1. A method for generating a z-effective value 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 data of the object; 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 specified threshold.