Venous blood effects in spin-echo fMRI of human brain

The spin-echo response to visual activation was studied as a function of sp atial resolution at a field of 1.5 T. The results showed that the increase in absolute T-2 upon activation was as large as 22.8 +/- 3.1% (P < 0.05) at the highest resolution (5.3 mm(3)), while it was as small as 3.5 +/- 0.2% (P < 0.05) at the lowest resolution (42.2 mm(3)). In addition, upon increas ing resolution, the spin-echo signal decay as a function of echo time chang ed from monoexponential to nonexponential. These data indicate that, when u sing the standard resolution for fMRI studies at 1.5 T,the effects of spin- echo changes in the draining veins are of major contribution to the total b lood oxygenation level-dependent (BOLD) signal changes measured in voxels e ncompassing the activated brain areas. The data can be quantitatively accou nted for using a model based on the intravascular origin of the spin-echo e ffect including both macrovascular and microvascular effects. Existing theo ries for the spin-echo BOLD effect based on diffusion through field gradien ts predict negligible spin-echo effects inside the large vessels and are th erefore incompatible with the data. Magn Reson Med 42:617-626, 1999. (C) 19 99 Wiley-Liss, Inc.