A method comprising: receiving (904 1, 904 2) first and second computed tomography scan data (40 1, 40 2) and generating first and second data (48 1, 48 2) being volumetric computed tomography data indicative of radiation attenuations (μ1, μ2) for each of a plurality of locations in a volume space at respective first and second energy spectra by a reconstructor; receiving, for each of the plurality of locations in the volume space, the first and second data and a) performing (906 1) a first polychromatic correction based on the radiation attenuations (μ1) of the plurality of locations in the volume space at the first energy spectrum by a polychromatic corrector, wherein the polychromatic correction generates first corrected radiation attenuations of the plurality of locations in the volume space at the first energy spectrum (52 1), wherein the step of performing a first polychromatic correction includes generating and using a first attenuation map, and b) performing (906 2) a second polychromatic correction based on the radiation attenuations (μ2) of the plurality of locations in the volume space at the second energy spectrum by the polychromatic corrector, wherein the polychromatic correction generates second corrected radiation attenuations of the plurality of locations in the volume space at the second energy spectrum (52 2), wherein the step of performing a second polychromatic correction includes generating und using a second attenuation map; determining a material composition of each of the plurality of locations in the volume space as a function of its location in an attenuation space defined by the corrected radiation attenuation at the first energy spectrum and the corrected radiation attenuation at the second energy spectrum by a material classifier; generating an image of the volume space indicative of the material composition at the plurality of locations in the volume space by an image processor. receiving (904 1, 904 2) first and second computed tomography scan data (40 1, 40 2) and generating first and second data (48 1, 48 2) being volumetric computed tomography data indicative of radiation attenuations (μ1, μ2) for each of a plurality of locations in a volume space at respective first and second energy spectra by a reconstructor; receiving, for each of the plurality of locations in the volume space, the first and second data and a) performing (906 1) a first polychromatic correction based on the radiation attenuations (μ1) of the plurality of locations in the volume space at the first energy spectrum by a polychromatic corrector, wherein the polychromatic correction generates first corrected radiation attenuations of the plurality of locations in the volume space at the first energy spectrum (52 1), wherein the step of performing a first polychromatic correction includes generating and using a first attenuation map, and b) performing (906 2) a second polychromatic correction based on the radiation attenuations (μ2) of the plurality of locations in the volume space at the second energy spectrum by the polychromatic corrector, wherein the polychromatic correction generates second corrected radiation attenuations of the plurality of locations in the volume space at the second energy spectrum (52 2), wherein the step of performing a second polychromatic correction includes generating und using a second attenuation map; determining a material composition of each of the plurality of locations in the volume space as a function of its location in an attenuation space defined by the corrected radiation attenuation at the first energy spectrum and the corrected radiation attenuation at the second energy spectrum by a material classifier; generating an image of the volume space indicative of the material composition at the plurality of locations in the volume space by an image processor.