The Radiographic Postero-Medial Border of the Lung, or the Linear Thoracic Paraspinal Shadow

Under the title “The Linear Thoracic Paraspinal Shadow” an editorial appearing in the August 1942 issue of Radiology (page 229) draws attention to “a slender line of demarcation which is often seen in anteroposterior or sagittal roentgenograms of the bony thorax and upper abdomen. This line lies on the left side of the lower two-thirds of the thoracic spine and sometimes continues as far down as the plane of the first two lumbar segments. The shadow is frequently an enigma to radiologists and other clinicians viewing such roentgenograms. It is not visible on all films or projections of this portion of the body but is observed with such frequency that it must be the result of variation in the course and position of a normal structure situated therein.” Then follows a discussion in which it is stated that it is not bony, not due to unilateral spinal ligaments, descending aorta, or inferior vena cava, “but it might be an enlarged and elongated hemiazygos vein …. This editorial is written with the suggestion that this finding be referred to as the left thoracic paraspinal shadow and that its frequency be noted by various radiological observers. In this manner its true incidence may be ascertained and its apparent nature confirmed.” This radiographic line of demarcation is of great importance in the diagnosis of lesions involving the lower dorsal and upper lumbar spine. It is frequently seen on anteroposterior radiographs of the dorsolumbar area on the left side, but it is seen, though less frequently, on the right side as well. It extends upward, sometimes to the level of the 4th dorsal vertebra but only as far as the reflection of the pleura on to the diaphragm, for it marks the postero-medial border of the lung. As will be seen from the accompanying drawing of a cross-section of the body at the 9th dorsal level, the postero-medial border of the left lung, which has been more thickly outlined for emphasis, lies in a direct sagittal line for two or more inches. For the whole of this distance it is separated from the vertebral body by the pleura, fascia, and some areolar tissue. On a plane with the anterior surface of the body it is displaced laterally by the descending aorta and more anteriorly by the heart. On the right side the medial border of the lung extends partly over the anterior surface of the vertebral body and is displaced laterally by the esophagus and the heart. Anteroposterior radiographs of this area will therefore show the postero-medial boundary of the left lung as a straight line lateral to the outline of the vertebra because on the lung side of the boundary the tissue is relatively radiotransparent, whereas on the vertebral side the tissue is much more radiopaque. The descending aorta, being more opaque than the lung, will also be seen as a lineal shadow lateral to the postero-medial border of the lung, where it is superimposed over the postero-medial aspect of the lung.