Segmentation of Brain Metastases Using Background Layer Statistics (BLAST)

Key steps in defining the BLAST parameter space and performing segmentations with BLAST. Images and segmentations are shown in the left and middle columns, and the corresponding parameter space is shown in the right column. Each point in the scatterplot represents a voxel in the image. A, The statistics of the background layer (gray and white matter) are first defined using K-means clustering on a normal section of the brain. The origin of the parameter space is defined by the centroid of the background layer cluster and the axes expressed as z score relative to the background layer cluster SD. B, An ellipsoid approximating the background layer is applied to all slices (red ellipse). Voxels corresponding to enhancing brain tumor, edema, and CSF are found in the upper right, lower right, and lower left corner of parameter space, respectively. C, Thresholds in both T1 MPRAGE and T2 FLAIR are applied to exclude nontumoral voxels to perform segmentations. In the sample shown, thresholds in both parameters are set to the mean of the background layer (z score = 0). D, Voxels falling within the background layer ellipsoid are subtracted to further exclude the background layer. The segmentation is selected to save a mask of the 3D-connected enhancing BM voxels. An algorithm to fill in necrotic areas is applied for the final segmentation (not shown).