Quantitative differentiation between BOLD models in fMRI

Abstract

Several gradient‐echo fMRI blood oxygenation level‐dependent (BOLD) effects are described in the literature: extravascular spin dephasing around capillaries and veins, intravascular phase changes, and transverse relaxation changes of blood. This work considers a series of tissue compartmentalized models incorporating each of these effects, and tries to determine the model which is most consistent with the data. To isolate the different tissue contributions, a series of multi‐echo inversion recovery (IR) fMRI scans were performed. Visual stimulation experiments were performed at 1.5 T, one interleaved six‐echo and two IR six‐echo EPI scans (the latter to suppress gray matter (GM) and cerebrospinal fluid (CSF)). The tissue and vascular composition of activated areas was analyzed using independent spin‐echo IR MRI experiments and MR venography, respectively. This information was used to fit the multi‐echo fMRI data to the BOLD models. The activated areas almost always included a venous vessel visible on the venogram and consisted of GM and CSF. The fMRI signal changes were best described by extravascular dephasing 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.