1. A method to generate an x-ray tomosynthesis image of a breast of a patient comprising: operating an x-ray imaging apparatus to conduct two tomosynthesis scans in a chronological succession, each comprised of a plurality of projection images acquired at respective projection angles, with said x-ray imaging apparatus being operated at x-ray energies that differ from each other respectively in said two tomosynthesis scans; providing the projection images acquired in each of said two tomosynthesis scans to a computerized processor and, in said processor, for one of said two tomosynthesis scans, generating a synthetic projection image from at least two projection images acquired in said one of said two tomosynthesis scans, said synthetic projection image being a projection image at a projection angle that corresponds to a projection angle of one of the projection images acquired in the other of said two tomosynthesis scans; in said processor, subtracting said synthetic projection image and said one of said projection images from the other of said two tomosynthesis scans, to produce a difference image; in said processor, reconstructing an x-ray tomosynthesis image using said difference image; and making the x-ray tomosynthesis image available in electronic form at an output of said processor.