EXPERIMENTAL-DETERMINATION OF THE BOLD FIELD-STRENGTH DEPENDENCE IN VESSELS AND TISSUE

High resolution functional MRI (fMRI) experiments were performed in hu man visual cortex at 0.5, 1.5, and 4 T to determine the blood oxygenat ion level dependent (BOLD) field strength response within regions of o bvious venous vessels and cortical gray matter (''tissue''). T-2-weig hted FLASH images were collected in single- and multi-echo mode and us ed to determine the intrinsic BOLD parameters, namely, signal-to-noise ratio (Psi), the apparent transverse relaxation rate (R-2) and the c hange in R-2 (Delta R-2*) between the activated and baseline states. The authors find the average percentage signal change (Delta S/S, meas ured 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 si gnal-to-noise ratio in optimized, fully relaxed proton density weighte d 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 voxe ls containing vessels larger than the voxel itself and greater than li nearly in voxels containing a mixture of capillaries and veins/venules with a diameter less than that of the voxel.