Iterative material decomposition of multispectral image data includes providing a plurality of spectral images of a region of interest comprising a body part and a plurality of sets of material coefficients for a plurality of materials. Each spectral image is decomposed into a plurality of material images and an offset image. At least one of the material images for each spectral image is manipulated on the basis of at least one topological constraint relating to the body part to determine for each spectral image an updated plurality of material images and an updated offset image. A plurality of spectral images is then recomposed from the corresponding updated plurality of material images and the updated offset. Intensities at image locations are compared, and the updated plurality of material images is modified. The steps are repeated until convergence, and at least one of the recomposed spectral images at convergence is output.