What causes the Air Crescent Sign in the lung on chest radiograph or CT?
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The air crescent sign results from the formation of a crescent-shaped collection of air between a central intracavitary mass or necrotic tissue and the surrounding lung cavity wall. It is most commonly seen in fungal infections such as aspergilloma (a fungal ball caused by Aspergillus species) growing within pre-existing lung cavities. It can also occur due to necrotic pulmonary lesions where necrotic lung tissue detaches from the cavity wall, such as in recovering invasive pulmonary aspergillosis. The sign signifies partial separation of the mass or necrotic material from the cavity, often associated with underlying cavitary pulmonary diseases including tuberculosis, sarcoidosis, or neoplasms.
Why is it called so?
The name is descriptive rather than a classical etymology: “air” refers to the radiolucent gas that separates the intracavitary mass or necrotic tissue from the surrounding lung cavity wall, and “crescent” describes the curved, moon-like shape that this air collection takes as it partially outlines the mass. When the sign is caused specifically by a mobile intracavitary fungus ball (aspergilloma) within a pre-existing cavity, it is also known as the Monod sign, named after Pesle and Monod, who first described this finding in 1954.
Pathophysiology
The sign develops when air collects between the cavity wall and an adjacent mass or necrotic tissue that has begun to separate or retract. This separation occurs as the central lesion loses adherence from the cavity wall due to necrosis or fungal colonization, allowing air to fill the space and produce a characteristic crescentic radiolucency on imaging. The crescent shape reflects partial circumferential separation rather than complete detachment of the intracavitary lesion.
References
- Sevilha JB, Rodrigues RS, Barreto MM, Zanetti G, Hochhegger B, Marchiori E. Infectious and Non-Infectious Diseases Causing the Air Crescent Sign: A State-of-the-Art Review. Lung. 2017.
- Thilagavathy G, Sasankh RK, Sekary A, Prabhu SS N. A Case of Pulmonary MALToma Camouflaged Within Invasive Pulmonary Aspergillosis. Cureus. 2024.
- Abramson S. The air crescent sign. Radiology. 2001.
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