Development and Monte Carlo analysis of antiscatter grids for mammography

Mammography arguably demands the highest fidelity of all x-ray imaging applications, with simultaneous requirements of exceedingly high spatial and contrast resolution. Continuing technical improvements of screen-film and digital mammography systems have led to substantial improvements in image quality, and therefore improvements in the performance of anti-scatter grids are required to keep pace with the improvements in other components of the imaging chain. The development of an air-core honeycomb (cellular) grid using x-ray lithography and electroforming techniques is described, and the production of a 60 mm x 60 mm section of grid is reported. A crossed grid was constructed with 25 microm copper septa, and a period of 550 microm. Monte Carlo and numerical simulation methods were used to analyze the theoretical performance of the fabricated grid, and comparisons with other grid systems (Lorad HTC and carbon fiber interspaced grids) were made over a range of grid ratios. The results demonstrate essentially equivalent performance in terms of contrast improvement factor (CIF) and Bucky factor (BF) between Cu and Au honeycomb grids and the Lorad HTC (itself a copper honeycomb grid). Gold septa improved both CIF and BF performance in higher kVp, higher scatter geometries. The selectivity of honeycomb grids was far better than for linear grids, with a factor of approximately 3.9 improvement at a grid ratio of 5.0. It is concluded that using the fabrication methods described, that practical honeycomb grid structures could be produced for use in mammographic imaging, and that a substantial improvement in scatter rejection would be achieved using these devices.

Grid and slot scan scatter reduction in mammography: comparison by using Monte Carlo techniques

PURPOSE

To evaluate a comprehensive array of scatter cleanup techniques in mammography by using a consistent methodology.

MATERIALS AND METHODS

Monte Carlo techniques were used to evaluate the Bucky factor (BF) and the contrast improvement factor (CIF) for linear and cellular grids and for slot scan and scanning multiple-slot assembly (SMSA) systems.

RESULTS

For a 28-kVp molybdenum anode-molybdenum filter spectrum with a standard detector and a 6-cm-thick 50% adipose-50% glandular breast phantom, slot scan techniques delivered an ideal BF. For slot widths greater than 4 mm, however, the CIF was lower than that achieved by the high-transmission cellular grid with a grid ratio of 3.8:1. A tungsten-septa air-interspaced cellular grid with a 4:1 grid ratio outperformed the high-transmission cellular grid in both BF and CIF. The SMSA was shown to be efficacious when 4-mm-wide slots were separated by at least 20 mm. In comparison with the literature, 3.6% agreement was achieved with other Monte Carlo studies, 3.3% with an experimental study that used a digital detector, and 13%-29% agreement was demonstrated in comparison to film-based experimental studies.

CONCLUSION

With use of consistent methods for comparison, cellular grids were shown to substantially outperform linear grids but have slightly higher BFs compared with that of slot scan geometries at the same CIF.