SIGNAL UNDERSHOOTS FOLLOWING VISUAL-STIMULATION - A COMPARISON OF GRADIENT AND SPIN-ECHO BOLD SEQUENCES

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 significantl y different. In both cases, the ratio of the GRE/SE relaxation rate ch anges were consistent with BOLD effects in larger vessels despite the well-known lower sensitivity of the SE sequence to the extravascular c omponent of the BOLD effect in larger vessels. The most probable expla nation of this result is that a significant fraction of the observed c hanges originated from the intravascular component of the BOLD effect. The SE sequence depicted smaller areas of activation than the GRE seq uence with more than 85% of the pixels being depicted as significant b y the SE sequence being also significant in the GRE activation maps. H owever, for the reverse comparison, an overlap of only 35% was observe d, with many of the strongly correlated GRE pixels showing weak correl ations in the corresponding SE activation image. Our results, together with the fact that signal undershoots have not been observed by group s using MR sequences that measure absolute flow changes for similar st imulation paradigms, suggest that the undershoot may be due to alterat ions in the blood volume and/or hematocrit during stimulation that nor malize at a slower rate than the changes in blood flow after the cessa tion of the stimulation, leading to a poststimulation signal undershoo t.