Comparison of matched BOLD and FAIR 4.0T-fMRI with [O-15]water PET brain volumes

Valid comparisons of functional activation volumes from fMRI and PET requir e accurate registration, matched spatial resolution, and if possible matche d noise. We coregistered 4.0T-fMRI and PET volumes, using a series of linea r and nonlinear transformations applied to the PET volumes. Because of the limited number of fMRI slices that were available, PET volumes were transfo rmed to the fMRI space. Since 4.0T-fMRI and 4.0T-MRI volumes have significa nt spatial distortion due to magnet inhomogeneities, high resolution 1.5T-M RI volumes were nonlinearly transformed to 4.0T-MRI volumes as part of the transformation chain. The smoothing effects of these registration transform ations were measured, in order to match the spatial resolution of the coreg istered fMRI and PET volumes. Spatial resolution of the transformed PET vol umes in the fMRI space was degraded by up to 60% due to the transformation process. Due to both the image acquisition characteristics and the coregist ration process, the transformed PET volumes had a spatial resolution that w as lower than that of fMRI. Therefore, significant smoothing of fMRI volume s was necessary to match their spatial resolution with that of the transfor med PET volumes. Matching the spatial resolution of the fMRI volumes to tho se of the transformed PET volumes was achieved by matching the shape of the ir point spread functions. In order to do this, Gaussian kernels were emplo yed to smooth the fMRI volumes. We were unable to simultaneously match the resolution and noise of fMRI and PET signals in the motor cortex. Activatio n maps derived from transformed PET and smoothed fMRI volumes were compared . Contralateral motor cortex was active in all modalities but there were la rge variations in the size of the activated region and its signal to noise ratio across BOLD, FAIR, and PET images within each subject. Nevertheless, the relative CBF changes measured by FAIR were consistent with those determ ined by PET. (C) 1999 American Association of Physicists in Medicine. [S009 4-2405(99)01608-9].