Abstract: A photon-counting imaging device for single x-ray counting includes an assembly of a number of photon-counting imaging modules which are connected to generate a flat detector plane. Each photon counting imaging module includes an L-shaped support structure connectable with adjacent support structures, a monolithic pixellated semiconductor detector layer and a high gain, low noise readout unit bump-bonded to the detector layer. The detector layer and the readout unit are sandwiched to a lower surface of a horizontal leg of the L-shaped support layer. A module control board is configured to collect data and to control the readout unit, wherein the module control board is disposed on an inner surface of a vertical leg of the L-shaped support layer. The module control board and the readout unit are connected via at least one opening which is disposed in the horizontal leg of the L-shaped support structure.