Assessment of material identification errors, image quality and radiation doses using small animal spectral photon-counting CT

Photon-counting CT offers the potential to provide new diagnostic information. In this study, we sought to determine the interplay between material identification errors, image quality indicators, and radiation doses using photon-counting spectral CT, and to determine whether this relationship is replicated in spectral CT scans of mice. Custom-built Perspex phantoms were used to measure signal-to-noise ratio and spatial resolution, and to measure radiation dose using thermoluminescent dosimeters. A multi-contrast calibration phantom containing inserts with different concentrations of gadolinium (1, 2, 4 and 8 mg/mL), hydroxyapatite rods (0, 54.3, 104.3, 211.7, 402.3 and 808.5 mg/mL) along with water and lipid was used to assess material identification errors. Image acquisition was performed using the MARS photon-counting scanner with four energy channels (30- 45, 45-60, 60-78 and 78-118 keV) at four different tube currents (24, 34, 44 and 55 µA). As increased tube current showed no significant effect on material characterisation, small animal dosimetry was performed with 24 µA tube current using two non-contrast mice and one mouse injected with gadolinium. Results demonstrated that a tube current increase from 24 to 54 µA improved the signal-to-noise ratio and spatial resolution by <10%, gadolinium identification by <20% (for 1 mg/mL) but radiation dose increased by >160%. Imaging results of the mice showed no obvious artefacts, and the mean absorbed dose measured for the three mice was 27.3±2.4 mGy. The results suggest that the energy resolving capability of photon-counting CT maintains diagnostically relevant image quality with high levels of material discrimination at reduced radiation dose.