The Postero-mesial Pleural Line

Several months ago there appeared in Radiology an editorial (1) entitled “The Linear Thoracic Paraspinal Shadow.” This editorial dealt with the question of the correct interpretation of a slender marginal line of demarcation sometimes visible in sagittal roentgenograms of the lower posterior thorax, especially on the left side. It was suggested that the true nature of this line had not been clarified and that it might possibly be due to an anomalous vascular structure. Within a few weeks some 6 letters were received offering various alternative explanations for the line in question. These were kindly forwarded by the Editor to the persons who perpetrated the editorial in question! The first letter reads: “We dissected a cadaver twenty years ago, taking straight anteroposterior films with each stage of dissection, removing successively the sternum, heart, aorta, esophagus, veins, lymphatics, etc. The paravertebral line on the left in this case was due to the fascia in which the esophagus and posterior mediastinal structures lie. We called this a paravertebral fascia line, but mediastinal-pleural line would be just as good a definition.” Well, we ourselves had made radiographic studies of cadavers some fourteen years ago and compared the postmortem films with the anatomical and pathological findings at autopsy. We did not feel that the method was a completely satisfactory one for identifying structures of the type under review but, as a matter of fact, we were not at the time studying that particular question. The next correspondent stated that the line was unquestionably due to a pleural reflection and cited the illustration appearing in Schinz's textbook of Roentgen Diagnosis (page 485, 1928 edition). This illustration does show a paravertebral line, but one which is separated from the spine by a distance of some 10 or 12 mm. and does not at all resemble the line we were discussing. A third correspondent, from Canada, reported that he had found no satisfactory explanation for the line and hoped that further contributions would be offered in regard to its correct identification. Two other correspondents made much the same comment. The sixth correspondent (2) identified the line as the posterior margin of the visceral or parietal pleura and submitted a most informative reprint in support of his thesis. He states therein that the posterior mediastinal boundaries of the lungs and pleurae appear in the lower thoracic region as longitudinal linear shadows closely paralleling the lateral borders of the spinal column, and are more commonly seen on the left than on the right. These lines are produced by tangential projection of the pleura lying in a sagittal plane against adjacent air-containing lung. Their frequent absence on properly exposed films is due to the fact that in these cases only insufficient portions of the pleura lie in a plane parallel to the central beam.