Role of imaging in COVID-19
On the frontlines of fighting the COVID-19 epidemic, chest imaging (particularly chest CT) now has an irreplaceable role in the early diagnosis of COVID-19 as well as monitoring the disease’s clinical course. COVID-19 has characteristic manifestations in the lung that are readily detected on chest CT as described in some recent publications [15, 24, 25]. In several prior reports, chest CT showed progression of disease that corresponded with worsening clinical symptoms and also disease resolution as the patients clinically recovered [21, 22, 27]. In a recent report [20], the predominant pattern of abnormality in COVID-19 observed on the basis of the interval between symptom onset and the first CT scan has some differences. Lesions quickly evolved from focal unilateral to diffuse bilateral ground glass opacities that progressed to or co-existed with consolidations within 1–3 weeks, indicating the change through clinical course which was also reflected on the number of involved lung segments. In a prior study of 51 patients, the absolute number of lung findings increased with the time from symptom onset and lesions with consolidations including ground glass opacities with consolidation and pure consolidation showed mildly positive correlation with the time between symptom onset and the CT [25]. Another report demonstrated mild or moderate progression of disease as manifested by increasing extent and density of lung opacities during follow-up [24]. In the context of typical clinical presentation and exposure history, some patients may present with negative results of RT-PCR for COVID-19 but have characteristic imaging features of COVID-19 on chest CT. Subsequently, these patients whose labs were initially negative for COVID-19 did test positive later with repeat swab tests. Thus, in patients at high risk for COVID-19, chest CT evidence of viral pneumonia may precede negative RT-PCR test results and present as an important warning signal. A combination of repeated swab tests and CT scanning may be helpful for individuals with high clinical suspicion of COVID-19 but negative RT-PCR screening [28, 29]. Chest CT has a high sensitivity for diagnosis of COVID-19. In a report of 51 patients with chest CT and RT-PCR assay performed within 3 days, the sensitivity of chest CT was greater than that of RT-PCR for COVID-19 at initial patient presentation (98% vs 71%, respectively, p < 0.001) [30]. In another study of 1014 patients, 59% had positive RT-PCR results, and 88% had positive chest CT scans. The sensitivity of chest CT in suggesting COVID-19 was 97% based on positive RT-PCR results. Between 60% and 93% of cases had initial positive CT consistent with COVID-19 prior (or parallel) to the initial positive RT-PCR results [31]. In addition, chest CT can evaluate disease severity. In terms of the percentage of pneumonia lesions in the entire lung volume, the difference between the earlier stage patients and the severe stage patients is statistically significant [32]. In an initial prospective analysis of the clinical features of 41 patients, chest CT images of ICU patients on admission showed bilateral multiple lobular and subsegmental areas of consolidation, and overall had more imaging abnormalities than non-ICU patients [1]. Older patients may have more systemic symptoms, more extensive lung involvement, and worse prognosis than younger patients [33]. Thus, the typical CT findings of COVID-19 can not only help early screening suspected cases, but can also monitor the clinical progression and may predict severe complications such as acute respiratory diseases. With high sensitivity for COVID-19, chest CT would play a very important role in the diagnosis and treatment of COVID-19 in the high-risk regions and countries. In some extreme situations in very high-risk areas (Wuhan, China), CT was the one of main diagnostic criteria for this disease due to the relatively high false negative of RT-PCR. For some low-risk regions and countries, positive predictive value of CT alone or adding CT to RT-PCR should be adjusted. The proposition of CT could be tempered to some degree. Therefore, to be more accurate, the role of chest CT in COVID-19 should be assessed based on the prevalence of the disease in each area.