Air crescent sign

The air crescent sign is a radiological finding
characterized by a
- - crescent-shaped radiolucency that
  - forms between
    - a mass or nodule and the surrounding lung tissue.
  - It is often associated with
    - infectious or inflammatory conditions like
      - invasive aspergillosis,
        
        especially in immunocompromised patients, where damaged lung and necrotic tissue
        
        begins to retract,
        
        leaving a space filled with
        
        air (cavitation).
  - Structurally, the air crescent sign indicates
    - cavitary lesions or areas of necrosis,
  - Clinically
    - presents with symptoms like
      - cough and hemoptysis.
  - Diagnosis is made through
    - clinical symptoms,
    - imaging (primarily CT), and
    - laboratory tests, including
      - cultures or
      - serology for infectious agents like
        Aspergillus.

Axial CT with Right Upper Lobe Aspergilloma
66- year-old malnourished, immunodeficient male presents with a chronic cough.
CT in the axial plane shows a 3.2cms right upper lobe mass with a rim of a crescentic accumulation of air in the dependant portionof the mass (magnified in the lower image) while the aspergilloma “sinks” to the most dependent portion of the cavity . This finding reflects an air -crescent sign and is consistent with a diagnosis of an aspergilloma
Ashley Davidoff TheCommonVein.net 293Lu 113528c

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

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.)