CT characteristics of COVID-19: reversed halo sign or target sign?

Dear Editor,

In their Letter to the Editor, Görkem and Çetin (1) reported reversed halo sign (RHS) as an important finding observed in chest computed tomography (CT) of a 15-year-old girl with a confirmed diagnosis of COVID-19. The RHS corresponds to a focal rounded area of ground-glass opacity, surrounded by a ring of consolidation. A wide variety of diseases may be related to this sign, including pulmonary involvement by COVID-19. Nevertheless, in our opinion, a closer look at the images provided by Görkem and Çetin (1) shows some morphological aspects that differ from those of the RHS, such as the central punctiform component and the polygonal aspect of the lesion periphery. These findings are more likely related to a new tomographic sign, also described recently in patients with COVID-19, characterized by peripheral ring-like opacities and a central rounded ground-glass opacity (2, 3). Muller and Muller (2) suggested that these ring-like opacities indicate a pattern of organizing pneumonia reaction. They named this chest CT finding “the target sign”, as it morphologically resembles a shooting target.

On detailed inspection, the peripheral wall of the CT target sign has a polygonal appearance in most patients. This pattern differs from the RHS, which generally has rounded or oval boundaries. The polygonal boundary is considered to represent the perilobular pattern, a characteristic finding of organizing pneumonia, consisting of thick, irregular polygonal or arcade-like opacities, with a peripheral distribution closely related to the inner surface of the interlobular septa (4). Despite its morphological distinctiveness, however, the target sign is often misinterpreted as the RHS.

Although the target sign and RHS have the same significance and represent organizing pneumonia, they must be differentiated, as the target sign seems to be more specific, described related only to infection with COVID-19. Special care should be taken when interpreting the RHS in patients with COVID-19. When an internal hypodensity is seen, with or without reticulation, pulmonary infarction should be suspected, and not organizing pneumonia (5).