Resolution properties of a large-area quasi-monochromatic x-ray source for mammography

Abstract

Quasi-monochromatic X-rays with energy in the 16 - 24 keV range have been produced in our laboratory by making use of a conventional W-anode X-ray tube and a monochromator optical system via Bragg diffraction. The optical system is based on an array of mosaic crystals which produces an irradiation field obtained with adjacent reflected beams. The field size is about 10.5 X 12.0 cm 2 in the image plane. The introduction of an active optical component such as a mosaic crystal array may modify the imaging performances of the system. In order to investigate how the resolution properties of the quasi-monochromatic source are affected by the use of adjacent beams and the Bragg diffraction phenomenon, images of a hole pattern and a slit camera have been obtained with different magnification factors. The images of the hole pattern show duplication effects due to the partial superposition of adjacent beams. Slit camera images show an unexpected increase of the focal spot blurring along the diffraction plane. This behavior could be explained by considering each crystal monochromator of the array as a secondary source and consequently as a focal spot of the beam coming from it. Along the orthogonal direction the image blurring depends on the focal spot size of the X-ray tube.