Discussion of Halo sign
- The halo sign refers to a zone of ground-glass attenuation surrounding a pulmonary nodule or mass on CT images.
- The presence of a halo of ground-glass attenuation is usually associated with hemorrhagic nodules.
- In severely neutropenic patients, the halo sign is highly suggestive of infection by an angioinvasive fungus, most commonly Aspergillus.
- Vascular invasion by this fungus results in thrombosis of small- to medium-sized vessels, which causes ischemic necrosis.
- At pathologic examination, the nodules represent foci of infarction, and the halo of ground-glass attenuation results from alveolar hemorrhage.
- Although it is less common, the halo sign may also be observed in nonhemorrhagic nodules, in which case either tumor cells or inflammatory infiltrate account for the halo of ground-glass attenuation.
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 References for Halo sign
- Pedro S. Pinto. The CT Halo Sign. Radiology 2004 230: 109-110.