In this study, we evaluated the temporal relationship between the appearance and resolution of CT findings and the RT-PCR evidence of COVID-19 virus presence and investigated lung imaging changes in COVID-19 pneumonia over time. This study’s major findings were twofold: first, the initial chest CT abnormality time was significantly shorter than the initial RT-PCR positive results, and the lung CT improvement time was shorter than the RT-PCR conversion to negative; second, chest CT is a reliable, non-invasive, rapid tool to monitor the occurrence, deterioration, and improvements in COVID-19 pneumonia.
The confirmation of COVID-19 infection depended on viral nucleic acid test using RT-PCR assays, which determined whether patients should be hospitalized or quarantine at home. But the RT-PCR test had distinct limitations due to its low sensitivity, insufficient stability, and relatively long processing time to obtain results. Some studies reported that the positive rate of RT-PCR assays for throat swab samples was 30–60% [17, 18]. Many factors can affect the RT-PCR results, including sampling operations, specimen sources (upper or lower respiratory tract), sampling timing, and the detection kit’s performance . A recent study showed that the salivary viral load was highest during the first week after symptom onset , which could account for the rapid spread of this pandemic at the early stage. Patients without timely isolation and therapy due to initial negative RT-PCR results definitely cause COVID-19 to spread.
Chest CT is a non-invasive, rapid, convenient imaging diagnostic tool and can detect mild lung abnormalities at the early stage of COVID-19 pneumonia. Our study showed that the initial chest CT abnormality time (3.23 ± 3.04 days) was significantly shorter than that of the initial RT-PCR positive results (5.84 ± 3.23 days). Our study was in agreement with previous research  that revealed that 29% (15/51) of patients had negative RT-PCR and positive CT results at initial presentation and 2% (1/51) demonstrated the opposite . Therefore, CT plays a vital role in the early detection of COVID-19 pneumonia, especially for patients with initial negative RT-PCR results. However, the specificity of CT for diagnosing COVID-19 pneumonia varies. Tao et al  reported that the sensitivity and specificity of chest CT for indicating COVID-19 infection was 97% and 25%, respectively. Bai et al reported that seven radiologists demonstrated sensitivities of 80%, 67%, 97%, 93%, 83%, 73%, and 70% and specificities of 100%, 93%, 7%, 100%, 93%, 93%, and 100% . Although the RT-PCR test is the gold standard for the diagnosis of COVID-19 infection, chest CT examination is essential for the early identification of potential patients and helps to determine treatment strategies. COVID-19 infection was upgraded from epidemic to pandemic by the World Health Organization (WHO) on March 11, 2020 . Thus, chest CT will certainly play a crucial role in the early detection of the disease to contain the pandemic’s spread.
Our study also found that the lung CT improvement time (11.58 ± 4.59 days) was significantly shorter than that of RT-PCR conversion to negative (14.38 ± 5.78 days). This indicates that CT is very useful for monitoring the course of COVID-19 pneumonia to direct clinicians to adjust the therapeutic strategy.
In our study, dynamic serial chest CT examination (4–6 times) with a relatively large patient population provided reliable data to observe the disease course. We divided the disease into 3 stages according to the time since the onset of the initial symptoms. At the early stage, the CT features of COVID-19 are predominantly GGO and small-vessel thickening. GGO is the main CT feature, and small-vessel thickening is the secondary feature. As the disease progresses, GGO evolves to consolidation. Crazy paving signs are also markedly enhanced as the disease progresses. At stage 2, air bronchogram signs are also dramatically increased as secondary signs of consolidation. Small-vessel thickening is still abundant as the secondary sign of GGO and consolidation. We also noticed that fibrotic lesions were gradually added, which represents the reparative process. At stage 3, fibrotic lesions significantly increased, accompanied by more consolidation, GGO, and crazy paving signs. This indicates the reparative and progressive process of COVID-19 pneumonia simultaneously, and various CT features coexist at stage 3. Our study demonstrated that the lung improvement time was 11.58 ± 4.59 days. The CT feature changes during follow-up in our study were in agreement with other reports [22, 23]. The recently published proposed COVID-RADS and common lexicon would improve the communication of imaging findings and facilitate the diagnosis and management of COVID-19 patients .
The lung CT appearance may be consistent with the pathology. The autopsy and histological examination of COVID-19 pneumonia showed bilateral diffuse alveolar damage with proteinaceous exudate, cellular fibromyxoid exudate, pulmonary edema, reactive hyperplasia of pneumocytes, desquamation of pneumocytes, hyaline membrane formation, fibroblastic plugs in airspaces, and interstitial mononuclear inflammatory infiltration [24, 25], which may be in accordance with the appearance of GGO, consolidation, and crazy paving signs. Tian et al reported patchy pneumocyte hyperplasia and interstitial thickening indicating an ongoing reparative process at the early stage of COVID-19 pneumonia . Notably, when the patients met the discharge criteria, there was still patchy consolidation left in the lung. This suggests that patients should quarantine at home for an additional period (2 weeks in China) after hospital discharge.
Our study also showed that typical COVID-19 pneumonia started as unilateral or bilateral sub-pleural GGO, then evolved to consolidation involving the sub-pleural and central areas. We also found that 12.9% of patients demonstrated normal chest CT images at the early stage. Then the number of lung lobes involved increased at stage 2. However, the number of lung lobes involved remained high at stage 3 and demonstrated no significant differences compared to stage 2. Residual GGO, scattered consolidation, and sub-pleural parenchymal bands persisted at stage 3. Thus, patients still need to be followed up after hospital discharge.
This study had limitations that merit mention. First, this is a short-term retrospective study, and long-term radiological follow-up is needed to monitor the pulmonary outcome due to the novel coronavirus infection. Second, this study lacked severe COVID-19 pneumonia patients (including respiratory failure requiring mechanical ventilation, shock, multiple organ failure necessitating ICU care) , so the changes in these patients’ CT findings need to be investigated in future research. Third, the CT scans’ radiation doses were not evaluated, but it is more important to detect infected patients, understand the disease course, manage patients, and contain the pandemic.
In conclusion, chest CT plays a vital role in the early detection of COVID-19 pneumonia, monitoring lesion progression, and improving treatment strategies. Our results indicated that CT showed lung abnormalities earlier than the time of initial positive nucleic acid tests with RT-PCR for the detection of COVID-19 infection, and demonstrated lung improvements earlier than the RT-PCR conversion to negative time. Chest CT should be considered as a useful tool to perform in the early stage of suspected COVID-19 infection, especially during the pandemic.