Atoll sign (lungs)

Last revised by Madeleine - Sanza on 12 Mar 2024

The atoll sign or reversed halo sign refers to focal ground-glass opacity with a peripheral ring of consolidation which may be complete or incomplete. The sign was originally described in organizing pneumonia but can also occur in infections, infarction, granulomatous disease, inflammation and tumor.

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

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

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.

ADVERTISEMENT: Supporters see fewer/no ads

Updating… Please wait.

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

 Thank you for updating your details.