Calculation of dose and contrast for two mammographic grids

To aid in selecting optimal conditions for screening mammography practice in Sweden, the performance of two mammographic grids (one moving and one stationary) has been investigated. Monte Carlo techniques were used to simulate photon transport in the breast. Transport through the breast support, grid covers, grid and image receptor (33.9 mg cm-2 Gd2O2S) was treated analytically. The contrast of a 100 microns calcification has been evaluated for three tissue compositions (adipose, glandular, 50:50 fractions by weight of adipose and glandular tissue) as a function of breast thickness (2-8 cm) and potential difference (25-30 kV, Mo anode). Contrast for a 5 cm 'average' breast at 28 kV was improved by 40% using the moving grid and by 30% using the stationary one; the corresponding increases in breast absorbed dose, keeping the energy imparted to the image receptor constant, were 90% and 150%, respectively. The superior properties of the moving grid were due to (i) equal scatter rejection ability and higher transmission of primary photons yielding lower scatter-to-primary ratios behind the grid, and (ii) less attenuation and filtering of the primary photons in the interspace material yielding lower degradation of primary contrast.