Reversed halo sign (lungs)

Last revised by Dr Mohammad Taghi Niknejad on 30 Nov 2021

Reversed halo sign, also known as the atoll sign, is defined as central ground-glass opacity surrounded by denser consolidation of crescentic shape (forming more than three-fourths of a circle) or complete ring of at least 2 mm in thickness 8. It was initially described on chest high resolution CT (HR CT).

This is in contrast to the more well-known halo sign of invasive fungal infection. 

The central area (ground-glass opacity) corresponds to alveolar septal inflammation and cellular debris in alveolar spaces, while the crescent or ring-shaped peripheral airspace consolidation corresponds to granulomatous tissue within the distal airspaces 5.

The sign is classically seen in organizing pneumonia (OP), which is most commonly cryptogenic (COP) but can also be secondary to other causes. However, the sign is only seen in about one-fifth of patients with COP 1.

Whilst the relatively high specificity for the aforementioned entity is preserved, the sign has also been described on CT with the following pathologies 3,4,7:

In severely immunocompromised patients, the sign has been demonstrated as highly suggestive of early infection by an angioinvasive fungus. Suggesting the diagnosis might prove life-saving in patients with prolonged neutropenia or graft-vs-host disease 3,4.

When associated with nodular walls, nodules inside the reversed halo or even centrilobular nodules and pattern of endobronchial spread (tree-in-bud sign), active pulmonary tuberculosis should be high on the list of differential diagnoses 2,3.

Integrating the ancillary radiological and clinical data (as exemplified above) should enable substantial narrowing of differential diagnoses. Providing a presumptive final diagnosis may obviate the need for biopsy in selected cases, especially when dealing with immunocompromised patients 3.

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Cases and figures

  • Case 1: with pulmonary eosinophilia
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  • Case 2: with cryptogenic organizing pneumonia
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  • Case 3: with cryptogenic organizing pneumonia
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  • Case 4
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  • Case 5
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  • Case 6: with COVID-19
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  • Case 7: due to COVID-19 pneumonia
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  • Case 8: COVID-19 pneumonia
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  • Case 9: COVID-19 pneumonia
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