Strategies to improve the detective quantum efficiency of silicon photon counting detectors

Silicon (Si) is an attractive material for photon counting detectors (PCDs) in CT applications for its electrical properties and its widespread availability, but its x-ray attenuation properties are poor. We sought to characterize possible strategies for improving detection in Si PCDs as well as assess their ultimate limits using Monte Carlo simulations. A variety of Si PCDs were modeled in GEANT4 and irradiated by a 120 kV spectrum filtered by 30 cm of water. The figure of merit was the detective quantum efficiency at zero frequency, DQE(0), analyzed with a single energy threshold. This corresponds to a non-spectral detection task. While spectral performance is also an important figure of merit, it is outside the scope of this work. The baseline Si PCD included tungsten foils 0.02 mm thick between silicon wafers 0.48 mm thick, was 60 mm thick (deep), and used a lowest energy threshold of 8 keV (corresponding to a reported electronic noise of 1.6 keV, using five standard deviations to avoid dark counts). This achieved a DQE(0) of 0.60. From here, we tested several individual improvements: (1) increasing the PCD thickness to 600 mm, to fully compensate for the low stopping power of Si, improved the DQE(0) to 0.63; (2) using perfect anti-coincidence logic, eliminating any double counting effects, improved the DQE(0) to 0.65; (3) decreasing the lowest energy threshold to 1 keV, enabling better detection of Compton events, improved the DQE(0) to 74; (4) implementing all of the above simultaneously improved the DQE(0) to 0.87. The final losses in DQE occur primarily from Rayleigh scattering (~10%) and the dead space from the tungsten foil (4%). We conclude that decreasing the electronic noise of the Si PCD could be the best immediate path to improving DQE(0). Rayleigh scattering presents an ultimate limit to dose efficiency because photons that undergo Rayleigh scattering and are subsequently absorbed by the tungsten foil leave no evidence and cannot be counted.