Reversed halo sign

1. Universidade Federal Fluminense, Niterói (RJ) Brasil. 2. Universidade Federal do Rio de Janeiro, Rio de Janeiro (RJ) Brasil. 3. Faculdade de Medicina de Petrópolis, Petrópolis (RJ) Brasil. 4. Santa Casa de Misericórdia de Porto Alegre, Porto Alegre (RS) Brasil. 5. Universidade Federal de Ciências da Saúde de Porto Alegre, Porto Alegre (RS) Brasil. A 35-year-old man presented to the emergency room with chest pain accompanied by dyspnea. He reported having sustained a lower limb fracture and having been immobilized for 30 days. A CT scan showed the reversed halo sign (RHS) with a reticular pattern, and the final diagnosis was pulmonary infarction. The RHS found on HRCT of the chest is defined as a rounded area of ground-glass attenuation surrounded by a ring of consolidation. This sign was initially described as a sign specific for organizing pneumonia (OP). Later studies identified the RHS in a wide spectrum of infectious and noninfectious diseases. In Brazil, the most common infectious causes of the RHS are tuberculosis, paracoccidioidomycosis, and invasive fungal diseases (invasive pulmonary aspergillosis and mucormycosis). Among the noninfectious causes, OP, both idiopathic and secondary, is the most common. Other important causes are pulmonary infarction and sarcoidosis. Although the RHS is considered a nonspecific sign, a careful analysis of its morphological characteristics can narrow the differential diagnosis, helping the attending physician to make a definitive diagnosis. Two imaging patterns should be taken into account in order to make the diagnosis more specific: the presence of nodules on the wall of or within the halo (nodular RHS); and a reticular pattern within the halo (reticular RHS).