1. A combo x-ray system for acquiring 2D and 3D images of the breast in a single compression, comprising: an x-ray source, a flat panel digital x-ray imager, and a breast support and a compression paddle between the source and the imager, said compression paddle configured for selectively compressing the breast against the breast support; a motion control configured for the source relative to a patient's breast while the breast remains immobilized in compression between the breast support and the compression paddle, and selectively energizing the source to emit x-rays through the breast to the imager at selected angular positions of the source relative to the breast; a processor configured to control the imager to derive therefrom projection images of the compressed breast when the source is energized at each of said positions, wherein (i) at least one of the projection images is a mammogram image taken at a same or similar x-ray dose to the breast as a conventional mammogram and (ii) each of a multiplicity of the other projection images is a tomosynthesis projection image taken at an x-ray dose to the breast that is substantially less than that for a conventional mammogram; said processor being further configured to process the multiplicity of the tomosynthesis projection images into tomosynthesis slice images representing slices of the patient's breast conforming to planes that are selectively the same or different from that of the mammogram image; and a display configured for displaying said mammogram image and one or more of said tomosynthesis slice images selectively side by side or superimposed.