The quadratus lumborum muscle: a possible source of confusion in sonographic evaluation of the retroperitoneum.

The quadratus lumborum muscle has been noted to appear as a hypoechoic structure on sonograms of the abdomen. On occasion its appearance may simulate a pathologic fluid collection such as an abscess or retroperitoneal hematoma. Illustrative examples of this confusing appearance, as well as ways of avoiding this potential pitfall, are presented.