Air gap effect on mammography image quality

One of the techniques of reducing scatter in mammography imaging is by introducing an air-gap (AG) between the detector and the breast. We studied the effect of AG size on image contrast and resolution. Monte Carlo simulation code (MCNP5) was used to simulate the dimensions and materials that closely mimic clinical mammography. The Scatter to Primary Ratio (SPR) was calculated for different x-ray tube peak voltages, breast phantom thicknesses, and AG sizes. A Molybdenum target/ Molybdenum filtered x-ray spectra was used for a semicircular shape breast phantom made of 50% adipose and 50% glandular tissue equivalent materials. We also analyzed the effect of AG on focal spot limited resolution. The SPR decreased with the size of the AG. For 3, 5, and 8 cm phantom thicknesses, the SPR decreased by 21±3% by increasing the AG form 1.5 to 5 cm. Increasing the AG to 10 cm decreased the SPR by 45 ±3%. For AGs larger than 10 cm, the SPR differential improvement was not as large as for AGs smaller than 10 cm. The resolution also decreases significantly with AG size due to the increase in magnification. However, even with a 15 cm AG it is possible to obtain a minimum resolution of 13 line pairs/mm. To maintain the same level of exposure to the film-screen detector, the exposure needs to be increased by 1.16 times for 5 cm and 1.33 times for 10 cm AGs. We have simulated the effect of increasing AG on scatter in mammography imaging and found very significant improvement that are comparable to those obtained with antiscatter grids with much less increase in exposure. However, the resolution is also reduced by the AG and should be measured to satisfy minimum resolution requirements.