Evaluation of Low-contrast Detectability of Photon-Counting-Detector CT Using Channelized Hotelling Observer and an ACR Accreditation Phantom.

The purpose of this work is to evaluate the low-contrast detectability on a clinical whole-body photon-counting-detector (PCD)-CT scanner and compare it with an energy-integrating-detector (EID) CT scanner, using an efficient Channelized Hotelling observer (CHO)-based method previously developed and optimized on the American College of Radiology (ACR) CT accreditation phantom for routine quality control (QC) purpose. The low-contrast module of an ACR CT phantom was scanned on both the PCD-CT and EID-CT scanners, each with 10 different positionings. For PCD-CT, data were acquired at 120 kV with two major scan modes, standard resolution (SR) (collimation: 144×0.4 mm) and ultra-high-resolution (UHR) (120×0.2 mm). Images were reconstructed with two major modes: virtual monochromatic energy at 70 keV and low-energy threshold (T3D), each with filtered-backprojection (Br44) and iterative reconstruction (Br44-3) kernels. For each positioning, 3 repeated scans were acquired for each scan mode at a fixed radiation dose setting (CTDIvol = 12 mGy). For EID-CT, scans (10 positionings × 3 repeated scans) were performed at a matched CTDIvol, and images were reconstructed using the same kernels with FBP and IR. A recently developed CHO-based method dedicated for QC of low-contrast performance on the ACR phantom was applied to calculate the low-contrast detectability (d') for each scan and reconstruction condition. Results showed that there was no significant difference in low-contrast detectability (d') between the UHR mode and SR mode (p = 0.360-0.942), and the T3D reconstruction resulted in 7.7%-14.6% higher d' than 70keV (p < 0.0016). Similar detectability levels were observed on PCD-CT and EID-CT. The PCD-CT: UHR-T3D had 6.2% higher d' than EID-CT with IR (p = 0.047) and 4.1% lower d' without IR (p = 0.122).