Coronary artery disease (CAD) is the leading cause of morbidity and mortality in the western world. Due to its excellent sensitivity and negative predictive value, coronary CT angiography (cCTA) has emerged as a non-invasive alternative to cardiac catheterization for the diagnosis of CAD. Despite remarkable technical advances, cCTA still faces challenges today that hamper its broader clinical application and limit its clinical relevance. These include insufficient spatial resolution for the examination of patients with severe coronary calcifications or with smaller coronary stents.
The NAEOTOM Alpha®, a newly developed dual-source CT scanner with photon-counting detectors (QuantaMax®), has the potential to address some of the challenges of cCTA. It provides energy-resolved CT data without electronic noise at improved spatial resolution.
The diagnostic image quality of energy-resolved dual-source photon-counting detector cCTA for CAD was first evaluated in a multicenter study, a collaboration between University of Freiburg, University of Tuebingen, and University Medical Centre Mannheim, in Germany. Dr. Isabelle Ayx from the University Medical Center Mannheim was the corresponding author for the publication.  The authors generally found high image quality, leading to good assessability of the coronary vessels and segments, even in cases of calcified plaques and stents. However, this initial study did not yet consider the potential increase in diagnostic performance provided by ultra-high resolution cCTA (Quantum HD Cardiac). Recently, another study has been published that focuses precisely on this aspect.  Who better to comment on this study than Dr. Isabelle Ayx, who wrote the following editorial.
Thomas Flohr, PhD., Siemens Healthineers
Diagnostic accuracy of ultra-high resolution coronary CT angiography using photon-counting CT
Coronary CT angiography (cCTA) has gained recognition as a valuable imaging modality for ruling out coronary artery disease (CAD) in patients with low or intermediate pretest probabilities. However, its use in a high-risk population, such as those indicated for transcatheter aortic valve replacement (TAVR), poses challenges in view of excessive coronary calcifications and stents. Invasive coronary angiography (ICA) is currently the gold standard for CAD screening in TAVR candidates, however non-invasive alternatives would be beneficial for the patients.
A recent, IRB-approved study investigated the diagnostic accuracy of ultra-high resolution (UHR) cCTA, utilizing the dual-source photon-counting detector CT (PCCT) NAEOTOM Alpha, in detecting CAD in a high-risk TAVR population. 
The study demonstrated promising results regarding image quality and vessel sharpness. Most analyzed coronary segments were rated as diagnostic, with good or excellent image quality. Objective evaluations showed favorable contrast-to-noise and signal-to-noise ratios. Importantly, UHR CCTA exhibited high diagnostic accuracy in detecting coronary stenosis of 50% or greater, with an area under the curve (AUC) ranging from 0.92 to 0.94 at different levels of analysis. 
The overall diagnostic accuracy of UHR cCTA in detecting coronary stenoses of 50% or greater was 88% on a participant-based level, 91% on a vessel-based level, and 95% on a segment-based level. Specificities were high, while sensitivities showed a slight decrease at the segment-based level. UHR cCTA also demonstrated accuracy in detecting stenoses of at least 70% (82 % on a participant-based level, 89 % per vessel, and 94 % per segment). Interobserver agreement for CAD diagnosis was substantial (κ = 0.76, P < .001). 
In the subgroup analysis, UHR cCTA showed promising results in participants with an Agatston score of at least 1000 and those prior to stent implantation, even though there was a reduction of accuracy in the identification of stenoses of 70% or greater in these subgroups (accuracy ranging from 71% till 87% (stenosis 70% or greater) versus ranging from 83% till 93% (stenosis 50% or greater). An additional limitation is the higher radiation dose of 13.3 mSv for the UHR cCTA scan. Nevertheless, these findings highlight the potential of UHR cCTA using PCCT as a non-invasive alternative to ICA in TAVR candidates. 
The use of UHR cCTA through the implementation of PCCT enables improved image quality and resolution, reducing the blooming artifacts associated with coronary stents and calcifications. This technology holds promise for accurate CAD detection, even in a high-risk population, including subjects with severe coronary calcification or prior to stent placement. It helps to guide clinical decision making in TAVR planning, as ICA could have been avoided in 54% of the participants in this study. 
Further research and validation are necessary to establish the widespread use of UHR cCTA in clinical practice. Further reduction in radiation dose is especially important for implementation in the clinical routine. This can likely be achieved through an optimization of the ECG-pulsing window, which was set to 20%–80% of the R-R interval in the study. Nonetheless, these findings contribute to the growing body of evidence supporting the potential of non-invasive imaging modalities in the evaluation of CAD, particularly in high-risk populations such as TAVR candidates.
Isabelle Ayx, MD, University Medical Center Mannheim, Germany