Comparison of Threshold Energy Calibrations of a Photon-Counting Detector and Impact on CT Reconstruction

Photon-counting detectors (PCDs) require a more complicated calibration process than the standard energy-integrating detectors. The first step of a PCD calibration is the threshold energy calibration. This step determines the linear pixel-by-pixel relationship between the output voltage of a PCD and the incoming photon energy. It can be performed in several ways. Three different methods were implemented, requiring only a standard X-ray source, or an X-ray source combined with K-edge materials or a PCD response model. The three resulting calibrations were first evaluated on monoenergetic and projection measurements. The impact of the threshold calibration on computed tomography (CT) images was also studied. The calibration method fitting a detector response model to polyenergetic measurements presented the best spectral accuracy when estimating monoenergetic peaks. This calibration also produced a better pixel homogeneity in an air projection. However, no significant improvement was observed in the noise power spectrum (NPS) and ring artifact evaluation of a conventional CT image for this calibration compared to a calibration providing lower spectral accuracy or lower pixel homogeneity.