Fig. 2From: Chest imaging using signs, symbols, and naturalistic images: a practical guide for radiologists and non-radiologistsAir crescent sign. CT scan of a patient with pulmonary aspergillosis showing a necrotic and cavitated lesion (white arrow) in left upper lobe. The air filling the cavitation looks like the shadow of a crescent moon—which is demonstrated in the embedded figureBack to article page