The anechoic crescent in abdominal aortic aneurysms: not a sign of dissection.

In six cases, abdominal aortic aneurysms simulated dissections because an anechoic crescent was seen peripheral to the echogenic intraluminal thrombus. Thrombus can easily be identified by sonography, but the layered echogenic and anechoic appearance has not been described. Three of these cases are presented to illustrate this sonographic pattern of thrombus so that it will not be misinterpreted as a dissection. CT scans were performed on all three patients and an arteriogram and surgery in one, all of which confirmed that these were not dissections but two layers of thrombus with the intima peripheral to the anechoic layer. At surgery, the anechoic layer medial to the intima in one case proved to be serosanguinous fluid and, therefore, likely liquefaction of the clot.