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 intrinsio BOLD parameters, namely, signal-to-noise ratio (Ψ), the apparent transverse relaxation rate (R₂*) and the change in R₂* (ΔR₂*) between the activated and baseline states. The authors find the average percentage signal change (ΔS/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 (ΔS/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.