This study assessed the effect of different iodine contrast concentrations on image quality during ultra-high-resolution (UHR) coronary CT angiography (cCTA) performed with photon-counting detector CT (PCD-CT). A total of 123 patients were retrospectively analyzed and evenly assigned to three groups receiving either Iomeron 400, Iomeron 350, or Ultravist 370. Baseline characteristics and injection flow rates were comparable among groups, but the iodine delivery rate was significantly higher with Iomeron 400. This higher-concentration agent achieved significantly greater intraluminal attenuation in distal coronary arteries and showed a trend toward higher attenuation in the proximal coronary arteries and ascending aorta. No significant differences were observed in signal-to-noise ratio, contrast-to-noise ratio, or image noise across groups, and all scans were rated as good or excellent in overall image quality. Importantly, the increased attenuation obtained with Iomeron 400 was not associated with higher radiation dose or increased heart rate variability. These findings suggest that Iomeron 400 may offer enhanced coronary attenuation in UHR cCTA performed with PCD-CT, potentially improving the confidence of coronary stenosis assessment without compromising image quality or dose parameters.