Definition of typical CT features of COVID-19
Combined with the typical imaging diagnostic standards of COVID-19 that were reported by domestic imaging diagnosis guidelines and some literatures (16-18), We define the cases with the following three CT features as typical COVID-19 cases and the others as atypical cases in this study . These three features are as follows: Firstly, density: Multiple GGOs(Ground glass nodules) or consolidations are present. Secondly, size: Pulmonary lobular or fusion lobule. Thirdly, distribution: Subpleural distribution with major axis nearly parallel to pleura and perpendicular to the bronchovascular bundle.