1) A new semi-automatic method is proposed to segment cortical gray matter from MRI scans in order to better visualize brain activity measured by fMRI. 2) The method involves 4 steps: segmenting white matter, selecting the white matter cortex component, verifying its topology matches anatomy, and growing gray matter classification out from the white matter boundary in a constrained way. 3) The semi-automatic method takes only 15 minutes compared to 18 hours for manual segmentation, and ensures the segmented gray matter is anatomically correct without tears or intersections.