A photon-counting imaging device for single x-ray counting, comprising: an assembly of a number of photon-counting imaging modules; each photon counting imaging module comprising: i) an L-shaped support structure being connectable with adjacent L-shaped support structures; said L-shaped support structure having a first leg and a second leg wherein the angle between the first leg and the second leg is in the range from 30° to 150°, preferably in the range from 80° to 100° ii) a monolithic pixellated semiconductor detector layer and a readout unit being bump-bonded to the detector layer; said detector layer and said readout unit being sandwiched to the outer surface of the first leg of the L-shaped support layer, wherein the detector layer covers the complete outer surface; iii) a module control board for data collection and controlling the readout unit, wherein the individual photon-counting imaging modules are supported by a frame having separate holding means and cooling channels, characterized in that the number of photon-counting imaging modules being connected in such manner as to generate a flat detector plane in order to get the individual semiconductor detector layers lie in parallel to the imaging plane; said frame comprising means to adjust the relative position of each individual module in order to achieve an overall flatness of the accumulated detector area and an even distribution of the positions of the individual modules; said module control board being disposed on the inner surface of the second leg of the L-shaped support layer; said module control board and said readout unit being connected via at least one opening which is disposed in the first leg of the L-shaped support structure. i) an L-shaped support structure being connectable with adjacent L-shaped support structures; said L-shaped support structure having a first leg and a second leg wherein the angle between the first leg and the second leg is in the range from 30° to 150°, preferably in the range from 80° to 100° ii) a monolithic pixellated semiconductor detector layer and a readout unit being bump-bonded to the detector layer; said detector layer and said readout unit being sandwiched to the outer surface of the first leg of the L-shaped support layer, wherein the detector layer covers the complete outer surface; iii) a module control board for data collection and controlling the readout unit, wherein the individual photon-counting imaging modules are supported by a frame having separate holding means and cooling channels, characterized in that the number of photon-counting imaging modules being connected in such manner as to generate a flat detector plane in order to get the individual semiconductor detector layers lie in parallel to the imaging plane; said frame comprising means to adjust the relative position of each individual module in order to achieve an overall flatness of the accumulated detector area and an even distribution of the positions of the individual modules; said module control board being disposed on the inner surface of the second leg of the L-shaped support layer; said module control board and said readout unit being connected via at least one opening which is disposed in the first leg of the L-shaped support structure.