Ultra-High-Resolution Photon-Counting CT Excludes a Bilateral Carotid Web Mimic

Carotid web is a shelf-like fibrointimal projection of the carotid bulb, increasingly recognized as a cause of embolic stroke, whose diagnosis rests almost entirely on a characteristic filling defect at CT angiography (CTA). The posterolateral bulb is also a physiologic site of boundary-layer flow separation and recirculation, in which blood of differing contrast density mixes. We describe a bilateral, reproducible pseudolesion at this site that closely simulated a carotid web. In a patient imaged twice over five months, a shelf-like filling defect was present bilaterally on a dual-energy energy-integrating-detector CT angiogram and on a photon-counting-detector CT angiogram obtained at presentation with acute stroke symptoms, but was absent on a dedicated ultra-high-resolution photon-counting-detector CT angiogram three days later. Targeted reconstructions tested whether reconstruction parameters could account for this discrepancy: the appearance persisted when the presenting study was reconstructed at 0.4mm, and could not be elicited when the ultra-high-resolution study was reconstructed at 0.8mm with a smooth kernel or as 55-keV virtual monoenergetic images. The discrepancy was therefore not reversed by the tested reconstruction changes, supporting an acquisition-dependent rather than reconstruction-dependent explanation. This temporal variability, together with the acquisition dependence, is most consistent with an acquisition-dependent flow phenomenon rather than a fixed anatomic lesion. Recognition of this pitfall may help prevent unnecessary invasive angiography or revascularization.