Solid state detector for slit-scan mammography

The presentation concerns evaluations that were made to demonstrate the feasibility of a solid state detector intended to produce digital images in slit scan mammography equipment. The detector designed for this modality consists of a multilinear array of x ray sensitive elements. It operates in time delay integration mode. A CsI phosphor screen is directly coupled to a TDI CCD circuit. Typical cell size is 50 micrometers X 50 micrometers . The typical number of lines in the TDI direction is 200. The image format is 4800 X 6000 for a 24 cm X 30 cm field of view. Image acquisition time is 5 seconds. The output signal is converted into a 12 bit coded digital signal. Achievable performance has been demonstrated by performing experiments on a 1024 X 1024 CCD matrix. Cell size is 19 micrometers X 19 micrometers . Cells are electronically binned to achieve a 57 micrometers X 57 micrometers pixel dimension. The matrix is operated in TDI mode along columns, over a 10 mm wide sensitive area. A CsI phosphor layer is applied on the chip surface. Screen density is 65 mg/cm2. The measured conversion efficiency is 190 electrons per absorbed x-ray quantum, 20 keV in energy. The detective quantum efficiency exceeds 50%. The modulation exceeds 10% at a spatial frequency of 8.8 lp/mm (Nyquist frequency of the experimental device). Experiments were conducted to estimate the performance of a Gd202S-based detector. The conclusion is that Gd202S is prohibited in such a detector structure. The major limitation lies in the noise contribution associated with the direct interaction of x-ray quanta in the silicon itself. This contribution is negligible in the case of a CsI-based detector.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.