Can ultrasound miss an abdominal aortic aneurysm?

Not likely. Some are not ideal if too much gas. Or poor prep or obesity , but usually are repeated if technical issue. The infra renal aorta and iliacs are well visualized in most.
Possibly. If the u/s was not performed for the express purpose of looking at the aorta, or if there were technical considerations like intestinal gas or body habitus that made imaging difficult, then AAA could be missed. Ct will never miss the finding.

Related Questions

Could an ultrasound miss detecting abdominal aortic aneurysm?

Yes. If the patient is large and/ or has a lot of gas in the bowel then the aorta might not be well seen. Such an exam would be described as sub-optimal and could not rule out an aneurysm. If the aorta was well seen from above the celiac artery to the aortic bifurcation then no significant aortic aneurysm should be missed. Read more...
Dx of AAA. Ultrasound is one of the typical studies used to diagnose aaa. It is optimal to let the tech doing the study know if you are trying to look specifically for an aneurysm. The bigger the aneurysm, the more likely it is to be picked up. In other words a very small one would have a higher likelihood to be missed. Read more...