1. A method of computing an atomic number of a material examined by a radiological modality, the method comprising: accessing a first monochromatic image and a second monochromatic image of an object, the first monochromatic image acquired at a first energy level and the second monochromatic image acquired at a second energy level; obtaining a ratio of mass attenuation coefficients between the first monochromatic image and the second monochromatic image; and computing the atomic number for a material of the object based upon the ratio of mass attenuation coefficients.