1. A method for producing a CT image comprising: acquiring CT data of a region-of-interest (ROI) of a subject, the ROI having constituent components with known characteristics; reconstructing the CT data into sub-images corresponding to each of the constituent components by iteratively minimizing a model that relates the acquired CT data to estimates of the sub-images and accounts for the known characteristics of the constituent components; and combining the reconstructed sub-images to produce the CT image.