1. A method for generating a z-effective values for a set of voxels in a CT density image for a radiography imaging modality, comprising Acquiring the CT density image of an object; Acquiring sparse multi-energy projection date of the object; Wherein the sparse multi-energy projection data comprising data insufficient for generating a multi-energy image; Identifying one or more sets of voxels in the CT density image; Setting an initial z-effective value for respective sets of voxels to generate a CT z-effective image; Forward projecting the CT z-effective image to generate synthetic multi-energy projection data; Comparing the synthetic multi-energy projection data to the sparse multi-energy projection data to determine a measure of similarity between z-effective information of the synthetic multi-energy projection data and z-effective information of the sparse multi-energy projection data; and Updating the initial z-effective value for one or more sets of voxels to update the CT z-effective image when the determined measure of similarity between the z-effective information of the synthetic multi-energy projection data and the z-effective information of the sparse multi-energy projection data is below a specific threshold.