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Radiology of the superior segment of the lower lobe: a regional perspective introducing the B6 bronchus sign.

The radiological findings of atelectasis, consolidation, or mass lesions in the superior segment of the lower lobe of the lung (B6 in Boyden's classification) are analyzed. Cases were collected over several years and supplementary series were assembled to study the configurations of the segmental bronchi, the right oblique fissure, and the left pulmonary artery in lateral projection. The upper part of the right oblique fissure could be distinguished from the horizontal fissure on the frontal radiographs by position and angulation; its visualization indicated atelectasis or displacement of the superior segment. Visualization was much less common on the left. Consolidation produced a visible superior segment air bronchogram (the B6 sign) much more often on the left, where the bronchus is usually larger. Orientation and position of the bronchus permit its identification. Other key anatomic landmarks were the descending aorta on frontal radiographs and the posterior margins of the left pulmonary artery and bronchus intermedius on lateral views.