Assessment of Multienergy Interpixel Coincidence Counters (MEICC) for Charge Sharing Correction or Compensation for Photon Counting Detectors with Boxcar Signals

Recently, multienergy interpixel coincidence counter (MEICC) has been proposed for charge sharing (CS) correction and compensation for photon counting detectors (PCDs), which uses energy-dependent coincidence counters to record coincident events between multiple energy windows of a pixel-of-interest and those of neighboring pixels. A Monte Carlo (MC) simulation study was performed to assess the performance of MEICC; however, the performance might have been overestimated in a previous study. The CS increases the number of photons recorded at a PCD pixel at the expense of the spatial resolution, and therefore, when spatially uniform flat-field X-ray signals are used, it gives PCDs with CS more signals than a PCD without CS. In this article, we propose to use spatially contained boxcar signals for evaluating the performances for high spatial frequency tasks because they provide consistent signals regardless of the presence or absence of CS. The flat-field signals must be used for low spatial frequency tasks. We assessed the performances of MEICC and other PCDs with both flat-field signals and boxcar signals, with optimal threshold energies, and with two different pixel sizes. As it is expected, normalized Cramér-Rao lower bounds (nCRLBs) measured with the boxcar signals were worse than those with flat-field signals in general. The nCRLBs of MEICC with 225-μ m pixel were close to the current 450- μ m PCD. We studied a combination of flat-field signals and N× N super-pixels, where the output of N× N pixels were added, using an MC simulation and a simple CS counting model. The study showed that CS had two opposing impacts on the conventional computed tomography imaging - a negative impact with double-counting among N× N pixels and a positive impact with single-counting spill-in and spill-out across the super-pixel boundary - and the positive impact diminished with increasing N. A use of large N× N super-pixels such as N≥ 25 was suggested to approximate the zero-frequency detection quantum efficiency of PCD with CS.