Purpose: To characterize a photon-counting CT (PCCT) bench-top system with a high-flux photon-counting cadmium zinc telluride (CZT) detector with 6 energy bins to determine the limiting spatial resolution and the smallest detectable concentration of multiple high-Z contrast agents imaged simultaneously.
Methods: Spatial resolution in 2D-radiographic images was determined by imaging a QC3 phantom, a QCkV-1 phantom, a custom CT resolution phantom, and a 0.33 mm thick sheet of Cu to obtain a straight edge. For the phantom data, the modular transfer function (MTF) was determined using the Droege and Morin method (1982). For the Cu edge, an oversampled edge spread function was obtained and the MTF calculated. In CT images, limiting spatial resolution was determined using the custom resolution phantom to obtain an MTF. The smallest detectable concentrations of the contrast agents were imaged in 0.2 ml vials placed in a 10-cm cylindrical polyethylene phantom at 120 kVp and 2mA with 2 mm of Al filtration. K-edge images were obtained using K-edge subtraction to separate the contrast agents. The contrast-to-noise ratio was measured in CT images to determine the lowest discernible concentration, which was determined when the CNR fell below 4.
Results: The limiting spatial frequency in radiographic images was 2.02 lp/mm, determined at 20% of the MTF of the Cu edge. The MTF data from the three phantoms supported this finding. The limiting spatial frequency was found to be 1.31 lp/mm in CT images. The two contrast agents were well separated using K-edge subtraction and the limiting concentrations discernible in CT images were 1.47 mg/ml for gold and 1.62 mg/ml for gadolinium.
Conclusion: The presented bench-top PCCT system with a spatial resolution of 1.31 lp/mm and the capability to separate multiple contrast agents distinguishable at low concentrations in a single image is a promising tool for future clinical applications.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Natural Sciences and Engineering Research Council, Canada Research Chairs, and Redlen Technologies.