Reversed halo sign (lungs)
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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.
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.
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
- other infections
- pulmonary infarction due to pulmonary thromboembolism
- granulomatosis with polyangiitis (formerly known as Wegener granulomatosis)
- lipoid pneumonitis
- pulmonary neoplasms
- radiofrequency/microwave ablation of pulmonary malignancies
- radiation pneumonitis 8
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.
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.
- 1. Kim S, Lee K, Ryu Y et al. Reversed Halo Sign on High-Resolution CT of Cryptogenic Organizing Pneumonia: Diagnostic Implications. AJR Am J Roentgenol. 2003;180(5):1251-4. doi:10.2214/ajr.180.5.1801251 - Pubmed
- 2. Marchiori E, Zanetti G, Irion K et al. Reversed Halo Sign in Active Pulmonary Tuberculosis: Criteria for Differentiation from Cryptogenic Organizing Pneumonia. AJR Am J Roentgenol. 2011;197(6):1324-7. doi:10.2214/AJR.11.6543 - Pubmed
- 3. Godoy M, Viswanathan C, Marchiori E et al. The Reversed Halo Sign: Update and Differential Diagnosis. Br J Radiol. 2012;85(1017):1226-35. doi:10.1259/bjr/54532316 - Pubmed
- 4. Georgiadou S, Sipsas N, Marom E, Kontoyiannis D. The Diagnostic Value of Halo and Reversed Halo Signs for Invasive Mold Infections in Compromised Hosts. Clin Infect Dis. 2011;52(9):1144-55. doi:10.1093/cid/cir122 - Pubmed
- 5. Voloudaki A, Bouros D, Froudarakis M, Datseris G, Apostolaki E, Gourtsoyiannis N. Crescentic and Ring-Shaped Opacities. CT Features in Two Cases of Bronchiolitis Obliterans Organizing Pneumonia (BOOP). Acta Radiol. 1996;37(6):889-92. doi:10.1177/02841851960373P289 - Pubmed
- 6. Legouge C, Caillot D, Chrétien M et al. The Reversed Halo Sign: Pathognomonic Pattern of Pulmonary Mucormycosis in Leukemic Patients with Neutropenia? Clin Infect Dis. 2014;58(5):672-8. doi:10.1093/cid/cit929 - Pubmed
- 7. Maturu V & Agarwal R. Reversed Halo Sign: A Systematic Review. Respir Care. 2014;59(9):1440-9. doi:10.4187/respcare.03020 - Pubmed
- 8. Chiarenza A, Esposto Ultimo L, Falsaperla D et al. Chest Imaging Using Signs, Symbols, and Naturalistic Images: A Practical Guide for Radiologists and Non-Radiologists. Insights Imaging. 2019;10(1):114. doi:10.1186/s13244-019-0789-4 - Pubmed