The SenoScan full-field digital mammography scanner uses a scanning slot detector that is 10 mm wide and 220 mm long. The X-ray beam is collimated to just outside the area of the detector. One important advantage of slot scanning is its inherent scatter rejection. As previously reported, the SenoScan slot scatter rejection is better than that obtained using a 3.5:1 mammography grid, and somewhat worse than that with a 5:1 grid. Additional scatter reduction can potentially improve the contrast in images of thick breasts. We evaluate a custom-designed grid for the slot scanning system. The grid is one-dimensional, offering scatter rejection along the longitudinal axis of the detector. We evaluate the reduction in scatter fraction, grid absorption and changes in the signal-difference-to-noise ratio (SDNR). Based on phantom studies, our results show effective scatter reduction by the grid with minimal reduction of SDNR. Grid absorption and scatter elimination do not necessarily lead to an increase in patient dose, especially if there is a improvement in the number of digital values in the image that are within the useful dynamic range of the detector. A benefit of removing the scatter contribution is an improvement in system dynamic range, because electronic detector gain adjustments can compensate for the drop in the digital pixel values.
The FDA has approved the SenoScan slot-scanning Full-field Digital Mammography system. A high power Tungsten-target x- ray tube enables breast imaging with 0.22 s effective exposure time. A 21-cm X 29-cm area is scanned in less than 6 seconds, at a typical clinical technique of 30 kVp, 170 mA. The detector comprises a Thalium-doped Cesium Iodide scintillator coupled to a combination of four CCDs abutted along their narrow dimension to from a 10-mm by 210-mm slot. With the CCDs operated in time-delay-and-integration mode along the narrow dimension, the system functions in a continuous scanning mode. The MTF in the standard and high- resolution modes extend to 10-cycles/mm and beyond 14 cycles/mm respectively. The Detective Quantum Efficiency curve starts at 50 percent at DC and extends to 10 cycles/mm in Standard model. Accordingly the SenoScan system enables screening and diagnostic breast imaging with a limiting resolution approaching that of film-based systems. The overall system design and intrinsic scatter rejection efficiency directly translate in high DQE characteristics that enable screening at a significantly reduced patient dose.