Assessing study-specific regional variations in fMRI signal

In this paper, we present a post hoc method for identifying regions where f unctional MRI data are subject to signal reduction that may compromise sens itivity to activations. The motivation for developing this technique derive s from recent language studies that showed responses in the inferior tempor al lobe that could be detected by PET but not by fMRI. Reduced signal is du e mostly to susceptibility artifacts and can be identified by comparing the T2* images (which are subject to susceptibility artifacts) with T2 images (which are not). However, T2 images are not usually acquired in fMRI studie s. Therefore, we propose that areas with reduced signal can be identified b y comparing T2* images that are corrected for nonuniformity with the origin al uncorrected images. The technique provides a voxel-wise characterization of signal reduction that pertains to the particular data that enter into a statistical model. It requires only the functional data and can thus be ap plied post hoc and without any additional scans. (C) 2001 Academic Press.