A comparison of doubly curved crystals and polycapillary optics for monochromatic beam production from a clinical source

Monochromatic beams produced with synchrotron sources are known to give higher contrast for mammography than clinical broadband sources. Monochromatic beams could also be achieved with clinical x-ray sources, by diffraction off of flat monochromator crystals, but monochromatic intensities are too low for imaging because only a small fraction of the incident beam is at the right energy and angle. With the use of polycapillary optics, monochromatic intensities could be increased. Two different x-ray optics schemes were tested to provide high monochromatic intensity from conventional divergent sources. A polycapillary collimating optic was employed to collect a large solid angle and redirect it into a parallel beam, which can be efficiently diffracted from a flat crystal. Measurements were performed for crystals of varying angular acceptance because there is a trade-off between intensity and resolution. Alternatively, doubly curved crystal (DCC) optics can be used to collect and focus monochromatic x rays from a divergence source. Higher monochromatic intensity can be obtained because the DCC optic diffracts and focuses the incident beam across the whole area of the crystal. For both methods, monochromatization occurs before the patient, resulting in a potential dose reduction as well as significant measured contrast enhancement. Measurements were made of contrast, resolution and intensity for the two techniques, and were compared to each other and to theoretical calculations.