[Colonic interposition between kidney and psoas muscle: anatomical variation studied with CT]

Interposition of the colon between kidney and psoas muscle may represent a benign anatomical variant that can be recognized on Computed Tomography (CT) images. In our series including 428 abdominal CT examinations, the position of the bowel in the pararenal space was studied with respect to the kidney and the psoas muscle. To determine the colon position in the pararenal space, three CT scans, at the upper pole, midkidney and lower pole, for each side, were selected. The colonic interposition between the kidney and the psoas muscle was then correlated with patient's age, sex and amount of perinephric fat. The ascending colon was interposed between the kidney and the psoas muscle in 6/428 cases (1.4%), appearing more frequently in adult men. Of these 6 cases, CT showed decreased perinephric fat in three cases, normal in one and increased in two. No case of descending colonic interposition between the kidney and the psoas muscle was demonstrated. CT is a valuable tool to depict this anatomical variation, preventing misdiagnoses and complications arising from interventional procedures.