• J M Oja, J Gillen, R A Kauppinen, M Kraut, and P C van Zijl.
• Department of Radiology, Johns Hopkins University Medical School, Baltimore, Maryland. oja@messi.uku.fi
• Magn Reson Med. 1999 Oct 1; 42 (4): 617-26.

AbstractThe 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 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 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.Copyright 1999 Wiley-Liss, Inc.

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