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Venous blood effects in spin‐echo fMRI of human brain

The spin‐echo response to visual activation was studied as a function of spatial resolution at a field of 1.5 T. The results showed that the increase in absolute T2 upon activation was as large as 22.8 ± 3.1% (P < 0.05) at the highest resolution (5.3 mm3), while it was as small as 3.5 ± 0.2% (P < 0.05) at the lowest resolution (42.2 mm3). In addition, upon increasing resolution, the spin‐echo signal decay as a function of echo time changed from monoexponential to nonexponential. These data indicate that, when using 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 blood oxygenation level‐dependent (BOLD) signal changes measured in voxels encompassing the activated brain areas. The data can be quantitatively accounted for using a model based on the intravascular origin of the spin‐echo effect including both macrovascular and microvascular effects. Existing theories for the spin‐echo BOLD effect based on diffusion through field gradients predict negligible spin‐echo effects inside the large vessels and are therefore incompatible with the data. Magn Reson Med 42:617–626, 1999. © 1999 Wiley‐Liss, Inc.