Imaging features
According to current diagnostic criteria, identification of the viral pathogen via nucleic acid detection (usually from swab test) is considered as the gold standard and formative assessment for the diagnosis of COVID-19 [12]. However, due to various problems of virus detection in the clinical setting such as a shortage of supply test kits, irregular sampling of samples, laboratory error, insufficient viral material in the specimen, improper extraction of nucleic acid from clinical materials, and contaminatory and technical problems, there have been false negatives. Health care workers on the front line have found the diagnostic value of imaging to be increasingly valuable, which has translated over into clinical diagnosis of COVID-19 patients [12]. In the clinical work of this epidemic, the radiologists play a crucial role in the rapid identification and early diagnosis of a suspected patient—this can be of great benefit not only to the patient but also to the larger public health surveillance and response systems.

Chest radiograph
Chest radiograph of COVID-19 is not routinely recommended in clinical practice currently because they are insensitive to detecting COVID-19 in the early stage. However, chest radiography may have some utility, with the potential to serve as a screening tool on the frontlines in medical settings with limited resources or in cases where the patient’s physical condition does not allow for transport to the radiology department CT scanner. As the disease progresses beyond the early stage, chest radiography can detect multiple patchy opacities throughout the lungs. These opacities eventually become confluent and severe cases may appear as a “whited out lung” [15]. In more advanced cases, in addition to the ground glass opacity and consolidation, even pleural fluid (in a severe case) has been reported on the chest radiographs [2, 16]. Progressive opacities and consolidation have been observed on chest radiographs over the time course of the illness [17]. In the first case of COVID-19 reported in the USA, there were no obvious abnormalities on the chest radiograph during the early stages after the onset of symptoms. However, on day 9 of the illness, an increased area left basilar opacity was visible on chest radiography. Then, stable streaky opacities in the lung bases were visible, and the opacities have steadily increased over time [18].

Chest computed tomography
Computed tomography (CT) imaging is strongly recommended because it is very sensitive to detecting early disease, assessing the nature and extent of lesions, and discovering subtle changes that are often not visible on chest radiography. The imaging features of lesions are always described with the following factors: distribution, quantity, shape, pattern, density, and concomitant signs [19].
The typical chest CT imaging characteristics of COVID-19 include multiple, peripheral, bilateral, patchy, sub-segmental, or segmental ground glass opacities and areas of consolidation, which are mostly distributed along the bronchovascular bundles and subpleural space. The presence of associated interlobular septal thickening in the areas of ground glass opacity can give a crazy paving appearance. Air bronchograms with the areas of consolidation and bronchial wall thickening are often present. More rarely, there is a thickening of the adjacent pleura or interlobar pleura, and a small amount of pleural effusion. There is no obvious lymphadenopathy [15] (Tables 2, 3). The imaging features mentioned above are consistent with the findings from 81 patients in a recently reported article, in which the predominant pattern of abnormality observed was bilateral, peripheral, ill-defined, and ground glass opacification, mainly involving the right lower lobes [20]. Furthermore, in the currently available reports, the most common chest CT findings in COVID-19 patients are the peripheral areas of ground glass opacity/consolidation (without subpleural sparing) which are bilateral in distribution [21–23]. In one study, of 21 patients, most had more than two lobes affected (15 of 21, 71%) with bilateral involvement (16 of 21, 76%) [24]. Another study showed that the most common patterns of COVID-19 on thin-section CT images are pure ground glass opacity, ground glass opacity with intra- and/or interlobular septal thickening, and ground glass opacity with consolidation and consolidation, with prominent distribution in the posterior and peripheral part of the lungs [25]. In a large research cohort study, chest CT findings showed a bilateral distribution of patchy ground glass opacity and consolidation in 138 patients [26]. Typical chest radiograph and chest CT of COVID-19 are shown in Figs. 2 and 3.
Table 2 The typical features on CT imaging of COVID-19
Parameter Characteristic manifestations on CT imaging
Density Ground glass opacity and consolidation, possible interlobular septal thickening
Shape Patchy, sub-segmental, or segmental
Distribution Mid and lower lungs along the bronchovascular bundles with bilateral involvement
Location Peripheral and subpleural areas of the lung parenchyma
Concomitant signs (variable) Air bronchogram, a small amount of pleural effusion, no obvious lymphadenopathy
Table 3 Frequency of chest CT findings in COVID-19
CT signs Frequency Stage
Ground glass opacity ++++ E/A/S
Consolidation without ground glass opacity ++ S
Ground glass opacity and crazy paving ++ E/A/S
Ground glass opacity with consolidation +++ E/A/S
Patchy ground glass opacity +++ E
Bilateral distribution ++++ E/A/S/D
Peripheral distribution +++ E
Air bronchogram ++ E/A/S
Pleural effusion + S
Strip-like opacity + D
The appearance frequency of each CT characteristic is described in order from low to high as (+~++++); E, A, S, and D stand for stage early, advanced, severe, and dissipation
Fig. 2 Chest radiograph (a) in a 61-year-old man shows bilateral patchy, somewhat nodular opacities in the mid to lower lungs [16]. Unenhanced computed tomography (CT) images (b) in a 33-year-old woman., Images show multiple ground glass opacities in the periphery of the bilateral lungs. The bilateral, peripheral patterns of opacities without subpleural sparing are common and characteristic CT findings of the 2019 novel coronavirus pneumonia [22]. Chest CT image of a 71-year-old male (c) shows consolidation in the peripheral right upper lobe and a patchy area of ground glass opacity with some associated consolidation intra- and interlobular septal thickening within the left upper lobe [25]
Fig. 3 Typical CT findings of COVID-19. Chest CT (a) in a 75-year-old male show multiple patchy areas of pure ground glass opacity (GGO) and GGO with reticular and/or interlobular septal thickening [25]. Chest CT image of a 38-year-old male (b) shows multiple patches, grid-like lobule, and thickening of interlobular septa, typical “paving stone-like” signs [19]. An axial CT image obtained in 65-year-old female (c) shows bilateral ground glass and consolidative opacities with a striking peripheral distribution [23]. CT image of a 65-year-old male (d) shows large consolidation in the right middle lobe, patchy consolidation in the posterior and basal segment of right lower lobe, with air bronchogram inside [19]
In “Expert Recommendations from the Chinese Medical Association Radiology Branch,” chest CT manifestations of COVID-19 are divided into three stages: early, advanced, and severe, based on the extent of lesion involvement [15]. This current clinical guideline recommends dividing COVID-19 into four stages according to the time of onset and the response of body to the virus with additional dissipation stage (Fig. 4).Early stage: Chest CT shows single or multiple scattered patchy or conglomerate ground glass opacities, predominantly in the middle and lower lungs and along the bronchovascular bundles. These ground glass lesions are often located in peripheral and subpleural areas of the lung. Intra- and interlobular septal thickening sometimes present in the areas of ground glass opacity can give a crazy paving pattern. The pathological process during this stage is dilatation and congestion of the alveolar septal capillary, exudation of fluid in alveolar cavity, and interlobular interstitial edema [19]. One patient had normal chest CT at initial presentation; however, 3 days later the disease progressed and developed a solitary rounded ground glass lesion in the right lower lobe, indicating this pattern may represent the very first radiologically visible manifestation in patients infected with SARS-CoV-2 [24].
Advanced stage: At this stage, chest CTs will show new lesions that are similar to the earlier lesions described above. In addition, findings from the early stage of disease increase in density and extent, coexisting with the new areas of disease. As areas of consolidation grow, air bronchograms are often present in the areas of consolidation. A previous case report described a patient who had evolved into a mixed pattern of ground glass opacities and consolidation by 15 after onset of symptoms [27]. The pathological features in this stage are the accumulation of a cell-rich exudate in the alveolar cavity, vascular expansion, and exudation in the interstitium. The fibrous exudation connects each alveolus through the interalveolar space to form a fusion state [19].
Severe stage: As the disease further progresses, chest CT shows diffuse consolidation of the lungs of varying density secondary to the fibrous exudate into the alveolar cavity, air bronchograms, and bronchial dilation. Non-consolidated areas of the lung appear as patchy ground glass opacity. When most of the lungs are involved, the lungs appear as a “whited out lung.” The pleura is thickened and there can be a small amount of pleural effusion.
Dissipation stage: Images show gradual resolution of the ground glass opacity and consolidation in the lungs with some residual curvilinear opacities compatible with fibrosis.
Fig. 4 CT manifestations of different stages of COVID-19
After isolation and treatment, a majority of patients infected by COVID-19 stabilize and gradually recover and on imaging the areas of disease decrease correspondingly. However, in a small number of patients with underlying disease or advanced age, the disease progresses during the treatment with the scope of the lesions in the lungs expanding and increasing density, eventually appearing as “whited out lungs.”