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The BOLD signal with the typical gradient-echo (GE) sequence comes, to large extent, from the draining veins located at the cortical surface, reducing the spatial specificity of fMRI. Left: BOLD signal in GE data shows strongest signal at the cortical surface, which houses the large draining veins. Right: spin-echo (SE) data show strongest signal at the middle of the cortex and a lesser amount at the surface. Cat cortex. Reprinted from Zhao et al. (2006) with permission from Elsevier.
