Ra. Jones et al., SIGNAL UNDERSHOOTS FOLLOWING VISUAL-STIMULATION - A COMPARISON OF GRADIENT AND SPIN-ECHO BOLD SEQUENCES, Magnetic resonance in medicine, 40(1), 1998, pp. 112-118
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.