We describe a high-resolution digital x-ray detector suitable for producing high quality mammographic images. The detector consists of an array of 3584 by 4096 pixels on 70 micrometer centers covering an area of 25 cm by 29 cm. The conversion layer of the detector is 250 micrometer thick amorphous selenium. Each pixel of the array consists of a storage capacitor for collecting x-ray signals and an amorphous silicon switching transistor. The signal is read out by custom high-speed, low-noise electronics. The integration of this detector with a mammographic x-ray system and acquisition console is described, as well as algorithms for calibration of the full system. We review characterization of the imaging performance of our system based on quantitative analyses of MTF and DQE data, and compare experimental results with theoretical calculations. We compare the performance of our direct conversion system with that of screen/film analog systems and indirect conversion digital detectors, such as LORAD's CsI/CCD detector, operated under similar conditions. MTF degradation mechanisms and system noise sources and their effect on DQE are discussed. We review qualitative aspects of image quality from our detector and present preliminary observer performance characteristics on clinical studies run with our system.
Direct conversion of x-ray energy into electrical charge has been extensively developed into imaging products in the past few years. Applications include general radiography, mammography, x-ray crystallography, portal imaging, and non-destructive testing. Direct methods avoid intermediate conversion of x-rays into light prior to generating a measurable electrical charge. This eliminates light scattering effects on image sharpness, allowing detectors to be designed to the limit of the theoretical modulation transfer function for a discrete-pixel sensor. Working exposure range can be customized by adjusting bias and thickness of sensor layers in coordination with readout-electronics specifications. Mature amorphous selenium technology and recent progress on high-quality Thin-Film Transistor (TFT) arrays for computer displays have allowed development of practical large-area high-resolution flat-panel x-ray imaging systems. A variety of design optimizations enable direct-conversion technology to satisfy a wide range of applications.