A project for digital mammography based on a GaAs pixel detector and on a self-triggering single photon counting acquisition system

The standard detector for mammography is the screen-film system, which consists of a photographic film coupled to an intensifying fluorescent screen; the latter improves the detection efficiency, but reduces the spatial resolution. The limited dynamic range of the film and other factors like inhomogeneities in the emulsion, variations in the chemical development of the film contribute in putting a lower limit (about 3 %) in the minimum detectable contrast, a very important parameter for an early detection of tumors. A direct digital approach can be performed using an integrating read-out or a single photon counting system. The former finds its most popular example in the CCDs. In the other type of digital detectors the detected photons are individually counted. In this paper we describe the project of a detection system for digital mammography based on a GaAs pixel array coupled, via bump-bonding, to a VLSI front-end chip which includes, for each channel, an amplifier, a discriminator and a scale and which performs the single photon counting. The imaging performance of this system in terms of the minimum detectable contrast and of the related dose to the patient is discussed and a comparison with the traditional screen-film system is made.