The anterior bronchus sign refers to the appearance of the anterior segmental bronchus of the upper lobes as seen on a frontal chest radiograph.
The anterior segment bronchus of the upper lobes courses anteriorly and laterally. When the orientation is predominantly anteriorly the bronchus is seen end on as a ring shadow. This occurs ~50% of the time.
Upper limit of the adjacent soft tissue thickness
Normally there is at most 3 mm of soft tissue lateral to the bronchus. Presence of more than 6 mm of soft tissue implies the presences of a mass, and warrants cross-sectional imaging.