What is the halo sign on chest CT indicative of?

• A. Pleural effusion
• B. Lung consolidation
• C. Halo sign
• D. Nodular granulomas

Answer: (C)

A halo sign on chest CT is a nodular lesion with a surrounding ground-glass halo, which reflects hemorrhage around a nodule caused by angioinvasive infection. The classic scenario is invasive pulmonary aspergillosis in immunocompromised patients, where fungal hyphae invade pulmonary vessels, causing vessel thrombosis and hemorrhagic infarction around the nodule. That blood-tinged, hazy ring around the nodule is what the halo represents. It’s a strong clue in the right clinical setting, though not absolutely specific. It’s not simply pleural fluid (which would outline the chest cavity), not just consolidation (which is dense alveolar filling without a halo), and not a solitary granuloma (which wouldn’t have the surrounding hemorrhagic halo).