To the best of our knowledge, of all the longitudinal CT performance studies of COVID-19 pneumonia [4,5,6,7,8], the present study analyzed the largest number of patients. Through the study of 589 COVID-19 CT images of 182 patients, we found that the total lesion CT score proposed by others only considers the area of involvement of the lesions, but not the nature of the lesion [4, 6, 8, 20,21,22], which may lead to delays of imaging evaluation in clinical practice . Therefore, according to the nature and area of the lesions, we first evaluated consolidation, linear opacity, and GGO scores. Second, although the total lesion score peaked during days 9–11, similar to previous reports [6, 8, 9], we found it interesting that the peak of the total lesion score was highest during days 17 and 18, because this is not reported by others. Third, the peak times of the four CT scores were basically consistent during the 10 stages. However, if the interval was extended to 1 week, the peak time of the total CT score lagged behind those of the consolidation and GGO scores, suggesting that a useful scoring system should include comprehensive assessment of the extent and histopathology of the lesion.
The peak times of the total lesion score differ among previous studies [5, 6, 8, 9]. These studies define the last stage of pneumonia as days 14 or 15 after disease onset. However, the average time from illness onset to discharge reported by other investigators is approximately 28 days [23, 24], and more than 12 days after illness onset is a critical period for disease progression . Therefore, we speculate that the staging method of these studies reported the average of the true peak of the total lesion score. Clearly, in our research, the 10-stage curve confirmed our prediction that the total lesion score peaked during days 9–11 as well as on days 17–18. The latter peak corresponds to week 3 of the five-stage curve, while the previous peak was averaged. In addition, the peak time of the consolidation score was earlier than that of the total score of the five-stage change, and the curve of the consolidation score conformed to a quadratic function. Moreover, the decreased time of the total lesion score is later than the improvement time of clinical parameters , and greater consolidation may indicate disease progression . Therefore, the consolidation score may reflect the severity of disease more promptly than the total lesion score, although the changes of the two scores may tend to be associated with the increase of the number of stages.
Among the typical features of each stage, here we show that COVID-19 pneumonia was mainly characterized by bilateral, multiple, and ill-defined lesions, with a GGO pattern, located in the posterior and peripheral regions of both lungs, similar to previous findings [5,6,7, 9]. In the predominant CT pattern of COVID-19 pneumonia, the GGO and linear opacity patterns increased rapidly from week 3 after disease onset. Histologically, GGO represents interstitial mononuclear inflammatory infiltrates and edema , while during the absorption stage, the absorption of exudate cells and hyaline membrane in the lung tissue forms other lesions such as consolidation and the crazy-paving pattern, gradually transforming into GGO. Evidence indicates that linear opacity is an organized feature during the late stage of disease . Therefore, good prognosis may be associated with increases in the proportions of the GGO and linear opacity patterns during the later stage of disease. However, the increases in the total score and GGO score during stage 5 (after week 4) may indicate an extension of hospital stay. Furthermore, the proportions of the consolidation pattern and the consolidation score peaked during week 2 after disease onset and gradually decreased, suggesting that the events that occur during week 2 may be critical for disease progression.
In the present study, the changing trends of the proportions of consolidation, GGO, and linear opacity were not completely consistent with the changing trends of their respective scores, suggesting that previous studies [7, 27], based only on the proportion of lesions used to judge the severity of the disease, may have generated unreliable results. Thus, the proportion of lesions and the area involved should therefore be simultaneously considered. Here, pleural thickening and interlobular septal thickening were relatively common CT signs during each stage. The crazy-paving pattern sign was relatively common during the first 2 weeks after illness onset, and pleural effusion appeared during the first 3 weeks.
Obviously, CT plays an important role in controlling the spread of COVID-19. However, CT has some insurmountable shortcomings, such as high cost, exposure to radiation, and their immobile nature, which can be overcome by lung ultrasound (LUS) [28,29,30,31]. Coupled with the fact that the lesions of COVID-19 tend to occur in subpleural regions, LUS has been suggested as a potential triage and diagnostic tool for COVID-19 . The latest research showed that LUS was a promising tool for early risk stratification in COVID-19, which may guide patients’ management strategies, as well as resource allocation in case of surge capacity [28, 31].
There were some limitations to our study. First, this was a single-center study, and the results require verification through a multicenter study. Second, the CT scores of this study may not be comparable with the quantitative scores determined using artificial intelligence , although visual scores are easier to determine. Finally, only patients with mild COVID-19 pneumonia were included, and the longitudinal CT performance of severe pneumonia must be determined using a larger sample size.