Reversed halo sign (lungs)

The reversed halo sign, also known as the atoll sign, on chest CT 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. The consolidation should be at least 2 mm in thickness ⁸.

The sign is distinct from the halo sign of invasive fungal infection.

Pathology

The central area (ground-glass opacity) corresponds to alveolar septal inflammation and cellular debris in alveolar spaces, while the peripheral consolidation corresponds to granulomatous tissue within the distal airspaces ⁵.

Etiology

The sign is most commonly seen in organizing pneumonia (OP), which is most commonly cryptogenic (COP) but can also be secondary to specific causes. However, the sign is only seen in about one-fifth of patients with cryptogenic organizing pneumonia ¹.

The sign has also been described on CT with the following pathologies ^3,4,7:

• fungal pneumonia
  • mucormycosis (most common)
  • invasive aspergillosis
  • paracoccidioidomycosis
  • histoplasmosis (uncommon)
  • cryptococcosis (uncommon)
  • pneumocystis pneumonia (rare)
• other infections
  • tuberculosis
  • community-acquired bacterial pneumonia
  • coronavirus in the context of COVID-19
• pulmonary infarction due to pulmonary thromboembolism
• granulomatosis with polyangiitis (formerly known as Wegener granulomatosis)
• sarcoidosis
• lipoid pneumonitis
• pulmonary neoplasms
  • non-small cell lung cancer
  • lymphomatoid granulomatosis
• iatrogenic
  • radiofrequency/microwave ablation of pulmonary malignancies
  • radiation pneumonitis ⁸

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.

Practical points

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

• bird's nest sign