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Coronary artery calcification (CAC) is an important indicator of coronary disease. Accurate volume quantification of CAC is challenging using computed tomography (CT) due to calcium blooming, which is a consequence of limited spatial resolution. Ex vivo coronary specimens were scanned on an ultra-high-resolution (UHR) clinical photon-counting detector (PCD) CT scanner, and the accuracy of CAC volume estimation was compared with a state-of-the-art conventional energyintegrating detector (EID) CT, a previous-generation investigational PCD-CT, and micro-CT.Approach: CAC specimens (n ¼ 13) were scanned on EID-CT and PCD-CT using matched parameters (120 kV, 9.3 mGy CTDI vol ). EID-CT images were reconstructed using our institutional routine clinical protocol for CAC quantification. UHR PCD-CT data were reconstructed using a sharper kernel. An image-based denoising algorithm was applied to the PCD-CT images to achieve similar noise levels as EID-CT. Micro-CT images served as the volume reference standard. Calcification images were segmented, and their volume estimates were compared. The CT data were further compared with previous work using an investigational PCD-CT.Results: Compared with micro-CT, CT volume estimates had a mean absolute percent error of 24.1% AE 25.6% for clinical PCD-CT, 60.1% AE 48.2% for EID-CT, and 51.1% AE 41.7% for previous-generation PCD-CT. Clinical PCD-CT absolute percent error was significantly (p < 0.01) lower than both EID-CT and previous generation PCD-CT. The mean calcification CT number and contrast-to-noise ratio were both significantly (p < 0.01) higher in clinical PCD-CT relative to EID-CT.Conclusions: UHR clinical PCD-CT showed reduced calcium blooming artifacts and further enabled improved accuracy of CAC quantification beyond that of conventional EID-CT and previous generation PCD-CT systems.