A comparison of dual kV energy integrating and energy discriminating photon counting detectors for dual energy x-ray imaging

Abstract

Dual energy imaging enables material decomposition and requires attenuation measurements with at least two different energies. Today's clinically available implementations use two separate exposures, at a low and high tube voltage (dual kV). Photon counting x-ray detectors (PCXDs) are an alternative technology that takes advantage of an x-ray source's broad spectrum by counting the number of transmitted photons at each energy from a single exposure. The richness of the information contained in these measurements can depend heavily on the detector's energy response, itself dependent on count rate. We compare the material decomposition precision of dual kV with energy integrating detectors to that of realistic PCXDs. We model the three primary effects that degrade PCXD performance: count rate limitations, energy resolution, and spectrum tailing. For example, the high flux rates required for clinical imaging pose a serious challenge for PCXDs. The Cram´er-Rao Lower Bound is used to predict the best possible material decomposition variance as a function of these detector imperfections. For dual kV and photon counting, we determined the optimal kV, mAs, and filtration for a broad range of imaging tasks, subject to dose and tube power constraints. We found that a well-optimized dual kV protocol performs on par with the estimated performance of today's PCXDs. For dual kV protocols, it is helpful to increase the energy separation between the spectra by increasing the kV separation and adding filtration. For PCXDs, the detector's count rate capabilities must be increased and the spectrum tailing reduced for photon counting to become a competitive technology at the high intensities of clinical imaging.