Ultra-high-resolution photon-counting CT argues against a suspected bilateral carotid web: a case report

Purpose Carotid web is diagnosed almost entirely from a shelf-like filling defect at CT angiography (CTA), yet the posterolateral carotid bulb at which webs arise is also a site of flow separation in which blood of differing contrast density mixes. We describe a bilateral web-like appearance at this site and determine whether it arose from reconstruction or acquisition. Methods A 75-year-old woman with a right-hemispheric transient ischemic attack had three carotid CTA examinations: a dual-energy energy-integrating-detector CT (EID-CT) angiogram five months before presentation, a stroke-protocol photon-counting-detector CT (PCD-CT) angiogram at presentation, and a dedicated ultra-high-resolution PCD-CT angiogram three days later. Each photon-counting acquisition was additionally reconstructed with other section thicknesses, kernels or energies, and attenuation across the appearance was measured. Results A bilateral shelf-like filling defect closely simulating a carotid web was present on the EID-CT and presenting PCD-CTF angiograms but absent on the ultra-high-resolution study. The tested reconstructions neither abolished the appearance nor elicited it. The ultra-high-resolution study was acquired 16.6 s after the end of injection at less than half the table speed, versus 10.3 s on the presenting study. Attenuation at the corresponding position measured 62%, 54% and 87% of luminal attenuation, respectively. No carotid intervention was undertaken. Conclusion An acquisition-dependent bilateral carotid bulb pseudolesion can mimic a carotid web on both EID-CT and PCD-CT angiography and is most consistent with flow-related mixing of blood of differing contrast density. A slower repeat acquisition of the carotid bulb with a few seconds more delay may prevent unnecessary revascularization.