Purpose: Energy-discriminating, photon counting detectors (PCDs) are an emerging technology for spectroscopic imaging in CT. Current PCDs suffer from poor spectral separation due to charge sharing effects. We propose the coincidence bin as a mechanism for charge sharing compensation. The coincidence bin simply counts coincidences in the lowest level discriminator (LLD) of adjacent pixels. The estimator uses the coincidence bin as if it were an energy bin to improve material decomposition estimates. Unlike other charge sharing compensation mechanisms, it does not reduce the count rate capability of the detector. We compare the impact of the coincidence at low flux against charge summing. Because charge sharing is often under the LLD threshold, we examined the incremental benefit of lowering the LLD from a standard (30 keV) to reduced threshold (10 keV).
Methods: The interaction of photons with the CdTe substrate was simulated using GEANT4. Separate Monte Carlo simulations are used to model the charge collected in pixels, modeling these pixels as perfect collectors. Noise in material decomposition images was then estimated using the Cramer-Rao Lower Bound. We assumed a 120 kVp spectrum, a 20 cm thickness water background, 5 energy bins equally spaced from the LLD to 110 keV, and used 10,000 noise realizations to calculate the CRLB.
Results: The relative dose efficiency for an iodine material decomposition task is 32%, 54%, and 100% for a 5-bin simulated PCD without charge sharing compensation, with the coincidence bin, and with analog charge summing. When the LLD is reduced from 30 keV to 10 keV, the dose efficiencies change to 36%, 68%, and 98%. Benefits are somewhat less for a water material task.
Conclusion: The coincidence bin is a simple and effective mechanism for charge sharing compensation. With a carefully selected LLD threshold, it can nearly double spectral performance in certain tasks.