1. A system for developing a radiotherapy treatment plan, the system comprising: a data storage device configured to hold MR image data acquired by an MRI system; at least one processor configured to: receive the MR image data from the data storage device; apply corrections to the MR image data to produce a series of corrected image data; assemble the series of corrected image data to generate a set of relaxometry maps; perform a segmentation of a plurality of regions of interest using the set of relaxometry maps; classify the plurality of regions of interest, using the set of relaxometry maps, to yield a plurality of classified structures; assign electron density values to the classified structures using an assignment process; generate, using the electron density values of the classified structures, a set of corrected synthetic electron density images; and perform a dose calculation using the corrected synthetic electron density images to develop a radiotherapy treatment plan.