Bragg-diffraction-based quasi-monochromatic source for mammography using mosaic crystals

Abstract

The existence of an optimal energy range for mammography has been demonstrated by several authors. Improvement in image contrast and reduction of patient dose can be achieved using narrow energy band X ray beams in the 16 - 24 keV range. Quasi-monochromatic X rays in the mammographic energy range have been produced via Bragg diffraction by making use of a conventional W-anode, Be-window X ray tube and a monochromator optical system based on a set of mosaic crystals. The mosaic crystals are high oriented pyrolytic graphite (002) which provide an interesting choice for monochromators because of their high integrated reflectivity compared to perfect crystals. The monochromator optical system consists of an array of ten crystals (2.8 X 6.0 cm<SUP>2</SUP> of size) which are assembled so as to produce in the image plane an irradiation field obtained with adjacent reflected beams. A scanning technique of the optical system has been applied in order to remove the spatial non- uniformities of the entire irradiation field. The source has been characterized in terms of beam size and monochromaticity, photon flux and exposure rate, field uniformity, capability in low contrast detection, dose reduction, and spatial resolution properties. The system provides a large field (10.5 X 12.0 cm<SUP>2</SUP>) of quasi- monochromatic X rays ((Delta) E/E equals 12%) at the energy of 18 keV. The spatial resolution capabilities of the sources are affected by the introduction of an active optical element such as a mosaic crystal monochromator. They may be optimized by choosing the proper irradiation geometry. The mean glandular dose delivered to the standard breast by the quasi-monochromatic source is about a half of those delivered by the conventional mammography units.