The reversed halo sign (RHS) — sometimes called the atoll sign — is a focal rounded area of ground-glass opacity surrounded by a more or less complete ring of consolidation. It is a pattern, not a diagnosis. The same appearance carries very different weight in a neutropenic transplant recipient and in a previously well adult with a subacute cough, and the commonest error is to anchor on the first disease that comes to mind.
What the sign actually describes
Describe the two components separately: the central ground-glass component and the peripheral ring. Note the number of lesions, their distribution (peripheral versus central, upper versus lower zone), the thickness and completeness of the ring, and whether there is associated consolidation, pleural effusion, cavitation or a vascular filling defect. Reverse windows and thin sections help: a ring that disappears on 1 mm sections was probably dependent atelectasis or motion.
The term is purely descriptive. It was originally reported in cryptogenic organising pneumonia (COP), which is why many descriptions still imply that diagnosis, but the literature since then has attached the pattern to infection, infarction, malignancy and treatment effects.
Causes worth knowing
| Condition | Typical setting | Clues that separate it |
|---|---|---|
| Cryptogenic organising pneumonia | Middle-aged adult, subacute cough and breathlessness, often after a viral illness | Multiple bilateral RHS, peripheral and lower-zone predominance, migratory over weeks, prompt response to corticosteroids |
| Invasive pulmonary aspergillosis | Prolonged neutropenia, stem-cell or solid-organ transplant | RHS tends to appear as the neutrophil count recovers; earlier imaging may show a nodule with a ground-glass halo or a wedge-shaped infarct |
| Pulmonary mucormycosis | Uncontrolled diabetes, haematological malignancy | Rapid progression, pleural effusion, reverse halo less frequent than in aspergillosis but reported |
| Pulmonary infarction | Pulmonary embolism, sickle cell disease, vasculitis | Pleural-based wedge, vascular filling defect, no response to antibiotics or steroids |
| Tuberculosis | Endemic setting or relevant exposure | Cavitation, tree-in-bud nodularity, upper-lobe scarring, satellite nodules |
| Organising pneumonia after viral infection | Weeks after COVID-19 or influenza | Peripheral RHS with residual ground-glass, steroid-responsive, no organism identified |
| Less common causes | Sarcoidosis, lipoid pneumonia, radiotherapy, metastases | Each has additional features — nodal enlargement, fat density, a radiation field, known primary |
How to work it up
Start with the host. In a neutropenic or transplant patient, the RHS is a fungal pattern until proven otherwise and should trigger urgent microbiology, galactomannan or beta-D-glucan testing and discussion with the treating team; do not wait for a positive culture. In an immunocompetent patient with a subacute course, organising pneumonia becomes likely, but infection still has to be excluded before steroids are started.
Then look at the vessels. A peripheral, pleural-based ring with a filling defect in a supplying artery is an infarct, and the management changes completely. Conversely, multiple bilateral rings that migrate on follow-up imaging are very difficult to explain by infection alone.
Finally, review the technique. Confirm that the ring persists on thin sections and in both lung windows, and that it is not a dependent artefact. On expiratory images, dependent lung can produce a pseudo-ring that resolves on inspiration.
Common pitfalls
- Confusing it with the halo sign. The halo sign is a nodule surrounded by ground-glass; the reversed halo is ground-glass surrounded by consolidation. They imply different stages of fungal disease.
- Calling every ring a reversed halo. A cavity with a thick wall, a target lesion, or a necrotic metastasis has a different wall and different enhancement.
- Ignoring the clinical tempo. A pattern that is stable over months is unlikely to be acute infection; a pattern that doubles in days is unlikely to be pure organising pneumonia.
- Treating the image rather than the patient. Steroids given to an unrecognised fungal infection can be harmful.
Key points
- Reversed halo = central ground-glass with a ring of consolidation; describe both components and the distribution.
- Organising pneumonia is the classic cause, but invasive fungal infection, infarction and tuberculosis are the ones that change management.
- In neutropenic patients, treat the pattern as angio-invasive fungal disease until excluded.
- Thin sections and inspiratory images separate true RHS from dependent artefact.
- Follow-up imaging tempo is a diagnostic tool: migratory multiple rings favour organising pneumonia.
Related: chest differential diagnosis index and teaching cases.