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Crescent sign (lung hydatid)

Last revised by Dr Daniel J Bell on 14 Dec 2021

The crescent sign is described in hydatid disease.  When the hydatid cyst erodes the adjacent bronchus or bronchiole, the trapped air between the pericyst and the laminated membrane of the endocyst gives a crescent-shaped rim of air around the cyst, thus is termed the crescent sign 1,2. It can be visible on x-ray, and CT 1,2. However this sign is not specific for hydatid disease, and may also be seen in lung carcinoma, mycetoma, blood clots, and Rasmussen aneurysm 1,2.

When the crescent-shaped rim of air is noted at the lower edge of the cyst, it is called inverted crescent sign 1,2.

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