Superior triangle sign (right lower lobe collapse)
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The superior triangle sign is seen with complete right lower lobe (RLL) collapse alone or combined with right middle lobe collapse on PA chest x-rays. This sign can be a useful indirect sign of right lower lobe collapse where typical features are absent.
This sign occurs due to displacement of the anterior mediastinal structures to the right of the mediastinum causing a triangular density in the right upper zone. The triangle has its apex pointing toward the right hilum and its base at the level of the clavicle. It can be seen in addition to the classic inferior triangle sign in the posteromedial right thorax or can be the sole radiographic evidence of right lower lobe collapse.
This sign should not be mistaken for right upper lobe abnormality like collapse, infiltrate, or even a mediastinal mass 1.