Reversed halo sign (lungs)

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. It was initially described on high-resolution 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 air spaces 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.

Share article

Article information

rID: 11129
System: Chest
Section: Signs
Synonyms or Alternate Spellings:
  • Atoll sign - chest
  • Atoll sign - CT chest
  • Reversed halo sign (RHS)
  • Reverse halo sign

Support Radiopaedia and see fewer ads

Cases and figures

  • Case 1: with pulmonary eosinophilia
    Drag here to reorder.
  • Case 2: with cryptogenic organizing pneumonia
    Drag here to reorder.
  • Case 3: with cryptogenic organizing pneumonia
    Drag here to reorder.
  • Case 4
    Drag here to reorder.
  • Updating… Please wait.

     Unable to process the form. Check for errors and try again.

     Thank you for updating your details.