[Reversed halo sign: chest CT findings in 5 patients and cause analysis]

Objective: To describe diseases that may present with the reversed halo sign (RHS) on high-resolution CT, and therefore to improve the recognition of this thoracic radiological feature. Methods: We retrospectively reviewed the HRCT scans and medical records of 5 patients who presented with RHS on chest CT from Peking University Third Hospital. With "reversed halo sign, fairy ring, atoll sign" as the Chinese and English keywords, 420 cases from the literatures were also reviewed through PubMed, Wanfang and CNKI databases until August 2015. Results: The 5 patients were all males, aging 19-55 years. The diagnosis of the diseases included cryptogenic organizing pneumonia (patient 1), pulmonary tuberculosis (patient 2), pulmonary infarction (patient 3), pulmonary edema (patient 4) and mycoplasma pneumonia (patient 5). Literature review showed that RHS was observed in 206 cases of infectious pulmonary diseases and 219 cases of noninfectious pulmonary diseases. Invasive fungal infection (including 59 cases of mucormycosis and 27 aspergillosis) and pulmonary tuberculosis (79 cases) predominated in infectious pulmonary diseases. However, in non-infectious pulmonary diseases, RHS was most frequently observed in organizing pneumonia (66%), sarcoidosis, lung cancer, pulmonary infarction, pulmonary edema and other interstitial lung diseases. There were some differences in the morphologic findings of RHS among invasive fungal infection, pulmonary tuberculosis and organizing pneumonia. In addition, our patient with mycoplasma pneumonia was the first case reported to present with RHS on HRCT. Conclusions: Although a wide spectrum of pulmonary diseases may present with RHS on chest CT, this sign is still a characteristic thoracic radiological feature, which is more commonly found in organizing pneumonia, pulmonary tuberculosis and invasive fungal infection, and is helpful in differential diagnosis.