Intersegmental (intersublobar) septum of the lower lobe in relation to the pulmonary ligament: anatomic, histologic, and CT correlations.

A thin horizontal linear area of hyperattenuation that extends laterally from the mediastinal surface of the lung within the area between the inferior pulmonary vein and the diaphragm is a common observation on computed tomographic (CT) scans of the chest. To determine the anatomic basis for this structure, the authors examined four normal human lungs (two right and two left lungs) fixed in an inflated state at pathologic and histologic examination and at CT. The structure in question was identified in one right and two left lungs. The linear area of hyperattenuation seen at CT is a septum of thin, loose intraparenchymal connective tissue that is bounded medially by the base of the pulmonary ligament, where the two sleeves of the visceral pleura appose one another, and laterally by a vertically oriented vein. The septum was identified in 39 of 50 (78%) normal chest CT scans. The pulmonary ligament often appeared to look like a beak at the mediastinal side of the lung. The septum, when visible (n = 39), was bounded medially by the beak in 28 CT scans (72%) and laterally by a vein in 24 scans (62%).