A 4096 pixel Photon Counting Chip (PCC) has been developed and tested. It is aimed primarily at medical imaging although it can be used for other applications involving particle counting. The readout chip consists of a matrix of 64 x 64 identical square pixels, whose side measures 170 mm and is bump-bonded to a similar matrix of GaAs or Si pixel diodes covering a sensitive area of 1.18 cm . The electronics in each cell comprises a preamplifier, a discriminator with variable threshold and a 3-bit threshold tune as well as 15-bit counter. Each pixel can be individually addressed for electrical test or masked during acquisition. A shutter allows for switching between the counting and the readout modes and the use of a static logic in the counter enables long data taking periods. Electrical tests of the chip have shown a maximum counting rate of up to 2 MHz in each pixel. The minimum reachable threshold is 1400 e with a variation of 350 e rms that can be reduced to 80 e rms after tuning with the 3-bit adjustment. Electical noise at the input is 170 e rms. Several read-out chips have been bump-bonded to 200 mm thick GaAs detectors. Tests with g-rays and b sources have been carried out. A number of objects have been imaged and 260 mm thick aluminium foil which represents a contrast to the surrounding aire of only 1.9% has been correctly imaged.