Reversed halo sign /Atoll sign

Reversed halo sign /Atoll sign

The reversed halo sign (also know as the atoll sign, reverse halo sign, and the fairy ring sign) is defined by central ground-glass opacity with a surrounding halo or crescent of consolidation .

This should not be confused with the halo sign which refers to central consolidation with a surrounding halo or rim of ground-glass opacity.

Furthermore, the reversed halo sign should be differentiated from cavitation (development of focal regions of internal gas) or pseudocavitation (likely internal bronchiolectasis described in subtypes of lung adenocarcinoma).

The reversed halo sign was first described and is most commonly associated with COP but is not specific to this disease. It has been reported in the setting of bacterial pneumonia, mucormycosis, paracoccidioidomycosis, tuberculosis, sarcoidosis, radiofrequency ablation, lymphomatoid granulomatosis, Wegener granulomatosis, tumor, and pulmonary infarcts.

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