Abstract
Digital mammography systems are likely to significantly differ in terms of the X-ray spectra, and the technologies for the X-ray detector systems used to acquire the image. As the case stands, the optimum imaging parameters are likely to be system specific rather than universal. Optimization of imaging parameters (target/filter and kVp) in digital mammography necessitates maximization of contrast while simultaneously minimizing patient radiation dose. The purpose of this work was to examine the impact of imaging parameters on the contrast-to-noise radio (CNR) and the average glandular dose (AGD) over a range of breast phantom thicknesses and compositions for a new system equipped with a tungsten anode in combination with rhodium (Rh) and silver (Ag) and to evaluate the characteristics of the physical image quality for a digital detector based on a direct detection flat-panel array design that employed an amorphous selenium converter. For determination of the optimal tube voltage, the analysis was based on the CNR from Al sheets positioned on homogeneous phantoms, and the figure of merit (FOM) was retrieved from the CNR and the AGD. Detective quantum efficiency (DQE) measurements followed IEC 62220-1-2 using an edge test device. The results show that, in general, the FOM is a function of kVp, with a stronger dependence on the breast phantom thickness than on the glandularity. In most cases, the tungsten/rhodium (W/Rh) combination is the best choice. The W/Ag combination provided the best results at thicknesses greater than 60 mm because the W/Ag combination produced more penetrating X-ray beam and then a lower value of the AGD. With an edge test device, the modulation transfer function (MTF) went down to 0.53 at the Nyquist frequency (5.88 cycles/mm). The low-frequency DQE value is approximately 64% with radiation quality RQA-M-4 (W/Rh, 28 kVp, 2-mm Al additional filtration) with neither a gird nor a compression paddle. The results of this study can be used to optimize the imaging parameters, and this approach should improve the performance of automatic exposure control (AEC) in digital mammography.
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