1. A method for determining absorption coefficients for an object composed of a plurality of material types differing with absorption attributes, comprising: recording a plurality of x-ray beam projections of the object with monochrome x-rays from different positions; determining a first set of the absorption coefficients using a reconstruction method based on the recorded projections; calculating a plurality of x-ray beam projections by a reprojection based on a formula-based description of a rule taking account of polychromy artifacts, wherein the rule involves a plurality of parameters to be determined, wherein the parameters vary based on different material types; segmenting the first set of the absorption coefficients, wherein the segmenting comprises identifying at least a first material type in a path of each beam projection and determining for each beam projection a thickness consisting of said at least first material type; in parallel with the calculating of the plurality of x-ray beam projections and the segmenting of the first set of the absorption coefficients, determining with the reprojection the plurality of parameters used by the rule to account for the polychromy artifacts and correcting the recorded projections based on the calculated projections; and determining a second set of the absorption coefficients corrected with the polychromy artifacts using the reconstruction based on the corrected projections.