Low contrast imaging with a GaAs pixel digital detector

A digital mammography system based on GaAs pixel detector has been developed by the INFN (Istituto Nazionale di Fisica Nucleare) collaboration MED46. The high atomic number makes the GaAs a very efficient material for low energy X-rays detection (10 - 30 keV is the typical energy range used in mammography). Low contrast details can be detected with a significant dose reduction to the patient. The system presented in this paper consists of a 4096 pixel matrix built on a 200 mu m thick Semi Insulating GaAs substrate. The pixel size is 170 x 170 mu m(2) for a total active area of 1.18 cm(2). The detector is bump-bonded to a VLSI Front-End:chip which implements a single-photon counting architecture. This feature allows to enhance the radiographic contrasts detection with respect to charge integrating devices. The system has been tested by using a standard mammographic tube. Images of mammographic phantoms will be presented and compared with radiographs obtained with traditional :film/screen systems. Monte Carlo simulations have been also performed to evaluate the imaging capability of the system. Comparison with simulations and experimental results will be shown.