Fast photon counting CdTe detectors for diagnostic clinical CT: dynamic range, stability, and temporal response

We report results from the development of a second-generation CdTe direct-conversion compound-semiconductor x-ray detector for photon-counting clinical CT. The first-generation detector has 512 pixels with a 1 mm pitch and is vertically integrated with the readout. A 32-row multi-slice CT system using first-generation detectors has been used for clinical low-dose CT applications. To provide adequate performance for whole-body diagnostic CT we have designed and fabricated new 0.25 mm(2) pixels to increase the maximum output to greater than 20 Mcps per mm(2) while preserving sufficient energy resolution for photon-counting CT. In addition to the need for dynamic range, CT places stringent uniformity and temporal response requirements on the detector. We have measured detector parameters including the dynamic range, energy resolution, noise floor, stability, and temporal response. Temporal response is determined by rapid cycling of the input flux with shutter driven attenuators. Cycling between high and low flux generates reproducible counts, within counting statistics, with a response time less than 1 ms. Stability is determined by measuring uniformity corrected flood images repeatedly over a time interval exceeding whole-body diagnostic CT scan times. Long exposure to uniform flux generates a number of counts which drift in some pixels slightly in excess of counting statistics. These results demonstrate the potential for these detectors to achieve whole-body CT.