the apparatus is an x - ray tube which generates collimated x - rays . the x - ray tube anode has an x - ray generating structure which is a single crystal , so that regardless of their locations of origin all the x - ray beams leave the structure at the same limited few angles . with the structure formed as a curve , one set of beams converges at the focal point of the curve , and with the structure flat , the beams illuminate an area with parallel , collimated , x - ray beams .