The posterior pleural reflections.

The posterior mediastinal pleural reflections, which contribute to the formation of the paraspinal and paraaortic lines, were studied by conventional linear tomography, computed tomography (CT), and photodensitometry in eight normal patients; these were compared with other patients with known or suspected abnormal features. The correlated evidence indicates that these lines do not depict anatomic structures as traditionally conceived, but instead represent optical phenomena--Mach bands--formed as a result of the particular anatomic shape, rather than tissue composition, of the lung-mediastinal interface. Consequently, a line may be identified as normal, on plain radiography, in the presence of either a normal or a pathologic posterior mediastinum or pleura. The determination of normalcy cannot be made solely on the basis of the appearance of these lines; CT is the only reliable method for this differentiation.