Spatio-energetic cross-talk in photon counting detectors: N×N binning and sub-pixel masking

Smaller pixels sizes of x-ray photon counting detectors (PCDs) have the two conflicting effects as follows. On one hand, smaller pixel sizes improve the ability to handle high count rates of x-rays (i.e., pulse pileups) because incident rates onto each PCD pixel decreases with decreasing the size. On the other hand, smaller pixel sizes increase chances of crosstalk and double-counting (or n-tuple-counting in general) between neighboring pixels, because, while the same size of electron charge cloud generated by a photon is independent of PCD sizes, the charge cloud size relative to the PCD size increases with decreasing the PCD size. In addition, the following two aspects are practical configurations in actual PCD computed tomography systems: N×N-pixel binning and anti-scatter grids. When n-tuple-counting occurs and those data are binned/added during post-acquisition process, the variance of the data will be larger than its mean. The anti-scatter grids may eliminate or decrease the cross-talk and n-tuple-counting by blocking primary x-rays near pixel boundaries or for the width of one pixel entirely. In this study, we studied the effects of PCD pixel sizes, N×N-pixel binning, and pixel masking on soft tissue contrast visibility using newly developed Photon Counting Toolkit (PcTK version 3.2; https://pctk.jhu.edu).