Home > Radiology > Atoll sign (Reversed halo sign)

Atoll sign (Reversed halo sign)

August 29, 2012

Atolls are circular, oval, or horseshoe-shaped arrays of coral reef islands that are perched around an oceanic volcanic seamount and encircle a shallow central lagoon. The small islands are separated from each other by channels that lead from the sea into the central lagoon.

The reversed halo sign (also known as the atoll sign) is defined as central ground-glass opacitysurrounded by denser consolidation of crescentic (forming more than three fourths of a circle) or ring (forming a complete circle) shape of at least 2 mm in thickness.

The reversed halo sign is a relatively specific sign for cryptogenic organizing pneumonia (COP), although it is only seen in of fifth of patients with the disease.

Crescentic or ring-shaped opacities surrounding an area of ground-glass opacification occasionally seen in the setting of organising pneumonia were first described by Voloudaki and later coined the ‘Atoll sign’ by Zompatori. Kim et al. documented it in 19% of cases of cryptogenic organising pneumonia. The word Atoll is derived from the Maldivian word ‘atholhu’ meaning an island consisting of a circular coral reef surrounding a lagoon . Although originally thought to be specific for organising pneumonia, this CT appearance has been described in a variety of other diseases including sarcoidosis, Wegener granulomatosis, lymphomatoid granulomatosis, pulmonary paracoccidiomycosis and non-specific interstitial pneumonia, and under different names including the ‘reversed-halo’ sign and the ‘fairy-ring’ sign. Histopathologically the ground-glass opacification centrally represents septal inflammation, and cellular debris within the airspaces and the peripheral crescentic or ring-shaped opacity represents organising pneumonia in the alveolar ducts. From a learning perspective, although this striking CT appearance is well documented in organising pneumonia, it should alert the radiologist and physician to other possible diagnoses, of which Wegener granulomatosis is particularly important.


Cryptogenic organising pneumonia

Cryptogenic organising pneumonia (COP) is a disease of unknown aetiology. A variety of infectious as well as non infectious causes have been associated.

The condition was previously termed bronchiolitis obliterans organizing pneumonia (BOOP) : (not to be confused with bronchiolitis obliterans per se.)

Epidemiology and clinical presentation

Presentation is commonest in the 55 – 60 age group. Patients present with short history (i.e less than ~ 2 months) of breathlessness, non productive cough, weight loss, malaise and fever.  There is no association with smoking.


In addtion to the alveolar inflammtory changes found with a normal pneumonia, there is also involvement of the bronchioles.

Histologically, it is characterized by the presence of buds of granulation tissue (Masson bodies) in the distal airspaces which may cause secondary bronchiolar occlusion due to extension of the inflammatory process. Hence, the reason for being perviously termed bronchiolitis obliterans organizing pneumonia (BOOP).

Radiographic features

Chest radiograph
  • consolidation
    • bilateral patchy areas ( commonest finding 3 ) : often migratory
    • can affect all lung zones
    • usually peripheral, sub-pleural, peribronchovascular 2
  • nodules
    • foci of granulation tissue up to 1 cm
    • may simulate neoplasm if > 5 cm in size
    • may be numerous in immunocompromised patients

The most common HRCT features include 6:

  • patchy consolidation with a predominantly subpleural and / or peribronchial distribution
  • small, ill-defined peribronchial or peribronchiolar nodules
  • large nodules or masses
  • bronchial wall thickening or dilatation in the abnormal lung regions
  • a perilobular pattern with ill-defined linear opacities that are thicker than the thickened interlobular septa and have an arcade or polygonal appearance
  • ground glass opacity or crazy paving

The reverse halo sign (atoll sign) is considered to be highly specific ,although only seen in ~ 20% of patients with COP 5


First described by Davison and colleagues in 1983.

Treatment and prognosis

Corticosteroids have been widely used and most patients recover completely 3-4.

Differential diagnosis

On plain film consider
On HRCT consider

Categories: Radiology
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