Coincidence Counters for Charge Sharing Compensation in Spectroscopic Photon Counting Detectors

Abstract

The performance of X-ray photon counting detectors (PCDs), especially on spectral tasks, is compromised by charge sharing. Existing mechanisms to compensate for charge sharing, such as charge summing circuitry or larger pixel sizes, increase and aggravate pileup effects. We propose a new mechanism, the coincidence counting bin (CCB), which does not increase pileup and which has implementation similarities to existing energy bins. The CCB is triggered by coincident events in adjacent pixels and provides an estimate of the double counts arising from charge sharing. Unlike charge summing, the CCB does not directly restore corrupted events. Nonetheless, knowledge of the number of coincident counts can be used by the estimator to reduce noise. We simulated a PCD with and without the CCB using Monte Carlo simulations, modeling PCD pixels as instantaneous charge collectors and X-ray energy deposition as producing a Gaussian charge cloud with 75 micron FWHM, independent of energy. With typical operating conditions and at low flux (120 kVp, incident count rate 1% of characteristic count rate, 30 cm object thickness, five energy bins, pixel pitch of 300 microns), the CCB improved dose efficiency of iodine and water basis material decomposition by 70% and 50%, respectively. An improvement of 20% was also seen in an iodine CNR task. These improvements are attenuated as incident flux increases and show moderate dependence on filtration and pixel size. At high flux, the CCB does not provide useful information and is discarded by the estimator. The CCB may be an effective and practical mechanism for charge sharing compensation in PCDs.