Reversed halo sign: from a clinico-radiologic-pathologic point of view

Reversed halo sign (RHS) is characterized by a central ground-glass opacity surrounded by a crescentic or ring-shaped consolidation. It was originally described as a pathognomonic imaging feature of cryptogenic organizing pneumonia (COP), but has subsequently been observed in a wide variety of pulmonary disorders, including infectious and non-infectious diseases. Although RHS is currently considered as a non-specific and non-pathognomonic sign, a careful analysis of its radiologic characteristics together with clinical features can greatly facilitate the differential diagnosis. The paper presents a state-of-the-art review of RHS, we retrospectively review the definition of RHS in chronological order and comprehensively summarize the spectrum of pulmonary diseases associated with RHS. In addition, the radiologic-pathologic correlations of RHS in different disease entities are systemically discussed and an integrated approach is proposed to establish the differential diagnosis of RHS.