What are the halo and air crescent signs, and what infection is classically associated with these signs?
Both signs are classically associated with angioinvasive aspergillosis, an opportunistic fungal infection commonly seen in the setting of bone or solid organ transplantation where the absolute neutrophil count is severely reduced. As the name implies, this infection affects blood vessels leading to areas of pulmonary infarction and hemorrhage. The typical imaging appearance is that of a nodule or masslike area of consolidation with a surrounding “halo” of ground glass opacity. In this setting, the halo represents pulmonary hemorrhage. Should the neutrophil count recover, the necrotic lung will begin to contract as part of the healing process, leaving a “crescent” of air. This is an important prognostic sign as it implies neutrophil recovery.
In the setting of neutropenia and fever , these radiographic signs are nearly specific for fungal infection, in particular angioinvasive Aspergillus , the most common offending organism. However, other processes can result in similar imaging appearances. The “halo” sign can be seen in other infections such as TB, pulmonary malignancies, and vasculitis.