Briefly, the segmentation procedure in SPM12 returns probability maps for each of the five tissues of our interest. For each tissue, a NIfTI file is generated with an image with same size and origin as in the input file (e.g. Colin27 atlas) and each voxel is set to the probability of being part of a given tissue. And the sum of the probabilities of a given voxel along head tissues plus air is 1. For example, voxel (x = 57, y = 126, z = 144) of Colin27 atlas corresponding to MNI coordinate (x = −34, y = 0, z = 72) presented probability 86.27% for skull and 13.73% for CSF.