Features.  For each of the intensity-corrected T1, T2, and FLAIR images (left), four 3D rotationally-invariant derivative-based local image filters were computed, consisting of the Gaussian, magnitude of the gradient, Laplacian of Gaussian, and magnitude of the Hessian (middle).  Each was computed at three spatial scales or levels (2, 4, and 8 times lower resolution), shown for the T1 image contrast (right), resulting in a 39-feature vector for each voxel.