Normal azygos arch: retrotracheal visualization on frontal chest tomograms.

Anteroposterior, linear tomograms of 78 adult subjects in the supine position revealed visualization of pleural reflections off the retrotracheal part of the normal azygos arch in 38 (49%). The course of the arch varied by about 3 cm. Five distinct patterns were found, mainly depending on the course of the inferior margin of the arch. The inferior margin varied from relatively superior retrotracheal positions to intermediate positions appearing to intersect the carina, to relatively inferior and right-sided positions posterior to the proximal right main bronchus. Recognition of these normal variations of the azygos arch should serve to differentiate them from abnormalities in this region.