Several gradient-echo fMRI blood oxygenation level-dependent (BOLD) effects
are described in the literature: extravascular spin dephasing around capil
laries and veins, intravascular phase changes, and transverse relaxation ch
anges of blood. This work considers a series of tissue compartmentalized mo
dels incorporating each of these effects, and tries to determine the model
which is most consistent with the data, To isolate the different tissue con
tributions, a series of multi-echo inversion recovery (IR) fMRI scans were
performed. Visual stimulation experiments were performed at 1.5 T, one inte
rleaved six-echo and two IR six-echo EPI scans (the latter to suppress gray
matter (GM) and cerebrospinal fluid (CSF)), The tissue and vascular compos
ition of activated areas was analyzed using independent spin-echo IR MRI ex
periments and MR venography, respectively. This information was used to fit
the multi-echo fMRI data to the BOLD models, The activated areas almost al
ways included a Venous vessel visible on the venogram and consisted of GM a
nd CSF. The fMRI signal changes were best described by extravascular dephas
ing effects in both GM and CSF around a venous vessel, in combination with
intravascular effects. The role of spin dephasing around capillaries in GM
appears to be insignificant, Magn Reson Med 45:233-246, 2001, (C) 2001 Wile
y-Liss, Inc.