The Reversed Halo Sign, also known as the "atoll sign", has been first considered to be specific to cryptogenic organizing pneumonia, but in last years has been described in a variety of pulmonary diseases, both infective or non-infective such as fungal infection, tubercolosis, sarcoidosis, pulmonary neoplasms and infarction.

The Reversed Halo Sign, also known as the "atoll sign", has been first considered to be specific to cryptogenic organizing pneumonia but in last years has been described in a variety of pulmonary diseases, both infective or non-infective such as fungal infection, tubercolosis, sarcoidosis, pulmonary neoplasms and infarction.

Pulmonary embolism (PE) refers to embolic occlusion of the pulmonary arterial system, manifesting with non specific clinical signs and symptomps such as dyspnea, chest pain and heamoptysis. The most important risk factors are primary hypercoagulable states, recent surgery, pregnancy and prolonged bed rest/immobility.

Radiographic features depend on whether PE is acute or chronic: CT pulmonary angiography show central filling defects within the pulmonary vasculature, surrounded or not by a thin rim of contrast, depending on occlusive or non-occlusive thrombosis. In case of chronic embolism, the filling defects are observed in the periphery of the affected vessel, with an obtuse angle with the vessel wall.

Pulmonary infarction (PI) occurs in the minority (10-15%) of patients with PE, due to the failure of both pulmonary and bronchial vascular systems, leading first to congestion and hemorrage and then, in some cases, to necrotic infarction.