The effects of minor degrees of obliquity on the lateral chest radiograph.

Minor degrees of obliquity affect the appearance of the lateral chest radiograph. For 100 normal subjects we compared findings on three left lateral projections: the "straight lateral", rotation of the subject's right side 10 degrees anterior ("right anterior"), and rotation of the patient's right side 10 degrees posterior ("right posterior"). A number of structures appeared generally best seen in the "right anterior" lateral projection, less well seen in the "straight posterior" lateral projection and least well seen in "right posterior" lateral projection. These structures were the epicardial fat stripe and pericardial line, the anterior margin of the ascending aorta, the hilar vessels, the origins of the upper lobe bronchi, and the posterior wall of the bronchus intermedius. The heart appeared largest in the "right anterior" lateral view, while in the "right posterior" lateral view it appeared smallest (whether assessed subjectively or by measurement of posterior left ventricular overlap of the inferior vena cava). Similarly, the hilar vascular shadows and major bronchi were most prominent when rotated apart in the "right anterior" projection and least prominent when more nearly superimposed in the "right posterior" projection. Slight rotation showed no significant effect on the anteroposterior diameter of the trachea, the width of the left pulmonary artery, or the diameters of the upper lobe bronchi. The azygos vein and a well visualized posterior tracheal band were seen in a few patients. They tended to be best and most frequently seen in the "right posterior" lateral view. Either obliquity tended to improve delimeation of the anterior pleural margins and the costophrenic angles. This was also true for the interlobar fissures, but confusing superimposition shadows were less likely in the "right anterior" than the "right posterior" lateral radiograph.