Full field images of mammographic phantoms obtained with a single photon counting system

As the use of digital radiograplic equipment in the morphological imaging field is becoming largely diffuse, the research of new and more performing devices from public institutions and industrial companies is in constant progress. Many of these devices are based on solid-state detectors as X-ray sensors. Semiconductor pixel detectors, originally developed in the high-energy physics environment, have been then proposed as digital detector for medical imaging applications. In this paper a digital single photon counting device, based on silicon and GaAs pixel detectors, is presented. The detector is a thin slab of semiconductor crystal equipped with an array of 64 by 64 square contacts, 170-mum side. The data read-out is performed by a VLSI integrated circuit named Photon Counting Chip (PCC), developed within the MEDIPIX collaboration. Each chip cell geometrically matches the sensor pixel. It contains a charge preamplifier, a threshold comparator and a 15 bits pseudo-random counter and it is coupled to the detector by means of bump-bonding. Most important advantages of such a system, with respect to a traditional X-rays film/screen device, are the wider, linear dynamic range (3x10(4)) and the higher performance in terms of MTF and DQE. Electronics read-out performance as well as imaging capabilities of the digital device will be presented. Images of mammographic phantoms acquired with a standard mammographic tube will be compared with radiographs obtained with traditional film/screen systems