Atoll sign (lungs)
Updates to Article Attributes
Reversed halo sign, also known as the atoll sign, is defined as central ground-glass opacity surrounded 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. It was initially described on high-resolution CT.
This is in contrast to the more well-known halo sign of invasive fungal infection.
Pathology
Histopathologically 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.
Practical points
Initially, it was thought of being highly specific for cryptogenic organising pneumonia (COP), although only seen in about one-fifth of patients with the disease 1.
Whilst the relatively high specificity for the aforementioned entity is preserved, the sign has also been described on standard resolution CT (multidetector CT, thin sections <3 mm 3,4) with the following pathologies:
- opportunistic invasive fungal infections (IFI)
- pulmonary mucormycosis (PM), most commonly in immunocompromised patients, published ranges from ~20% 4 to >90% 6
- invasive pulmonary aspergillosis (IPA)
- paracoccidioidomycosis
-
granulomatosis with polyangiitis (formerly known as Wegener
'sgranulomatosis) - sarcoidosis
- pneumocystis pneumonia
- tuberculosis
- community-acquired pneumonia
- lymphomatoid granulomatosis
- lipoid pneumonitis
- pulmonary neoplasms
- pulmonary infarction
- following radiation therapy and radiofrequency/microwave ablation of pulmonary malignancies
Clinical significance
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.
Role of the radiologist
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.
See also
-<a href="/articles/granulomatosis-with-polyangiitis">granulomatosis with polyangiitis</a> (formerly known as <a href="/articles/granulomatosis-with-polyangiitis">Wegener's granulomatosis</a>)</li>- +<a href="/articles/granulomatosis-with-polyangiitis">granulomatosis with polyangiitis</a> (formerly known as <a href="/articles/granulomatosis-with-polyangiitis">Wegener granulomatosis</a>)</li>
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