The invention concerns a system (1) comprising a X-ray source (1a) capable of supplying a conical X-ray beam at one first energy level called high energy, and a second energy level, called low energy; a two-dimensional X-ray detector (2) and electronic means (3a, 3b) for processing images supplied by said detector. The method is characterised in that it consists in acquiring a low energy image of a patient's anatomical zone (1c), acquiring a high energy image of said anatomical zone and in matching said acquired low energy and high energy images respectively, before constructing the map of bone densities of the anatomical zone.