 0.5, 1.5, and 4 T to determine the blood oxygenation level dependent (BOLD) field strength response within regions of obvious venous vessels and cortical gray matter (''tissue''). T-2*-weighted FLASH images were collected in single- and multi-echo mode and used to determine the intrinsic BOLD parameters, namely, signal-to-noise ratio (Psi), the apparent transverse relaxation rate (R-2*) and the change in R-2* (Delta R-2*) between the activated and baseline states. The authors find the average percentage signal change (Delta S/S, measured at TE = T-2*) to be large in vessels (13.3 +/- 2.3%, 18.4 +/- 4.0%, and 15.1 +/- 1.2%) compared with that in tissue (1.4 +/- 0.7%, 1.9 +/- 0.7%, and 3.3 +/- 0.2%) at 0.5, 1.5, and 4 T, respectively, The signal-to-noise ratio in optimized, fully relaxed proton density weighted gradient echo images was found to increase linearly with respect to the static magnetic field strength (B-0), The predicted upper bound on BOLD contrast-to-noise ratio (Delta S/R)(max) as a function of field strength was calculated and found to behave less than linearly in voxels containing vessels larger than the voxel itself and greater than linearly in voxels containing a mixture of capillaries and veins/venules with a diameter less than that of the voxel.