The Anterior Mediastinal Line on Chest Roentgenograms

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Abstract
The mediastinum supplies the interested radiologist with a treasure-house of radio-anatomic detail, so suitable is it for simple roentgenologic exploration because of the contrast in the respiratory system (air) and in the esophagus (air or opaque media). We have previously (1, 2) discussed the esophageal-pleural stripe in its anatomic and pathologic aspects, pointing out that this stripe could not possibly represent the contact of the right and left lung anteriorly, as maintained by McDonnel and his group working with supervoltage roentgenography at the Walter Reed Army Hospital (3), but that rather it represents the right wall of the esophagus with its pleural covering, in contrast, on the right, to air within the lung, and on the left, to air or an opaque medium within the esophageal lumen. The purpose of the present communication is to demonstrate the “contact” of right and left lung in the anterior mediastinum, forming the anterior mediastinal line on the standard chest roentgenogram and to prove its origin by laminagraphy. The anterior mediastinal line is visualized on the standard chest roentgenogram as afine linear shadowof water density about 2 to 3 inches long, starting cranially in the region of the joint of the manubrium with the body of the sternum, and extending caudally more or less longitudinally and medially over the mediastinum.3 As a very rough approximation, 1 of every 4 or 5 of all standard chest roentgenograms (Fig. 1) will show this feature. The assumption is reasonable that the line is denned on the right by air in the right lung, and on the left by air in the left lung, and that this approximation of the lungs lies in the anterior mediastinum because of the cranial and caudal limits of the line: cranially, under the manubrium, the lungs diverge; 2 or 3 inches caudally, they again diverge to accommodate the heart. Cranially or caudally to these points, therefore, no radiographically detectable linear shadow of the approximated lungs is to be expected. Most anatomic studies based on autopsy material portray a greater or less separation of the lungs as they approximate each other anteriorly. Gray (4), however, cannily foretold the living anatomy, as later established by the laminagraphic method, when he wrote: “In the front of the chest, where the parietal pleura is reflected backward to the pericardium, the two pleural sacs are in contact for a short distance. At the upper part of the chest, behind the manubrium, they are separated by an angular interval ; the line of reflection being represented by a line drawn from the sternoclavicular articulation to the midpoint of the junction of the manubrium with the body of the sternum.” Our assumption as to the derivation of the anterior mediastinal line was proved by a laminagraphic “cut” just deep to the sternum (Fig. 2).

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