1. A method for determining an optimal monochromatic energy for all or a portion of a simulated monochromatic image, the method comprising: accessing at least a first image acquired using a first polychromatic spectrum and a second image acquired using a second polychromatic spectrum; generating material density maps for at least two materials using the first image and the second image; generating a plurality of simulated monochromatic images using the material density maps, wherein each simulated monochromatic image is at a different monochromatic energy; and for the plurality of simulated monochromatic images, determining an optimal monochromatic energy at which dispersion is minimized with respect to distributions generated using the simulated monochromatic images.