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 generating a ratio image based on the ratio of mass attenuation coefficients, wherein the ratio image is a composite of the first monochromatic image and the second monochromatic image; determining a transfer function by computing the ratio of mass attenuation coefficients of two or more elements at the first and second energy levels based on a predetermined threshold value; applying the transfer function to the ratio image for computing the atomic number for a material of the object based upon the ratio of mass attenuation coefficients.