The disclosure relates to a system for image decomposition of an anatomical projection image. The system comprises a data processing system which implements a decomposition algorithm of an projection image which is generated by irradiating a part of a subject with imaging radiation. A body portion within the irradiated part is a three-dimensional attenuation structure of an attenuation of the imaging radiation, wherein the attenuation structure represents a member of a predefined class of attenuation structures of the decomposition algorithm, thereby representing a classification of the body portion. The data processing system decomposes the projection image data using the classification of the attenuation structure. The decomposition of the projection image data substantially separates the contribution of the classified body portion to the projection image from the contribution of a further body portion of the subject to the projection image. The further body portion overlaps with the classified body portion in the projection image.
