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Gradient-echo (GRE) and spin-echo (SE) EPI BOLD sequences were used to quantitate the effect of visual stimulation. Both sequences showed a positive BOLD response during stimulation and a negative BOLD response in the interstimulation intervals. The relaxation rate changes during stimulation were larger for the GRE sequence than for the SE sequence, whereas in the interstimulation intervals they were not significantly different. In both cases, the ratio of the GRE/SE relaxation rate changes were consistent with BOLD effects in larger vessels despite the well-known lower sensitivity of the SE sequence to the extravascular component of the BOLD effect in larger vessels. The most probable explanation of this result is that a significant fraction of the observed changes originated from the intravascular component of the BOLD effect. The SE sequence depicted smaller areas of activation than the GRE sequence with more than 85% of the pixels being depicted as significant by the SE sequence being also significant in the GRE activation maps. However, for the reverse comparison, an overlap of only 35% was observed, with many of the strongly correlated GRE pixels showing weak correlations in the corresponding SE activation image. Our results, together with the fact that signal undershoots have not been observed by groups using MR sequences that measure absolute flow changes for similar stimulation paradigms, suggest that the undershoot may be due to alterations in the blood volume and/or hematocrit during stimulation that normalize at a slower rate than the changes in blood flow after the cessation of the stimulation, leading to a poststimulation signal undershoot.

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