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.
The upper limit of the adjacent soft tissue thickness:
- normal: <3 mm of soft tissue lateral to the bronchus
- abnormal: >6 mm of soft tissue implies the presences of a mass, and warrants cross-sectional imaging