Azygos continuation of inferior vena cava: CT appearance.

Two cases demonstrating the computed tomographic appearance of azygos continuation of the inferior vena cava (IVC) are presented. One is associated with a retroaortic left renal vein and the other with a duplicated IVC. An enlarged azygos vein should not be confused with retrocrural lymphadenopathy or a mediastinal mass. It is a tubular structure extending from the diaphragm to the azygos arch, which together with the aorta forms a characteristic collar around the trachea. It enhances intensely after intravenous injection of contrast material.