At present, retrospective studies on various aspects of COVID-19 are being conducted. The infectivity of SARS-CoV-2 is high, the source of the pathogen is unknown, there is no specific treatment method, and the mortality rate is high[14], therefore, the early diagnosis of COVID-19 is a challenge. Correct and timely diagnosis is of great significance for treatment and prevention of dissemination. Due to the delayed laboratory results and limited sensitivity of the tests, there is still controversy regarding how to obtain an early clinical diagnosis [15]. Thoracic imaging plays an important role in early diagnosis. We believe that in epidemic areas, the key to diagnosis with imaging is to distinguish COVID-19 from other CAPs.
The data were similar to most clinical observations[16-18]. The age of COVID-19 group was mostly composed of young adults: 46 patients (71.9%) were <60 years old, and only 7 patients (15.2%) younger than 60 years were in the CAP group, This does not comply with the general spread of infectious diseases, that is, children and elderly populations are often susceptible groups, Young adults typically have the strongest resistance to infectious diseases and the incidence rate should be low. Among the 72314 cases noted by the Chinese Center for Disease Control and Prevention. The incidence rate in China for those under 10 years old was less than 1%[19]. The reason behind the high incidence rate in young adults cannot exclude the possibility of more virus contact and infection opportunities due to more social interactions. Of course, further studies on the epidemiology and pathogenesis of this disease are needed. Fever was the prominent clinical manifestation in both groups, but the increase in temperature was more obvious in the COVID-19 group than in the CAP group. In the early stage of the disease (within 7 days), the total number of leukocytes in the observed patients with COVID-19 was normal or low (< 4 × 109/L), or showed a decreasing trend, similar to that reported in the literature [20], which may be caused by the decrease in lymphocytes in the first few days after infection.. The symptoms of muscle soreness and fatigue in the COVID-19 group were significantly more prevalent than those in the CAP group, while the symptoms of expectoration in the latter group were significantly more prevalent than those in the former group,which may be related to the fact that CAP is prone to producing more purulent secretion due to bacterial infection[21]. The results showed that the incidences of heart disease, hypertension, chronic lung disease and diabetes in the CAP group were significantly higher than those in the COVID-19 group.
The chest CT changes of patients with COVID-19 usually showed ground-glass opacities shadow of different sizes 3-4 days after clinical onset. The size and shape of COVID-19 ground-glass opacities varied, among which the most typical was round or quasi-round ground-glass shadows, which were found in 32.1% of patients(53 cases) in this group and most of which were early changes in young patients. The statistics of this group showed that a single lesion was significantly more common in COVID-19 than that in CAP, which may be related to the early stage of SARS-CoV-2 infection. With the progression of the disease, the ground-glass opacities in some patients expanded and gradually developed into ground-glass shadows with multiple patches, large areas of fusion or large and small patches coexisting, but the density change was relatively small; this is in contrast with the rapid consolidation observed as CAP progresses, but the area of expansion was relatively not obvious. Of course, there are also some patients with mild CAP whose lung lesions do not expand until absorption. The pathological basis of ground-glass changes is mainly related to local severe pulmonary oedema, hyaline membrane formation and fluid exudation in the alveolar cavity in early lesions. Another 32.7% (54 cases) of patients with COVID-19 showed fine reticular shadows overlapping in the ground-glass shadow, which may be related to alveolar oedema in the lung lesion area and slight thickening of the alveolar septum with infiltration of monocytes, lymph and plasma cells[22]. Because some CAP patients also showed ground-glass opacities shadow (47 cases, 39.8%), which overlaps with the changes observed in COVID-19, the imaging manifestations must be combined with clinical findings and other lung changes.
In the CAP group, the new exudative inflammatory lesions were often accompanied by obvious fibrous components or texture aggregation and cord adhesion in other parts of the lung, which manifested as bronchitis or bronchial thickening and blurring of the outer edge, suggesting that these patients had experienced the process of lung inflammation in the past and that there were some remaining chronic inflammation or later changes. This is quite different from the acute onset, rapid progress and multiple manifestations of lung inflammation seen with COVID-19. Therefore, this study suggests that lung findings of fibrous cord and bronchial wall thickening can be used to exclude COVID-19. In addition, a certain proportion of emphysema, pulmonary bullae, reticular or honeycomb changes under the pleura, "mosaic" signs and/or small cavity shadows in the consolidation area were observed in the CAP group. In conclusion, the diversity of lung lesions and the coexistence of new and old lesions in the CAP group are helpful in distinguishing it from COVID-19.
In this group, 6 patients with COVID-19 underwent chest CT re-examinations approximately one week after the onset of the disease, The lesions showed "wandering" characteristics, which may indicate the heterogeneity of pathological changes in different lung areas; that is, the early changes and the changes in the organizing phase of diffuse alveolar injury can appear in different segments at the same time, and this was more common in young patients. Due to the small number of patients in this group, whether these findings have a good diagnostic specificity remains to be further confirmed.
The results of this study showed that there was a positive correlation among age, size of the lesion and total number of lung segments involved in the COVID-19 group. The correlation coefficient between age and the size of the lesion was 0.522, P < 0.001, and the correlation coefficient between age and the total number of segments involved was 0.531, P < 0.001. In this group, elderly patients had a significantly wider range of lesions at the first visit than young patients, and the number of involved segments was also significantly higher in the former group than that in the latter group. This may be due to the rapid development of the disease in elderly patients. The first CT scan showed involvement of multiple segments, which is also one of the factors that affects patient prognosis.
The follow-up of the COVID-19 group in the later period showed that patients aged older than 60 years had many changes remaining in the lung in the later period, and the most serious manifestation was extensive fibrous cord shadows in the lung. In this group, 32 patients (19.4%) were over 60 years old, and the prognosis of these patients was poor. The 2-3 month follow-up showed that the remaining changes in the lung could be absorbed, but the absorption was slow.
This study has several limitations. First, we retrospectively analyzed the imaging data of the two groups of patients. Our screening strategy has selection bias.. At present, a more balanced and large-scale prospective study of similar patients is still needed. In this study, the different situations of the disease in each centre and the different experiences of the radiologists had some influence on the diagnosis of the disease. In addition, some patients had positive results from the new crown pneumonia nucleic acid test. However, during the winter and spring seasons, these patients often have viral and bacterial pneumonia or other diseases, which may cause some interference with the performance of CT in evaluating new crown pneumonia.