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In this paper, we describe a novel CMOS X-ray active pixel sensor for indirect C.T. X-ray detection. Considerable noise reduction has been achieved by lowering the detector junction capacitance. For this purpose a new low capacitance, low dark current dot type photodiode based on minority diffusion has been developed. The dynamic range is expanded to 17 bit by the use of individual in-pixel automatic gain control. A photon noise limited detector exhibiting a 20 × 10 pixel array with a frame rate of 3000 frames/sec has been realized in a 1.2 μm CMOS process.