1. An image processing method for medical imaging including processing a radiographic image acquired with at least two energy levels, said method comprising the steps of: providing (S 1 ), to a processing circuitry and memory, energy-resolved image data, either from a detector or from an intermediate data storage, the energy-resolved image data representative of said radiographic image with at least two energy levels; and generating, by way of the processing circuitry and memory, at least one image representation for medical imaging by decomposing (S 2 ) said provided image data into at least one basis image representation, based on a model where a combination of at least two basis functions is used to express a representation of at least one linear attenuation coefficient, where at least one ordinary basis function models a physical material, and an NLPV basis function models the Non-Linear Partial Volume (NLPV) effect, wherein said at least one other basis function that models the NLPV effect is constructed by: selecting (S 11 ) a set of at least one ordinary basis function, such that the energy dependent linear attenuation coefficients of homogeneous materials in the image volume can be described by a linear combination of the basis functions in the set, determining (S 12 ) the fraction of transmitted photons as a function of energy for a detector element situated behind an inhomogeneous region of materials, as seen from the source, taking into account the spatially extended nature of either the detector, the source or both, and determining (S 13 ) at least one NLPV basis function as the linear attenuation coefficient of a fictious material which, if one unit of distance of a homogeneous slab of this material were traversed by an x-ray beam, would give the same transmitted photon fraction as a function of energy as said calculated fraction which is transmitted through an inhomogeneous region.