SEGMENTATION ANALYSIS IN FUNCTIONAL MRI - ACTIVATION SENSITIVITY AND GRAY-MATTER SPECIFICITY OF RARE AND FLASH

Brain activation is accompanied by local decreases in vascular deoxyhe moglobin. Theoretically, gradient-echo and spin-echo sequences show si milar sensitivity to capillary deoxyhemoglobin, but spin-echo sequence s should be less sensitive to venous deoxyhemoglobin. This is an impor tant distinction in the context of cortical localization. We report he rein a direct experimental comparison of a gradient-echo sequence (fas t low-angle shot [FLASH]) with a spin-echo sequence (rapid acquisition with relaxation enhancement [RARE]) for functional MRI (fMRI) in seve n subjects undergoing visual stimulation. A Student t test analysis wa s used to locate areas of significant activation, and then computerize d image segmentation was performed to determine the type of activated tissue. Contrary to previous reports, both sequences proved equally se nsitive to overall activation. RARE activation, however, was more spec ific for gray matter, as suggested by prior theoretical models.