A double -layer “edge-on” silicon microstrip detector has been designed and realized in the frame of the PICASSO (Phase Imaging for Clinical Application with Silicon detector and Synchrotron radiatiOn) project at the SYRMEP (SYnchrotron Radiation for MEdical Physics) beamline of Elettra (Trieste, Italy). The detector meets the requirements for synchrotron radiation mammography with patients inregarding: (a) size, since it covers the full beam width (210mm); (b) spatial resolution, determined by the 0.05mm strip pitch; (c) single-photon counting capabilities, because it is able to handle more than 10 6 photons/(pixel×s); (d) contrast resolution, thanks to a threshold trim DAC that equalizes the channel sensitivity; (e) efficiency, due to the high absorption in the 15–20mm sensor depth. Experimental measurements evidence charge sharing, though not compromising the spatial resolution.