BIMETAL ANODE WITH TUNGSTEN OR RHODIUM - COMPARATIVE-STUDIES ON IMAGE QUALITY AND RADIATION EXPOSURE IN SCREEN-FILM MAMMOGRAPHY

Purpose: The impact of different anode materials (tungsten and rhodium) on spatial resolution, image contrast and radiation exposure was studied. Materials and methods: Two mammographic systems providing bimetal x-ray tubes (Mo/W and Mo/Rh) were compared by imaging a breast radiography phantom with additional acrylic plates from 3 to 8 cm thickness. Spatial resolution was evaluated using a line bar pattern. Image contrast was assessed by measuring the ratio of optical densities in a acrylic step-wedge. The entrance dose was measured. with a low energy ionisation chamber. Results: The spatial resolution was about 13 lp/mm regardless of the beam quality The image contrast depended substantially on the thickness. A similar image contrast was found with Mo/Mo, Mo/Rh and Rh/Rh for simulated breast thicknesses of 4 to 6 cm and with Rh/Rh and W/Rh for 7 cm. In comparison to Mo/Mo the dose reduction was significant for Mo/Ph (35 %), Rh/Rh (50 %) and W/Rh (60 %). Conclusions: Bimetal x-ray tubes provide optimal conditions for screen film mammography of both normal and dense breasts, allowing good contrast and dose reduction by using the adequate anode/filter-combination.