Quantitative comparison of 68-keV monoenergetic, routine-dose, and low-dose 120-kVp CT for calcium-mimicking inserts in thoracic phantoms: effects of phantom size and reconstruction method

Abstract Objective 68-keV monoenergetic images from dual-energy CT are often compared with conventional 120-kVp CT, but quantitative performance may depend on phantom size, reconstruction method, and dose settings. We performed a technical phantom-based comparison of 68-keV monoenergetic, routine-dose, and low-dose 120-kVp CT for calcium-mimicking inserts. Materials and methods In this phantom study, a thorax phantom with a cardiac calcification insert was scanned using routine-dose 120-kVp CT, low-dose 120-kVp CT, and dual-energy CT with 68-keV monoenergetic reconstruction. Small- and large-phantom conditions were created without and with an extension ring. Images were reconstructed at 2.5-mm slice thickness using filtered back projection (FBP), iterative reconstruction (IR), and deep-learning reconstruction (DL). Mean attenuation, background noise, contrast-to-noise ratio (CNR), and figure of merit (FOM, CNR²/CTDIvol) were summarized descriptively. Repeated scans and repeated region of interest redraws were not performed. Results For both 5-mm and 3-mm inserts, attenuation generally increased with hydroxyapatite (HA) density. The with-ring condition showed higher background noise and lower CNR. DL showed the most favorable descriptive quantitative results. Under most conditions, 68-keV imaging showed the highest CNR, followed by routine-dose and low-dose 120-kVp CT; however, it was also associated with the highest CTDIvol. When the dose was evaluated using FOM, the differences among the three strategies were smaller. Conclusion In this technical phantom study, 68-keV monoenergetic imaging showed a descriptively higher CNR than routine-dose and low-dose 120-kVp CT, but this was accompanied by a higher radiation dose. Dose-normalized performance was more comparable across strategies. Findings should be interpreted as descriptive condition-level comparisons rather than statistically tested superiority. Key Points Question How do 68-keV monoenergetic, routine-dose 120-kVp, and low-dose 120-kVp CT differ in conspicuity and dose-normalized performance for calcium-mimicking targets? Findings 68-keV imaging generally yielded the highest CNR, but also the highest CTDIvol; when dose was considered using FOM, performance differences became smaller. Relevance statement This phantom study provides a technical framework for calcium-focused CT protocol optimization by comparing conspicuity, radiation dose, and dose-normalized performance across monoenergetic and conventional acquisitions.