Counting and integrating readout for direct conversion X-ray imaging concept, realization and first prototype measurements

A novel signal processing concept for X-ray imaging with directly converting pixelated semiconductor sensors is presented. The novelty of this approach compared to existing concepts is the combination of charge integration and single photon counting in every single pixel. Simultaneous operation of both signal processing chains extends the dynamic range beyond the limits of the individual schemes and allows determination of the mean photon energy. Medical applications such as X-ray computed tomography can benefit from this additional spectral information through improved contrast and the ability to determine the hardening of the tube spectrum due to attenuation by the scanned object. A prototype chip in 0.35-micrometer technology was successfully tested. The pixel electronics are designed using a low-noise differential current mode logic and provide configurable feedback modes, leakage current compensation and various test circuits. This paper will discuss measurement results of the prototype structures and give details on the circuit design