Ultra-high spatial resolution at photon-counting computed tomography: technical insights and sustainable applications in cardiothoracic imaging

The latest technological advancements in CT enable the exploration of unprecedented limits of spatial resolution in in vivo imaging. Nowadays, ultra-high-resolution imaging is available by using CT with detector elements at or smaller than 0.25 mm along the z-axis, like those used on photon-counting CT (PCCT) scanners. However, spatial resolution represents a complex criterion of imaging performance affected not only by detector elements, but also by other complex variables that can interact with each other. Knowledge of these variables and the metrics to evaluate spatial resolution is key to performing accurate cardiothoracic examinations with optimized CT protocols, which can eventually reduce acquisition times and radiation doses. This opens to a sustainable cardiothoracic radiology that permits accurate cardiac CT evaluations also in patients previously excluded, due to high calcium score, metallic stents or obesity, and allows to reduce radiation doses to never-seen levels. In this article, we review the technical advancements that allowed such an increase in spatial resolution in PCCT, along with all technical determinants of spatial resolution, the metrics to evaluate it, the clinical impact of UHR at PCCT and its challenges on cardiothoracic imaging. RELEVANCE STATEMENT: Knowledge of the ultra-high spatial resolution capabilities of new photon-counting CT technology is key to its best uses - performing accurate diagnostic examinations at unmatched low radiation doses and scanning patients previously excluded from cardiac CT examinations. KEY POINTS: Photon-counting CT scanners enable radiologists to evaluate cardiothoracic examinations with an exceptional spatial resolution, along with spectral information and dose reduction. The ultra-high spatial resolution in cardiovascular imaging enables accurate assessment in patients previously excluded, such as obese, those with extensive calcifications or metallic stents. Ultra-high spatial resolution empowers the visualization and follow-up of focal and diffuse lung diseases at unmatched accuracy and low radiation doses.