This means it's virtually impossible to get an accurate assessment of the aorta.

http://www.radiologyinfo.org/en/info.cfm?pg=abdominus

What are the limitations of Abdominal Ultrasound Imaging?
Ultrasound waves are reflected by air or gas; therefore ultrasound is not an ideal imaging technique for the bowel. Barium exams and CT scanning are the methods of choice for bowel-related problems in most cases.

Ultrasound waves do not pass through air; therefore an evaluation of the stomach, small intestine and large intestine may be limited. Intestinal gas may also prevent visualization of deeper structures such as the pancreas and aorta. Patients suffering from obesity are more difficult to image—this is because tissue attenuates (weakens) the sound waves as they pass deeper into the body.

http://www.aemj.org/cgi/content/abstract/10/5/572-a