Virtual Non-Iodine Coronary Calcium Scoring on Photon-Counting CT: Patient- and Plaque-Level Analysis

<b>Background/Objectives</b>: Whether PCCT-derived virtual non-iodine (VNI) images can replace true non-contrast (TNC) for coronary artery calcium scoring (CACS) remains uncertain, particularly for small, low-density plaques. We aimed to evaluate agreement between VNI and TNC for CACS at the patient and lesion levels and to quantify risk-category reclassification. <b>Methods</b>: In this retrospective single-center sample (May 2024-May 2025), 211 patients without prior coronary intervention and with nonzero CAC on TNC underwent PCCT. VNI (55 keV, QIR 1; 60 keV, QIR 4; PureCalcium) and TNC were reconstructed with matched section thickness/increment and kernel. Agatston and total calcified volume were recorded. Paired comparisons used Wilcoxon tests; reclassification across CAC categories (0, 1-99, 100-399, ≥400) and lesion-level false negatives (FNs) were assessed with TNC as the reference. <b>Results</b>: Low-keV VNIs (55-60 keV) underestimated CAC versus TNC. The median Agatston score decreased from 35.9 (IQR, 10.3-121.2) on TNC to 23.6 at 55 keV (<i>p</i> = 0.0006) and 22.2 at 60 keV (<i>p</i> = 0.0003); the total volume declined from 37.8 mm³ to 20.2 mm³ (<i>p</i> = 0.001) and 18.3 mm³ (<i>p</i> < 0.0001), respectively. More than half of patients were reassigned to a lower CAC category; despite no patient being CAC = 0 on TNC, 46.9% (55 keV) and 47.4% (60 keV) were labeled CAC = 0 on VNI. Because this study deliberately included only patients with nonzero CAC on the TNC reference, these CAC = 0 rates on VNI represent misclassification within a CAC-positive sample and should not be interpreted as population-level prevalence. At the lesion level, 95% of patients had ≥1 FN plaques (430 FN plaques total), typically small (median 8 mm³) and of low density (median Agatston 6). <b>Conclusions</b>: In this single-center sample with relatively low-burden calcification, low-keV VNI (55-60 keV) significantly underestimates CAC and down-classifies patients, with frequent "false-zero" assignments (defined as CAC_VNI = 0 despite CAC_TNC > 0) driven predominantly by small, low-density plaques.