Room: Track 1
Purpose: Photon counting detectors (PCDs) are an emerging technology for next generation CT and offer several advantages compared to conventional detectors. However, at high flux, PCDs suffer from pileup when two photons arriving in close spatiotemporal proximity are perceived as a single photon. This can cause the detector to paralyze, leading to large increases in noise. We investigated the utility of compensating electronics that could mitigate the deleterious effects of pileup. An example of compensation electronics is the pileup retrigger mechanism, which continues to count at regular intervals when the detector would otherwise be paralyzed. This has recently been incorporated into a commercial product.
Methods: We simulated four PCDs: (1) a baseline PCD without compensating electronics; (2) a PCD using the standard pileup retrigger mechanism; (3) a modified pileup retrigger mechanism with the retriggered counts rerouted into separate counters; and (4) an ideal detector without dead time or pileup effects. This comparison was performed using numerical simulations, estimating the performance of the PCD using the Cramer-Rao Lower Bound of the variance of the iodine basis material image.
Results: All of the non-ideal PCDs suffered from large increases in variance at high flux due to pileup. We defined the maximum acceptable flux as the point for which the noise exceeds the ideal detector at 10% of the characteristic count rate. We found that the baseline PCD reached the maximum acceptable flux at 40 Mcps/pixel (million counts per pixel per second per pixel), but that the standard pileup retrigger mechanism could extend the maximum acceptable flux to 75 Mcps/pixel. The modified pileup retrigger with separate counters further extended the maximum acceptable flux to 130 Mcps/pixel.
Conclusion: Compensating electronics can improve the performance of PCDs at high flux. The pileup trigger can be further improved by routing retriggered counts into separate counters.