Reversed halo sign (lungs)
Reversed halo sign, also known as the atoll sign, is defined as central ground-glass opacity surrounded by denser consolidation of crescentic shape (forming more than three-fourths of a circle) or complete ring 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.
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 .
The sign is classically seen in organizing pneumonia (OP), which is most commonly cryptogenic (COP) but can also be secondary to other causes. However, the sign is only seen in about one-fifth of patients with COP .
Whilst the relatively high specificity for the aforementioned entity is preserved, the sign has also been described on CT with the following pathologies :
- opportunistic invasive fungal infections (IFI)
- pulmonary infarction due to venous thromboembolism
- granulomatosis with polyangiitis (formerly known as Wegener granulomatosis)
- pneumocystis pneumonia
- community-acquired pneumonia
- lymphomatoid granulomatosis
- lipoid pneumonitis
- pulmonary neoplasms
- coronavirus in the context of COVID-19
- following radiation therapy and radiofrequency/microwave ablation of pulmonary malignancies
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 .
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 .
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 .