International Journal of Clinical & Medical Images

Clinical Image

Reversed halo sign, also known as the atoll sign, was initially described in 1996 by Voloudaki et al. It is defined as a focal round opacity of groundglass attenuation surrounded by a crescentic or ring of consolidation. The central ground glass opacity within the lesions corresponds to alveolar septal inflammation, and the annular consolidation corresponds to peripheral fibrin precipitation and formation of granulomatous process (Figure 1). On thoracic imaging, this sign is classically seen in cryptogenic organizing pneumonia (COP). However it can also be secondary to other etiologies such as: Infections especially coronavirus in the context of Covid 19, neoplasms and inflammatory diseases. Therefore, the patient’s history in conjunction with the clinical findings, and other radiological results can ultimately lead to an accurate diagnosis. In case of doubt a biopsy may be needed to provide a presumptive final diagnosis.

Keywords: Atoll sign; Thoracic imaging; Organizing pneumonia