This study evaluates the diagnostic accuracy and limitations of photon-counting computed tomography (PCCT) in characterizing coronary plaque, with comparisons to optical coherence tomography (OCT) and intravascular ultrasound (IVUS). The objective is to assess whether cardiac ultra-high resolution (UHR) PCCT -with its improved spatial resolution and superior soft tissue contrast relative to conventional CT- can serve as a reliable, non-invasive alternative for coronary plaque assessment and support clinical decision-making. A total of 100 patients with either acute or chronic coronary syndrome will be enrolled, including 40 individuals with suspected in-stent restenosis and 10 patients one year post-percutaneous coronary intervention (PCI) for chronic total occlusion (CTO). All participants will undergo invasive coronary angiography based on current European Society of Cardiology guidelines. Cardiac PCCT imaging will be conducted shortly before the angiographic procedure, accompanied by invasive OCT evaluation. Additionally, a subgroup of 10 patients will undergo both OCT and IVUS, allowing for direct comparison across imaging modalities. OCT, regarded as the gold standard for plaque characterization, offers near-histological resolution for identifying plaque features, while IVUS is particularly effective in evaluating plaque burden and volume.