1. A dual-energy x-ray imaging system for taking two-dimensional images of a subject, said system comprising: (a) in physical sequence from front to back, an x-ray source, a front two-dimensional x-ray detector, a beam selection means, and a rear two-dimensional x-ray detector, said subject being located between said x-ray source and said front detector; (b) said x-ray source being adapted to emit x-rays with two different energy spectra for passage through said subject, said x-rays including primary x-rays having their direction of travel unaltered by interaction with said subject and said x-rays including scatter x-rays having their direction of travel altered by interaction with said subject; (c) said front detector receiving said primary x-rays and said scatter x-rays; (d) said rear detector receiving those of said x-rays passed by said beam selection means and having a plurality of selected locations and a plurality of shadowed locations; and (e) said beam selection means preventing passage of said primary x-rays to said shadowed locations, allowing passage of said scatter x-rays to said shadowed locations, and allowing passage of said primary x-rays and said scatter x-rays to said selected locations.