Photon counting spectroscopic CT with dynamic beam attenuator

Purpose: Photon counting (PC) computed tomography (CT) can provide material selective CT imaging at lowest patient dose but it suffers from suboptimal count rate. A dynamic beam attenuator (DBA) can help with count rate by modulating x-ray beam intensity such that the low attenuating areas of the patient receive lower exposure, and detector behind these areas is not overexposed. However, DBA may harden the beam and cause artifacts and errors. This work investigates positive and negative effects of using DBA in PCCT. Methods: A simple PCCT with single energy bin, spectroscopic PCCT with 2 and 5 energy bins, and conventional energy integrating CT with and without DBA were simulated and investigated using 120kVp tube voltage and 14mGy air dose. The DBAs were modeled as made from soft tissue (ST) equivalent material, iron (Fe), and holmium (Ho) K-edge material. 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 were determined. Results: Simple PCCT exhibited major image artifacts and quantification errors when DBAs were used. The artifacts and errors were decreased with 2bin spectroscopic PCCT and nearly eliminated with 5bin spectroscopic CT. The photon starvation noise did present with Fe-DBA due to strong absorption of lower energy photons. The 5bin PCCT with ST-DBA and Ho-DBA were nearly free of artifacts and photon starvation noise. The Ho-DBA better preserved low energy photons due to its K-edge at 55.6keV, which decreased beam hardening artifacts and improved material decomposition. The Ho-DBA was miniature having 1.4mm thickness and 2cm length, which is much smaller than ST-DBA and 10 times smaller than Fe-DBA. Conclusion: If successfully implemented, DBA fabricated from Kedge material such as Ho can address count rate problem of PCCT and provide miniature size, minimal image artifacts, and improved material decomposition at lowest patient dose.