Objectives: Severe coronary artery calcification leads to blooming artifacts in conventional coronary CT angiography (CCTA), which may reduce diagnostic accuracy. Ultra-high resolution (UHR) photon-counting detector CT (PCD-CT) reduces calcium blooming. This study aimed to evaluate the diagnostic performance of UHR CCTA with PCD-CT for detecting coronary stenoses in patients with a high coronary calcium burden compared to invasive coronary angiography (ICA). Materials and methods: In this IRB-approved single-center study, patients with Agatston scores > 600 were included, who underwent ICA and coronary UHR PCD-CT angiography within 2 months. 94% of patients underwent CT prior to transcatheter aortic valve replacement. Coronary arteries were graded for diameter stenosis severity (> 50, > 70%) by two blinded, independent readers. Accuracy, sensitivity, and specificity of CCTA were calculated using ICA as the reference standard. Subgroup analyses were performed across Agatston score groups (600–999, 1000–1999, 2000–2999, and > 3000). Results: 62 patients (mean age, 76 ± 9 years, 16 women) were included. The median Agatston score was 2423 (IQR: 1085–3340). Across all Agatston categories, CCTA demonstrated high diagnostic accuracy and strong agreement with ICA (Cohen’s kappa 0.71–1.00). In patients with Agatston scores > 3000 ( n = 18), the diagnostic performance of coronary UHR PCD-CT angiography for detecting > 50% (sensitivity 92–100%, specificity 83–100%) and > 70% diameter stenosis (sensitivity 88–100%, specificity 89–100%) remained high for both readers. There was no correlation between the Agatston score and diagnostic error ( p > 0.05). Conclusion: UHR CCTA with PCD-CT provides high diagnostic performance even in patients with extensive coronary calcifications. Key Points: Question In patients with severe coronary calcification, conventional coronary CT angiography often suffers from blooming artifacts, reducing accuracy and causing unnecessary invasive procedures. Findings Ultra-high resolution coronary angiography with photon-counting detector CT showed high sensitivity and specificity for detecting stenoses, even at Agatston scores > 3000. Clinical relevance Ultra-high resolution coronary CT angiography enables accurate noninvasive detection of stenoses in heavily calcified vessels, potentially reducing invasive coronary angiography. Graphical

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