Air crescent sign
ChestCT / Radiograph
A crescent of air separates a rounded intracavitary mass from the surrounding cavity wall, often near the top of the cavity.
What it means
Typically an aspergilloma (fungus ball) within a pre-existing cavity, or the recovery phase of angioinvasive aspergillosis.
Mechanism
A mobile fungus ball or retracting necrotic tissue sits dependently within a cavity, leaving a crescent of air between it and the wall.
Associated conditions
- Aspergilloma
- Invasive pulmonary aspergillosis (recovery phase)
- Tuberculous cavity
- Cavitating neoplasm or haematoma
Pearls
- The intracavitary ball often moves dependently when the patient is repositioned.
- In neutropenic patients its appearance frequently coincides with neutrophil recovery.
References
- Radiopaedia: Air crescent sign
For educational use only. This dictionary is a study aid and reference, not a diagnostic tool, and must not be used to guide clinical decisions or patient care. Imaging appearances vary; always correlate with the full clinical picture and confirm with a qualified radiologist.