a method and apparatus which corrects the polychromatic distortion of ct images which is produced by the non - linear interaction of body constituents with a polychromatic x - ray beam . a ct image is processed to estimate the proportion of the attenuation coefficients of the constituents in each pixel element . a multiplicity of projections for each constituent are generated from the original image and are combined utilizing a multidimensional polynomial which approximates the non - linear interaction involved . an error image is then generated from the combined projections and is subtracted from the original image to correct for the poloychromatic distortion .