Small-field imaging properties of narrow energy band X-ray beams for mammography

X-ray imaging with low-energy, narrow-band, and tunable radiation offers the possibility of reducing dose and improving image contrast. The authors are investigating the production of narrow energy band X-ray beams with a standard X-ray tube by using Bragg diffraction on mosaic crystals for mammography application. Quasi-monochromatic X-ray beams (ΔE/E~0.1) have been produced in the mammography energy range. Small-field (1.1×3.0 cm²) radiographs of a plexiglas phantom 3.6 cm thick were obtained with 18, 20, and 22 keV quasi-monochromatic beams and a conventional film/screen combination for mammography. Images showed a non-uniformity in the irradiation field. A digital detector was used as imaging system to correct the phantom radiographs for this uneven illumination across the image. The overall contrast of the images decreases with increasing the energy of the beam from 18 keV to 22 keV. A measurement of the resolving power of the reflected beam has shown an asymmetric unsharpness along the two dimensions of the image plane. The actual focal spot has a size of about 0.2×0.05 cm²