Computed tomography (CT) is a key imaging modality, with rapidly increasing use in cardiovascular diagnostics. Coronary CT angiography (CCTA) provides high-resolution, non-invasive assessment of coronary anatomy and is now recommended as a first-line test for suspected coronary artery disease (CAD) due to its excellent sensitivity, allowing for the firm exclusion of the disease. CCTA, accompanied by coronary artery calcium (CAC) scanning have become, therefore, recommended as the first-line diagnostic modalities in a majority of patients with suspected CAD. It is thanks to the steady efforts to improve this modality's diagnostic performance further and reduce patients' exposure to radiation and contrast agents that have contributed to the successful implementation of this modality in routine clinical practice. However, further progress with conventional energy-integrating detector (EID) CT is limited by detector physics, as EIDs measure only total deposited x-ray energy.

By contrast, photon-counting CT (PCCT) introduces semiconductor detectors that register individual photons and their energies, enabling higher spatial resolution, improved contrast-to-noise, and intrinsic spectral imaging. Early PCCT systems have shown better visualization of small coronary structures and reduced blooming, though limitations such as degraded energy resolution at high speeds and cross-talk still restrict full spectral performance. Radiation dose reductions are possible but have not yet been consistently achieved compared with optimized EID-CCTA, which is critical if PCCT is to be used for risk stratification or screening. There remains a need to better exploit PCCT's spectral capabilities for plaque and stent evaluation while maintaining acceptable radiation and contrast exposure. By 2026, second-generation silicon-based scanners (Si-PCCT) are approaching, offering potential improvements over first-generation cadmium-telluride systems. The present study aims to evaluate the diagnostic performance of such devices and exploit their technical advantages to lower radiation dose.

Cardiovascular disease remains one of the leading causes of morbidity and mortality worldwide and represents a major burden for healthcare systems. In patients with suspected or known coronary artery disease (CAD), coronary CT angiography (CCTA) has become a cornerstone of contemporary clinical practice, owing to its high sensitivity, excellent negative predictive value, and non-invasive character. Current guidelines recommend CCTA as a first-line diagnostic test in a broad range of patients with suspected CAD, reflecting its established role in everyday cardiology (Eur Heart J. 2024 Sep 29;45(36):3415-537).

Despite continuous technical refinement, conventional CT systems based on energy-integrating detectors (EID-CT) have inherent physical limitations. These include suboptimal spatial resolution for small coronary structures, blooming artefacts in calcified plaques and stents, limited spectral information, and trade-offs between image quality and radiation and contrast dose. While state-of-the-art EID-CT can achieve very low radiation doses, further meaningful improvements in image fidelity and tissue characterization are increasingly constrained by detector physics (Diagn Interv Imaging. 2025 Feb;106(2):53-9).

Photon-counting CT (PCCT) represents a fundamental technological advance by enabling direct photon detection with intrinsic energy discrimination. Early, first-generation PCCT systems have demonstrated improved spatial resolution, reduced blooming artefacts, and the feasibility of spectral CCTA, with encouraging results for plaque assessment and coronary stenosis evaluation. However, published clinical studies have also highlighted relevant limitations, including compromised energy resolution during fast cardiac acquisitions, cross-talk effects, and radiation doses that often remain higher than those achieved with optimized EID-CT protocols. As a result, the full clinical potential of PCCT-particularly for low-dose cardiovascular imaging and quantitative plaque characterization-has not yet been realized, and robust evidence supporting broad clinical adoption is still lacking (J Cardiovasc Comput Tomogr. 2025 Jul-Aug;19(4):474-82).

A second generation of PCCT scanners based on silicon detectors (Si-PCCT) has recently been developed to overcome these limitations (Phys Med Biol.2021 Jan 29;66(3):03tr1). The physical properties of silicon detectors, combined with an edge-on detector geometry and depth-segmented pixels, provide superior count-rate capability, improved energy separation, and high spatial resolution. Preliminary phantom studies and early prototype investigations suggest that Si-PCCT may enable high-quality coronary imaging at substantially reduced radiation and contrast doses while offering reliable spectral information for advanced plaque analysis (Radiology. 2024 Jun;311(3):e231598). However, systematic clinical evidence in humans is currently sparse, particularly in the setting of cardiovascular imaging that requires fast acquisition and high temporal resolution.

The present clinical investigation addresses this critical knowledge gap. Its primary purpose is to evaluate the diagnostic performance, dose efficiency, and clinical feasibility of Si-PCCT for CCTA in direct comparison with established EID-CT and, where available, invasive coronary angiography. By prospectively evaluating low-dose acquisition protocols, comparing quantitative plaque burden and composition, and exploring the impact on diagnostic accuracy and functional assessment, this study aims to establish whether Si-PCCT can deliver clinically meaningful improvements beyond the current state of the art. The results are expected to provide essential evidence to guide safe clinical implementation of Si-PCCT and to inform future applications in cardiovascular risk stratification, prevention, and patient management.

Inclusion Criteria:

• Referred for clinically indicated cardiac CT
• Age ≥18 years
• Informed Consent signed by the subject

Exclusion Criteria:

• Known allergy to iodinated contrast
• Pregnancy
• Impaired renal function (eGFR<30ml/min/1.73m2)
• Any other common contraindication for routine cardiac CT or any other reason leading to the inability to successfully and completely perform routine cardiac CT with and without contrast-enhancement