CTICasesSlow-Flow Mixing Artifact (Pseudo-Thrombus) in the Aorta and Left Atrial Appendage
Imaging findings
On two CT studies performed months apart, an ill-defined area of intermediate attenuation is seen in the anterior proximal descending thoracic aorta, with Hounsfield unit measurements in the 180 to 220 range rather than true thrombus density. Sagittal images show intermixing linear streaks of opacified and unopacified blood along the lesser curvature, and a similar phenomenon is present as an apparent fluid level within the left atrial appendage.
Key takeaways
This represents a slow-flow contrast-mixing artifact, sometimes called black blood or a pseudo-thrombus phenomenon, related to fast modern CT scanners outrunning the contrast bolus, particularly along the lesser curvature of the aorta and in the left atrial appendage in older patients with slower flow. Recognizing this pattern, rather than an isolated attenuation value, prevents misdiagnosis of aortic or atrial appendage thrombus, dissection, or clot, an increasingly common pitfall as scanners get faster.
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