A system and method of density and effective atomic number imaging include a computer programmed to acquire projection data from the detector of an unknown material at the time of projection data acquisition. The computer is also programmed to generate a density image for the unknown material based on a calibration of two or more known basis materials and to generate an effective atomic number (Z) for the unknown material based on the calibration of two or more known basis materials and based on a function arctan of a ratio of atomic numbers of the two or more known basis materials. The density and effective atomic number images are stored to a computer readable storage medium.