The present invention comprises a method for generating a pseudo-CT image volume from multiple MRI scans through determining and then applying multi-variable polynomial mapping functions. An exemplary example extracts intensity values of voxels in images from acquired multiple MRI scans and CT scan from a plurality of training subjects to determine a multi-variable high-degree polynomial function for each of three regions, which is then applied to voxels in corresponding regions from images of multiple MRI scans of the target subject to generate the pseudo-CT image volume.