Abstract Diffraction-enhanced X-ray imaging (DEI) is one of a class of imaging techniques developed at a synchrotron that is based on contrast mechanisms other than absorption. This method uses perfect crystal optics to prepare and analyze beams that traverse the object imaged. The combination of a highly collimated beam along with an analyzer gives such system sensitivity to X-ray refraction and ultra-small-angle scattering contrast (extinction). The translation of the system used at the synchrotron to a conventional X-ray tube source has challenges that must be overcome for a practical system to be built. One of those restrictions is the propagation of unwanted energies through the parallel matched crystal system. This paper addresses a method of eliminating such unwanted energies from the imaging beam using a mismatched two crystal system to prepare the beam. The properties of this mismatched system are found along with the restrictions in applying this method to a practical DEI system.