A chest radiograph shows a dense opacity in the left upper zone with the aortic knuckle sharply outlined through it, and a crescent of lucent lung seen between the aortic arch and the medial edge of the collapsed left upper lobe. What is this sign called?

• A Hilum overlay sign
• B Comet tail sign
• C Silhouette sign
• D Luftsichel sign ✓

Explanation

The Luftsichel (air sickle) sign is seen in left upper lobe collapse. The hyperinflated superior segment of the left lower lobe migrates upward and medially, interposing a crescent of aerated lung between the mediastinum and the collapsed left upper lobe. The hilum overlay sign instead localises a mass anterior or posterior to the hilum by showing vessels through it, and the comet tail sign belongs to rounded atelectasis.

Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.

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