Air crescent sign

Air crescent sign

Air crescent sign. Frontal radiograph (A) of the chest shows cavitating lesion within the superior aspect of the right lower lobe (arrow). Follow-up enhanced CT of the chest (B) confirming the “air crescent sign” (arrow) in a patient with documented angioinvasive aspergillosis.
Source
Signs in Thoracic Imaging
Journal of Thoracic Imaging21(1):76-90, March 2006

The air crescent sign appears as a variably sized, peripheral crescentic collection of air surrounding a necrotic central focus of infection on thoracic radiographs (Fig. 1A) and CT (Fig. 1B).2–4 It is often seen in neutropenic patients who have undergone bone marrow or organ transplantation and is most characteristic of infection with invasive pulmonary aspergillosis. The fungus invades the pulmonary vasculature, causing hemorrhage, thrombosis, and infarction. With time, the peripheral necrotic tissue is reabsorbed by leukocytes and air fills the space left peripherally between the devitalized central necrotic tissue and normal lung parenchyma.5 Thus, the presence of the air-crescent sign heralds recovery of granulocytic function.4 Other causes of the air crescent include cavitating neoplasms, bacterial lung abscesses, and infections such as tuberculosis or nocardiosis.6

CT scan at the level of the right pulmonary artery shows a mass in the superior segment of the left lower lobe characterised by a c shaped or crescent shaped anterior rim of air with ball of soft tissue, reminiscent of the crescent sign most characteristic of an aspergillus infection of a cavity and resulting in an aspergilloma.
Ashley Davidoff MD
CT scan at the level of the right pulmonary artery shows a mass in the superior segment of the left lower lobe characterised by a c shaped or crescent shaped anterior rim of air with ball of soft tissue, reminiscent of the crescent sign most characteristic of an aspergillus infection of a cavity and resulting in an aspergilloma.
Ashley Davidoff MD

Links and References

Fleischner Society

air crescent

Radiographs and CT scans.—An air crescent is a collection of air in a crescentic shape that separates the wall of a cavity from an inner mass (,Fig 3). The air crescent sign is often considered characteristic of either Aspergillus colonization of preexisting cavities or retraction of infarcted lung in angioinvasive aspergillosis (,9,,10). However, the air crescent sign has also been reported in other conditions, including tuberculosis, Wegener granulomatosis, intracavitary hemorrhage, and lung cancer. (See also mycetoma.)