Learning Radiology xray montage

Monad's Sign and Air Crescent Sign

General Considerations

  • Sickle-shaped lucency, partially surrounding a soft tissue mass in a pulmonary cavity seen on conventional radiography and CT
  • Terminology can be confusing as a similar sign is described for both invasive aspergillosis and secondary, non-invasive, saprophytic aspergillosis
  • Invasive aspergillosis typically occurs in immunocompromised patients
    • It frequently begins with a solid-appearing peripheral soft tissue opacity that, through infarction and retraction, produces a cavity with a density within it
    • The density is necrotic lung tissue, not the fungus ball in non-invasive aspergillosis
    • Sputum cultures are not often positive (10%) so diagnosis may require bronchoscopy or percutaneous aspiration
  • Saprophytic aspergillosis (aspergilloma, mycetoma, fungus ball) occurs in a pre-existing cavity
    • It is more common than the invasive form
    • Usually occurs in a cavity from TB or sarcoid
    • The "ball" is composed of hyphae, mucous and cellular debris
    • Hemoptysis is a common symptom

Imaging Findings

  • In secondary, non-invasive (saprophytic) aspergillosis
    • The fungus ball is usually freely movable and will move to the dependent surface on decubitus radiographs or supine and prone CT studies
    • Rarely, the fungus ball may calcify
  • Invasive aspergillosis
    • On CT, there may be ground glass densities in the area around the nodular density called the "halo sign" and indicative of hemorrhage

Differential Diagnosis

  • Cavitating neoplasm
  • TB
  • Nocardiosis

Monad's sign. The axial CT scan of the chest, on the left, shows a nodule (black arrow) within an air-filled cavity with a crescent of air at the superior surface of the cavity (white arrow). In the close-up view of the left upper lobe from a chest radiograph of another person, on the right, there is again seen a thin-walled cavity (white arrow) containing a soft-tissue mass (black arrow). Both of these cavities were from old tuberculosis.
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Abramson, S. January 2001  Radiology,   218,  230-232

Saprophytic aspergillosis  Eurorad case 6672