1. A system for radiographic imaging of body tissue comprising: a) a cell membrane-permeable, radio-opaque imaging agent; b) means for administering said imaging agent to a patient; c) means for generating a plurality of X-ray beams with predetermined different energy spectra; d) means for acquiring radiographic images disposed on the side of said patient opposite said X-ray beam generating means; and e) means for performing a weighted combination of a plurality of said radiographic images to produce a single image.