Aim of this paper is to give a general idea of the characteristic of a system for medical digital imaging based on a GaAs pixel detector bump-bonded to a VLSI electronic chip. After a discussion on the characteristics of the GaAs detector, and its detection and charge collection efficiency properties, it follows a description of the high-density channel VLSI chip named Photon Counting Chip (PCC). Then, a description of the apparatus used to characterize the detector and to perform electrical measurements is made. Finally are discussed irradiation tests and the laboratory setup used at the laboratory course of the ICFA 99 school. ©2000 American Institute of Physics.