Perirenal spaces: CT evidence for communication across the midline.

The perirenal spaces may communicate across the midline, anterior to the lower aorta and vena cava. The connecting channel has a relatively narrow anteroposterior dimension on computed tomographic (CT) scans, both in vivo and in injected cadavers. It may therefore be difficult to visualize on abdominal radiographs and may be mistaken for unopacified bowel on CT scans. The midline extension of perirenal fluid is usually contiguous to the lower abdominal great vessels anteriorly but does not surround them completely, possibly because of fibrous septa within the perivascular fat. Hematomas from ruptured aortic aneurysms extend mainly into the perirenal spaces. Thus, the lower abdominal great vessels are located, in effect, within the midline extension of these spaces. Superiorly, the perirenal spaces extend to the diaphragm, abutting the lateral and anterior margins of the psoas and quadratus lumborum muscles and the bare area of the liver. Inferiorly, perirenal collections appear to diverge into the pelvis, along the psoas muscles, ureters, and iliac vessels.