than the other two. Although rarely shown in the early CT findings of all three viral pneumonias, the fibrotic changes were more frequent in SARS than COVID-19 and MERS </ = 6.275, P< 0.05). For other imaging signs, only the MERS pneumonia demonstrated tree-in-buds, cavitation, and its incidence rate of interlobular or intralobular septal thickening presented significantly increased as compared to the other two pneumonia <l = 22.412, P<O.05). No pneumothorax, pneumomediastinum and lymphadenopathy was present for each group.
Conclusions Imaging fmdings on early stage of these three coronavirus pneumonias showed similar basic lesion patterns, including GGO and consolidation, bilateral distribution, and predominant involvement of the subpleural area and the lower lobes. Early signs of COVID-19 pneumonia showed less severity of inflammation. Early fibrotic changes appeared in SARS only. MERS had more severe inflammatory changes including cavitation and pleural effusion. The differences may indicate the specific pathophysiological processes for each coronavirus pneumonia. S I NCE late December, 2019, an outbreak of a novel coronavirus disease (CQVID-19; previously known as 2019-nCoV) [1, 2] was reported in Wuhan, China, which has subsequently affected 26 countries worldwide. [2] As of now, although outbreak of the novel coronavirus pneumonia is not yet fully under control, many researches about the novel coronavirus and its pathogenic mechanism have been published continuously, and related clinical and therapeutical knowledge is updated on a weekly basis.
Medical imaging played an important role during the course of the pneumonia diagnosis and follow-up. CT has been an important imaging modality in assisting in the diagnosis and management of patients with CQVID-19 pneumonia, and reports on the radiological appearances of CQVID-19 pneumonia are emerging. Because chest radiography is not sensitive for the early detection of subtle changes such as groundglass opacity (GGQ) and may demonstrate normal findings in early stage of infection, chest CT functions more effectively in early detection of CQVID-19 pneumonia. Currently, CT is of unique importance as it is the main tool for screening, primary diagnosis, and evaluation of disease severttv.!" Dozens of imaging papers published revealed some relatively characteristic manifestations on chest CT images at early stage. [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] The virus is formally named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The genetic sequence analysis revealed that the SARS-CoV-2 belongs to the~-coronavirus genus, with a 79.0% nucleotide identity to severe acute respiratory syndrome coronavirus (SARS-CoV) and 51.8% identity to Middle East respiratory syndrome coronavirus (MERS-CoV.)YS] Latest histopathological report!": 17] presented that there are some similar pathological basis between the CQVID-19 pneumonia and other two types of pneumonias induced by SARS-CoV and MERS-CoV. Past imaging studies also revealed a certain similar radiological appearance both for the SARS-CoV and the MERS-CoV pneumonia. [18] However, there is no comparison study reported on CT imaging features among these three coronavirus pneumonias on its early phase, to the best of our knowledge.
In this study, we were trying to reveal similarities and differences of the early CT features among the three types of viral pneumonias infected by SARS-COV-2, SARS-CoV and MERS-CoV respectively through a systemic review.
We included all original articles and case reports presenting viral pneumonia induced by SARS-CoV-2, SARS-CoV and MERS-CoV. Relevant papers were identified by computerized searches of PUBMED, Web of Science and WANGFANG till February 24th 2020, and using different combination of search words as follows:
CT, imaging, radiologic, pneumonia, SARS, MERS, and CQVID-19. We also used the bibliography of retrieved articles to further identify relevant studies. The inclusion criteria were: (1) observational studies or case reports related to CT imaging features of pneumonia infected by the SARS-CoV-2, SARS-CoV and MERS-CoV; (2) pathology or cytology examinations or mechanism were used as the gold standard of diagnosis. Exclusion criteria were: (1) duplicate publications or data; (2) publications related to chest X ray or radiograph only;
(3) publication related to late follow up or convalescence; (4) publication in languages other than English or Chinese; and (5) insufficient data available in the original publication.
used for statistical analysis.
Articles excluded for incomplete data or non-applicable data retrieval and unmatched study design (n=56) General features of the included studies A total of 90 publications were initially identified, 88 of which were written in English and 2 in Chinese. After evaluation using inclusion and exclusion criteria, a total of 24 publications were included in the study ( Figure  1) . [3-13, 19, 21-32] The characteristics of the 24 studies are listed in Table 1 . The basic results of the pooled study and CT availability information are listed in Table 2 . 
Simple vote-counting method was employed to record the different radiological features for each included observational study. The CT exam and imaging finding data were collected from the included articles and recorded by three authors using same criteria. The recorded data included: (1) Age, gender, system of onset, availability of the early chest CT (For each patient, we defined the early CT scans as which obtained within 1 week after onset of symptoms). (2) Extraction and classification of imaging pattern and features, including distribution of lesions and the features as followed: GGO, consolidation, nodular lesion, inter-or intra-lobular septal thickening, fibrotic changes, tree-in-bud sign, crazy paving pattern, cavitation, organizing pneumonia, pneumomediastinum, pneumothorax, lymphadenopathy, and pleural effusion. Imaging pattern and features were defined as follows with reference to Ref 19_21. [19] [20] [21] The geographic distribution of lesions was defined as: unilateral or bilateral, focal or multifocal or extensive, predilection to lower lobes or no predilection to lower lobe. The transverse distribution of the abnormalities was categorized as central (i.e., peribronchovascular), peripheral (i.e., subpleural), or no transverse predilection. Nodular opacities were defined as focal round opacities. Fibrotic changes included traction bronchiectasis, reticulation, irregular linear opacities, and stellate or subpleural bands. Pleural effusion, other complications such as pneumothorax and pneumomediastinum, and other coexisting abnormalities (e.g., emphysema bullae) if any, were also recorded.
Disagreements were resolved by discussion among all authors during the process of data abstraction and classification. If the parameter could not be counted for certain cases included in the study, the parameter was defined as "not accessible" and the certain cases were excluded from the calculation of percentage.
Qualitative variables were presented as frequency and percentage. Presence or absence of other CT parameters (GGO, consolidation, cavitation, pleural effusion, tree-in-bud, crazy paving, and inter-or intra-lobular septal thickening) were compared between the two groups using a Chi-square test for categorical data.
A P value (two-tailed) less than 0.05 was considered statistically significant. SPSS software (22.0, IBM) was COVID-19 num and lymphadenopathy was present for each group.
Xie et al. [4] 2020 China Case reports 5 Lei et al. [5] 2020 China Case report 11
Fang et al. [6] 2020 China Case report 2
Shi etalY] 2020 China Case report 1
In this study, we designed the systemic review of literatures and found similarities and differences of the Pan et al. [8] 2020 China Article 52 early CT image features in three types of pneumonias Un et al. [9] 2020 [11] 2020 China Article 21
The early use of chest CT as a screening tool for Duan et al. [12] 2020 CT not only decrease missed diagnosis which might Ajlan et al. [3] 2015 Saudi Arabia Case reports 7 occurred on chest radiograph, but can also help to features on early CT of these three types of pneumo-Chan et al. [19] 2004 Hong Kong Article 27 nias caused by SARS-CoV-2, SARS-CoV and MERS-CoV Mulleret al. [32] 2004 Canada Article 29 were diffuse airspace opacities which present as GGO, CQVID-19: severe acute respiratory syndrome coronavirus consolidation or mixed GGO and consolidation. The 2; MERS: Middle East respiratory syndrome coronavirus; airspace opacities were often seen in subpleural or pe-SARS: severe acute respiratory syndrome coronavirus. ripheral area of bilateral lobes, starting from the lower lobes. Such imaging features are pathogenomic for compared to SARS and MERS pneumonias cl =22.412, coronavirus pneumonia in certain scenario. [14, 18] Abnor-P< 0.05). Although rarely shown in the early CT find-mal lung CT findings can be present even in asympings from all three viral pneumonias, the fibrotic tomatic patients, and lesions can rapidly evolve into changes were more frequent in SARS than COVID-19 a diffuse GGO predominance or consolidation pattern and MERS cl =6.275, P< 0.05). More detailed results within 1-3 weeks after onset of symptoms, peaking at are listed in Table 3 . around 2 weeks after onset. [14] GGO: ground-glass opacity. Percentage comparison among three groups or between two groups was performed using Chi-Square test. P value represents the comparison among the three groups unless specified.
"Not accessible" means the parameter cannot be counted for certain cases included in the study. On the other hand, differences were also noted among necrosis was also observed. These lesions comprise the pathologic basis for the respiratory failure and radiologic abnormalities of MERS-CoV lnfectlon.l": 40] It is not difficult to explain that the inflammatory process in MERS infection was more severe than COVID-19 and SARS.
The innovative point of our study is that we firstly reported the comparison of radiological features for the three coronavirus pneumonias on basis of a systematic literature review and analysis. COVID-19 still needs to be studied deeply in case it becomes a global health threat. In clinical practice, we already found early phase CT is a reliable and feasible test which provides differential diagnosis qulckly when combined with epidemiological, clinical and laboratory results for suspected patients. That is especially the case when real-time reverse transcription polymerase chain reaction is slow and not accessible widely.
We acknowledge three main limitations of our study. First, our study is of its retrospective nature and basis of literature review, some included cohort was small and sequential CT study was not performed; hence, we were not able to compare both the early and late CT findings in patients of all the three groups. Only early phase CT was included in our study, but at the same time, we believe the early CT signs are more pathognomonic as compared with progressive phase CT and even the severe phase as adult respiratory distress syndrome. Secondly, because of the year gap between the included studies, the technologies and intervention were evolving. There would be diverse uncontrolled factors affecting the collection and the comparison in the study and some bias is unavoidable.
Thirdly, the literature on clinical and chest CT findings of COVID-19 is still expanding very quickly. It is not easily feasible to get the most updated literatures on COVID-19 in this study. At the same time, we believe the case number included for COVID-19 is appropriate since it is balanced as compared with the case numbers included for MERS and SARS (147 vs. 37 & 85 cases respectively) in this systemic review using comparative study design.
In summary, imaging findings on early stage of these three coronavirus pneumonias showed similar basic lesion pattern, including GGO and consolidation, bilateral distribution, and predominate involvement of the subpleural area and the lower lobes. Early signs of COVID-19 showed less severity of inflammation. Early fibrotic changes appeared in SARS pneumonia only. MERS had more severe inflammatory changes including cavitation and pleural effusion. The differences may indicate the specific pathophysiological processes for each coronavirus pneumonia. Further investigation is warranted to elucidate the pathophysiological basis for the difference and the correlation with the clinical disease progress.