Discussions
COVID-19 is a new disease which is caused by betacoronavirus. The diameter of the virus particle is very small, about 60~140 nm; therefore, it is easy to reach the lung terminal structure, such as alveolar septum, alveolar wall, and interlobular septum, which causes extensive edema and lymphocyte infiltration in the lung interstitium; early alveolar exudation is not prominent, but the disease progresses rapidly [16].

Imaging features
In this study, the imaging features were consistent with the previous literature reports [9–13, 16–18] of viral pneumonia; most of the patients had ground-glass opacities and mixed ground-glass opacities; no patients demonstrated consolidation without ground-glass opacification. Subpleural distribution was common. It also occurred around the bronchovascular bundle. Air bronchograms and interlobular septal thickening were often present. No patients had cavitation, centrilobular nodules, and lymphadenopathy. Pleural effusion was rare and most of them occurred in severe cases. Most of the lesions involved both lower lobes (85.7%), most of them more than two lobes (74%), and bilateral involvement was common (83.3%); single lobe involvement was rare (10.3%).

CT visual quantitative evaluation
In a recent study done by Michael et al, they introduced a method to score the severity of inflammation on CT images based on summing up degree of acute lung inflammation lesions involvement of each lobe (including ground-glass opacity or consolidation or other fuzzy interstitial opacities) [9]. We used the same method to quantify pulmonary inflammation and correlate to the clinical classifications. There was significant difference in scores between common type and severe-critical type (p < 0.001). However, there was also a score overlap between the two groups, which showed that 8 cases in the common type had a higher score, and 5 cases in the severe-critical type had a lower score. Among 8 cases of common type, 7 cases had fibrotic lesions which indicated that the lesions began to be repaired, and all of the 8 patients were less than 70 years old (range 36–65, average 52.5 years), none of them had pulmonary complications. The specific situation of 5 patients in a severe-critical type which had a lower score was as follows: 3 of the 5 patients were over 70 years old; among them, 1 patient was a female smoker with diabetes, aged 80 years old, with moderate emphysema and a small amount of pleural effusion; 1 patient was a 70-year-old female, with emphysema and a small amount of pleural effusion as well; the other one was 75-year-old female with high blood pressure; the forth case was a 44-year-old male without any underlying disease; however, CT images showed only progressive lesions such as ground-glass opacification and consolidation without any fibrotic lesions (Fig. 5). The last case was a 58-year-old female without any underlying disease; further analysis was needed to find out the cause of clinical severity. Overall, we believe that many factors, such as advanced age, underlying diseases, and pleural effusions, would relate to the clinical severity, which call for a comprehensive evaluation.
Fig. 5 A 44-year-old male was admitted to the hospital 1 day after fever and cough with a body temperature of 39 °C. The leukocytes were normal and lymphocytes were decreased. He was living in Zhuhai and traveled to Macao 12 days before the onset of the disease and stayed in Macao for 1 week. He was healthy and nonsmoker. Chest CT (images a–c) on the 4th day after admission demonstrated bilateral peripheral ground-glass opacities without consolidation. TSS was 9. The clinical type was severe-critical type. Follow-up CT (images d, e) on the 22nd day after onset showed bilateral fibrotic changes with traction bronchiectasis and ground-grass opacities

High proportion of mild-type patients
Among the 78 cases, there were 24 cases of mild type (30.8%) which mean those patients had positive real-time RT-PCR SARS-CoV-2 tests, while chest CT was normal. In the review of several recent published literature of COVID-19 in Wuhan, China, all patients reported in articles had ground-glass opacities in the lungs. Huang et al reported 41 infected cases in Wuhan, all had pneumonias [1]. Chen et al reported 99 confirmed cases in Wuhan with 74 bilateral pneumonias and 25 unilateral pneumonias [19]. However, the most recent report from Guangzhou had similar findings, which showed 23.6% confirmed patients without abnormalities on chest CT [20]. To further explore our data, we found several characteristics. Nine cases had a short time interval from onset to the latest CT examination with a range of 0–7 days, which indicated that the chest CT could be normal at the early phase. Another 9 cases had a longer time interval from onset to the latest CT scan with a range of 8–19 days. The negative findings may not relate to the shorter onset time. It remains to be further explored whether the CT negativity may relate to the degree of infection and autoimmunity. Finally, the last 6 patients had no symptoms. These patients were negative in both clinical and imaging, suggesting that some cases were potential sources of infection, which should be paid more attention to.

Limitation
In this study, the number of cases between groups was significantly different because too few severe-critical patients were included in this study, which decreased the reliability of statistical results. Only image analysis was carried out without combining clinical information in this study; however, advanced age, underlying diseases, and pleural effusions may lead to a lower TSS but severe situation. In our next study, we will include more cases, and make a comprehensive evaluation combining the clinical characteristics and laboratory examination information.