Ultrasonography and focal vascular disease.

The major vascular structures in the retroperitoneum are valuable landmarks for the ultrasonographer. Two cases of focal disease, one involving the pancreatic region (an aneurysm of the superior mesenteric artery) and another involving the kidney (a renal arteriovenous fistula) are presented to emphasize the importance of considering a vascular origin in the differential diagnosis of obscure sonolucent masses.