1. A system comprising: an interface to: acquire Q (Q≧2) energy images of M (M≧2) different materials, each one of the Q energy images associated with a respective range of photon energies; and a processor to: determine R (R≧2) combinations of N (N≧2) energy images from the Q energy images; determine an overlap associated with each of the R combinations of N energy images, the overlap associated with a combination of first N energy images indicating an extent of overlap between distribution functions associated with each of the M different materials and determined based on the first N energy images; and identify one of the R combinations of N energy images associated with a smallest overlap of the determined overlaps.