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"). T2*-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 (R2*) and the change in R2* (deltaR2*) between the activated and baseline states. The authors find the average percentage signal change (deltaS/S, measured at TE = T2*) 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 (B0). The predicted upper bound on BOLD contrast-to-noise ratio (deltaS/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.