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.
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.
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)
- granulomatosis with polyangiitis (Wegener granulomatosis)
- pneumocystis pneumonia
- community-acquired pneumonia
- lymphomatoid granulomatosis
- lipoid pneumonitis
- pulmonary neoplasms
- pulmonary infarction
- 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 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.
- 1. Kim SJ, Lee KS, Ryu YH et-al. Reversed halo sign on high-resolution CT of cryptogenic organizing pneumonia: diagnostic implications. AJR Am J Roentgenol. 2003;180 (5): 1251-4. AJR Am J Roentgenol (citation) - Pubmed citation.
- 2. Marchiori E, Zanetti G, Irion KL 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 citation
- 3. Godoy MC, 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 - Free text at pubmed - Pubmed citation
- 4. Georgiadou SP, Sipsas NV, Marom EM et-al. 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 - Free text at pubmed - Pubmed citation
- 5. Voloudaki AE, Bouros DE, Froudarakis ME et-al. Crescentic and ring-shaped opacities. CT features in two cases of bronchiolitis obliterans organizing pneumonia (BOOP). Acta Radiol. 1997;37 (6): 889-92. Pubmed citation
- 6. Legouge C, Caillot D, Chrétien ML 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 citation
Interstitial lung disease
interstitial lung disease
- drug-induced interstitial lung disease
- hypersensitivity pneumonitis
idiopathic interstitial pneumonia (mnemonic)
- acute interstitial pneumonia (AIP)
- cryptogenic organising pneumonia (COP)
- desquamative interstitial pneumonia (DIP)
- idiopathic nonspecific interstitial pneumonia (NSIP)
- idiopathic pleuroparenchymal fibroelastosis
- lymphoid interstitial pneumonia (LIP)
- respiratory bronchiolitis–associated interstitial lung disease (RB-ILD)
- usual interstitial pneumonia / idiopathic pulmonary fibrosis (UIP/IPF)