Effect of the anti-scatter grid and target/filters in full-field digital mammography

Computer Analysis Of Mammography Phantom Images (CAMPI) is a method for making quantitative measurements of image quality. This paper reports on further applications of the method to a prototype: full-field digital mammography (FFDM) machine. The specific aim was to investigate the effect on speck Signal-to-Noise-Ratio (SNR) of grid vs. non-grid techniques and different target-filter imaging conditions, for 4-cm thick phantoms. Images of a 50-50 composition, 4-cm thick phantom containing a mammography accreditation phantom insert plate, were acquired on a General Electric FFDM machine under conditions of constant pixel value and constant mean glandular dose. They were obtained under conditions of grid and no-grid with the Mo-Mo target/filter. Also acquired were 4-cm phantom grid images at constant dose using different targets-filter combinations (Mo/Mo, Mo/Rh, Rh/Rh) and different phantom material glandular-fat compositions (percentages: 30-70, 50-50, 70-30). Analyses of the images yielded signal-to-noise-ratio (SNR) for the specks and a non-uniformity measure. The SNR was converted to a Figure of Merit (FOM) by dividing by the square root of the mean glandular dose. For the grid-non-grid study, the FOM plots showed a broad maximum at about 26-28 kVp, meaning that this range is optimal in dose efficiency for imaging a 4-cm breast of 50-50 composition. The non-grid FOM values were greater than the grid values, meaning that the former was more dose-efficient. For the Target/Filter study the FOM also showed broad maxima as a function of kVp. The overall trends were as follows: (1) the Mo-Rh combination was superior to the Mo-Mo combination for all tissue compositions and kVps, and was generally superior to Rh-Rh except for kVp >30. (2) The optimal kVp moves towards higher values for more glandular (dense) breast equivalent material; (3) At the optimal kVp, Mo-Rh is the superior combination, outperforming both Mo-Mo and Rh-Rh for imaging a 4-cm breast. The non-uniformity was higher by 60% for the non-grid condition, and this negated its dose efficiency advantage: In the non-grid mode the non-uniformity is quite dependent on the phantom dimensions and position, implying that scatter is a si,significant contributing factor. The Rh-Rh combination also yielded a higher non-uniformity than did Mo-Mo and Mo-Rh. The Mo-Rh target filter combination appears to be the best overall choice for imaging the 4-cm breast of arbitrary composition. In-spite of its superior dose efficiency, we recommend-against using the non-grid technique until a better uniformity correction procedure is adopted.