Atoll Sign/Reverse Halo Sign - Diagnostic or Clue to Differential Diagnosis

Atoll sign is a computed tomographic nding in which there was ground glass opacity (GGO) at the centre and ring of consolidation surrounding it. It is so named as it resembles tropical ring-shaped coral reefs partially or completely surrounds a central lagoon. Central area is due to alveolar inammation and cellular debris and the peripheral consolidation is granulomatous tissue because. It is also called Reversed Halo Sign (RHS) and considered it specic to cryptogenic organizing pneumonia (COP). Similar nding was seen in variety of other conditions like fungal infections, tuberculosis and sarcoidosis. Critical analysis of centre of RHS and the periphery along with associated CT ndings, clinical ndings and the immune status of the patient can help us to arrive at the diagnosis.