The aim of this paper is to investigate the properties of a clinical FFDM unit (Giotto -Image MD, IMS Italy). The digital detector consists of a flat panel using the amorphous selenium technology (ANRAD Corporation, Canada). The active area of the imager is 17.4 cm × 23.9 cm (2048×2816 pixels) with a pixel pitch of 85 µm. The direct conversion of X-rays into charge provides excellent imaging performance. In this work we present an objective and complete characterization of such system: detector response, MTF, NPS and DQE calculation will be presented. MTF and DQE at Nyquist frequency (5.88 lp/mm) are equal to 38% and 15%, respectively. The detector linearity is very good under the typical mammographic tested exposure conditions.