Photon Counting Detector CT-Based Virtual Noniodine Reconstruction Algorithm for In Vitro and In Vivo Coronary Artery Calcium Scoring: Impact of Virtual Monoenergetic and Quantum Iterative Reconstructions

Objectives: The aim of this study was to evaluate the impact of virtual monoenergetic imaging (VMI) and quantum iterative reconstruction (QIR) on the accuracy of coronary artery calcium scoring (CACS) using a virtual noniodine (VNI) reconstruction algorithm on a first-generation, clinical, photon counting detector computed tomography system. Materials and Methods: Coronary artery calcium scoring was evaluated in an anthropomorphic chest phantom simulating 3 different patient sizes by using 2 extension rings (small: 300 x 200 mm, medium: 350 x 250 mm, large: 400 x 300 mm) and in patients (n = 61; final analyses only in patients with coronary calcifications [n = 34; 65.4 +/- 10.0 years; 73.5% male]), who underwent nonenhanced and contrast-enhanced, electrocardiogram-gated, cardiac computed tomography on a photon counting detector system. Phantom and patient data were reconstructed using a VNI reconstruction algorithm at different VMI (55-80 keV) and QIR (strength 1-4) levels (CACS(VNI)). True noncontrast (TNC) scans at 70 keV and QIR "off " were used as reference for phantom and patient studies (CACS(TNC)). Results: In vitro and in vivo CACS(VNI) showed strong correlation (r > 0.9, P < 0.001 for all) and excellent agreement (intraclass correlation coefficient > 0.9 for all) with CACSTNC at all investigatedVMI andQIR levels. Phantom and patient CACS(VNI) significantly increased with decreasing keV levels (in vitro: from 475.2 +/- 26.3 at 80 keV up to 652.5 +/- 42.2 at 55 keV; in vivo: from 142.5 [7.4/737.7] at 80 keVup to 248.1 [31.2/1144] at 55 keV; P < 0.001 for all), resulting in an overestimation of CACS(VNI) at 55 keV compared with CACS(TNC) at 70 keV in some cases (in vitro: 625.8 +/- 24.4; in vivo: 225.4 [35.1/959.7]). In vitro CACS increased with rising QIR at low keV. In vivo scores were significantly higher at QIR 1 compared with QIR 4 only at 60 and 80 keV(60 keV: 220.3 [29.6-1060] vs 219.5 [23.7/1048]; 80 keV: 152.0 [12.0/735.6] vs 142.5 [7.4/737.7]; P < 0.001). CACS(VNI) was closest to CACS(TNC) at 60 keV, QIR 2 (+0.1%) in the small; 55 keV, QIR 1 (+/- 0%) in the medium; 55 keV, QIR 4 (-0.1%) in the large phantom; and at 60 keV, QIR 1 (-2.3%) in patients. Conclusions: Virtual monoenergetic imaging reconstructions have a significant impact on CACSVNI. The effects of different QIR levels are less consistent and seem to depend on several individual conditions, which should be further investigated.