By using the  deformation field  from nonlinear registration, we can build the correspondence between voxels in the template and each linearly-aligned image. For instance, for each voxel (x, y, z) in the template image, we can find its corresponding voxel (x + dx, y + dy, z + dz) in each linearly-aligned image, where (dx, dy, dz) is the displacement from the template image to the linearly-aligned image defined by the deformation field.