Optimal x‐ray spectra for screen‐film mammography

Abstract

Theoretical and experimental techniques were used to study optimal X-ray spectra for screen-film mammography. A simple model of mammographic imaging predicts optimum X-ray energies which are significantly higher than the K-characteristic energies of Mo. A subjective comparison of X-ray spectra from Mo-anode and W-anode tubes indicates that spectra produced by a W-anode tube filtered with materials of atomic number just above that of Mo are more suitable for screen-film mammography than spectra produced by the Mo-anode/Mo-filter system. The imaging performance of K-edge filtered, W-anode tube spectra was compared to the performance of Mo-anode spectra using phantom measurements and mastectomy specimen radiography. Optimal W-anode spectra can produce equal contrast with an exposure reduction of a factor of 2-3, a dose reduction of a factor of 2 and equal or reduced tube loading compared to Mo-anode spectra. A computer simulation was carried out to extend the initial, monoenergetic theory to the case of real, polychromatic sources. The effects of varying filter material and thickness, tube operating potential, and breast thickness were all studied. Since W-anode X-ray tubes are considered to be better for Xerox mammography than Mo-anode tubes, both Xerox and screen-film techniques can be performed optimally with a single, properly designed, W-anode X-ray tube.