The halo sign refers to a zone of ground-glass opacity surrounding a pulmonary nodule or mass on computed tomography (CT).

In severely neutropenic patients, the CT halo sign is highly suggestive of infection by an angioinvasive fungus, most commonly aspergillus. Most common risk factors for mucormycosis are uncontrolled diabetes mellitus, hematologic malignancies, treatment with corticosteroids, renal transplantation, and acquired immune deficiency syndrome (AIDS). The presence of a halo of ground-glass attenuation is usually associated with hemorrhagic nodules. Pulmonary Kaposi’s sarcoma typically manifests on chest CT images as ill-defined, flame-shaped nodules, predominantly seen in a peribronchovascular distribution.

Rarely, the halo sign can be seen in posttransplantation lymphoproliferative disorders and lung cancer, especially with lipidic growth seen in bronchioalveolar carcinoma.