Photon counting CT: Effects of dynamic beam attenuator on image quality

Purpose: Photon counting (PC) computed tomography (CT) can provide material-selective CT imaging at low patient dose, but it suffers from suboptimal detector count rate. A dynamic beam attenuator (DBA) can help with the count rate by modulating the x-ray beam intensity such that the low attenuating areas of the patient receive lower exposure, and the detector behind these areas is not overexposed. However, DBA may harden the beam and cause artifacts and errors. This work investigates the positive and negative effects of using DBA in PCCT. Methods: A simple PCCT with a single energy bin, spectroscopic PCCT with 2 and 5 energy bins, and a conventional energy integrating CT were simulated. The simulations were performed with and without using a DBA, at 120 kVp tube voltage and 14 mGy air dose. The DBAs were modeled by soft tissue (ST) equivalent material, iron (Fe), and holmium (Ho) with K-edge located around the middle of the x-ray spectrum. A cylindrical CT phantom and chest phantom with iodine and CaCO3 contrast elements were used. Image artifacts and quantification errors in general and material decomposed CT images were determined. Results: Simple PCCT exhibited major image artifacts and quantification errors when DBAs were used. The artifacts and errors were decreased with 2-bin spectroscopic PCCT and nearly eliminated with 5-bin spectroscopic CT. The photon starvation noise was higher with Fe-DBA due to strong absorption of lower energy photons. The 5-bin PCCT with ST-DBA and Ho-DBA were nearly free of artifacts and photon starvation noise. The Ho-DBA better preserved the low energy photons due to its K-edge at 55.6 keV, which also decreased beam hardening artifacts and improved material decomposition. The Ho-DBA has a potential to be substantially smaller than Fe-DBA and much smaller than ST-DBA. Conclusion: The DBA fabricated from low Z (such as soft tissue) and K-edge (such as Ho) materials can address the count rate problem of PCCT and provide minimal image artifacts and quantification errors. The DBA fabricated from high Z material with suboptimal K-edge location (such as Fe) can results in major beam hardening artifacts, photon starvation noise, and quantification errors. (C) 2017 Elsevier B.V. All rights reserved.