Radiographic features
The primary findings of COVID-19 on chest radiograph and CT are thos
e of atypical pneumonia or organizing pneumonia.
However imaging has limited sensitivity for COVID-19, as up to 18% demonstrate normal chest radiographs or CT when mild or early in the disease course, but this decreases to 3% in severe disease. Bilateral and/or multilobar involvement is common.
The current recommendation of the vast majority of learned societies and professional radiological associations is that imaging should not be employed as a screening/diagnostic tool for COVID-19, but reserved for the evaluation of complications.
Plain radiograph
Although less sensitive than chest CT, chest radiography is typically the first-line imaging modality used for patients with suspected COVID-19. For ease of decontamination, use of portable radiography units is preferred.
Chest radiographs may be normal in early/mild disease.
In those COVID-19 cases requiring hospitalization, 69% had an abnormal chest radiograph at the initial time of admission, and 80% had radiographic abnormalities sometime during hospitalization. Findings are most extensive about 10-12 days after symptom onset.
The most frequent findings are airspace opacities, whether described as consolidation or, less commonly, GGO. The distribution is most often bilateral, peripheral, and lower zone predominant 89.97. In contrast to parenchymal abnormalities, pleural effusion is rare (3%).
CT
The primary findings on CT in adults have been reported as
• ground-glass opacities (GGO): bilateral, subpleural, peripheral
• crazy paving appearance (GGOs and inter-/intra-lobular septal thickening)
• air space consolidation
• bronchovascular thickening in the lesion
• traction bronchiectasis
The ground-glass and/or consolidative opacities are usually bilateral, peripheral, and basal in distribution.
A retrospective study of 112 patients found 54% of asymptomatic patients had pneumonic changes on CT.
The following chest CT findings have been reported to have the highest discriminatory value (p<0.001).
peripheral distribution
ground-glass opacity
bronchovascular thickening (in lesions)
Atypical CT findings
These findings only seen in a small minority of patients should raise concern for superadded bacterial pneumonia or other diagnoses.
Temporal CT changes
Four stages on CT have been described
- early/initial stage (0-4 days): normal CT or GGO only
- halo half of patients have normal CT scans within two days of symptom onset
- progressive stage (5-8 days): increased GGO and crazy paving appearance
- peak stage (9-13 days): consolidation
- absorption stage (>14 days): with an improvement in the disease course, "fibrous stripes" appear and the abnormalities resolve at one month and beyond
Pediatric CT
In a small study of five children that had been admitted to hospital with positive COVID-19 RT-PCR tests and who had CT chest performed, only three children had abnormalities. The main abnormality was bilateral patchy ground-glass opacities, similar to the appearances in adults, but less florid, and in all three cases the opacities resolved as they clinically recovered.
On 18 March 2020, the details of a much larger cohort of 171 children with confirmed COVID-19, and evaluated in a hospital setting was published as a letter in the New England Journal of Medicine. Ground-glass opacities were seen in one-third of the total, whereas almost 16% of children had no imaging features of pneumonia.
Ultrasound
Initial work on patients in China suggests that lung ultrasound may be useful in the evaluation of critically ill COVID-19 patients. The following patterns have been observed, tending to have a bilateral and posterobasal predominance:
- multiple B-lines
- ranging from focal to diffuse with spared areas
- representing thickened subpleural interlobular septa
- may also manifest as a light beam sign, an evanescent, broad-based vertical reverberation artifact arising from a regular pleural line
- irregular, thickened pleural line with scattered discontinuities
- subpleural consolidations
- can be associated with a discrete, localized pleural effusion
- relatively avascular with color flow Doppler interrogation
- pneumonic consolidation typically associated with preservation of flow or hyperemia 65
- alveolar consolidation
- tissue-like appearance with dynamic and static air bronchograms
- associated with severe, progressive disease
- restitution of aeration during recovery
- reappearance of bilateral A-lines
Radiology report
The Radiological Society of North America (RSNA) has released a consensus statement endorsed by the Society of Thoracic Radiology and the American College of Radiology (ACR) that classifies the CT appearance of COVID-19 into four categories for standardized reporting language:
typical appearance
peripheral, bilateral, GGO +/- consolidation or visible intralobular lines (“crazy paving” pattern)
multifocal GGO of rounded morphology +/- consolidation or visible intralobular lines (“crazy paving” pattern)
reverse halo sign or other findings of organizing pneumonia
indeterminate appearance
absence of typical CT findings and the presence of
multifocal, diffuse, perihilar, or unilateral GGO +/- consolidation lacking a specific distribution and are non-rounded or non-peripheral
few very small GGO with a non-rounded and non-peripheral distribution
atypical appearance
absence of typical or indeterminate features and the presence of
isolated lobar or segmental consolidation without GGO
discrete small nodules (e.g. centrilobular, tree-in-bud)
lung cavitation
smoother interlobular septal thickening with pleural effusion
negative for pneumonia:
no CT features to suggest pneumonia, in particular, absent GGO and consolidation.
CO-RADS
In March 2020, the "COVID-19 standardized reporting working group" of the Dutch Association for Radiology (NVvR) proposed a CT scoring system for COVID-19. They called it CO-RADS (COVID-19 Reporting and Data System) to ensure CT reporting is uniform and replicable. This assigns a score of CO-RADS 1 to 5, dependent on the CT findings. In some cases a score of 0 or 6 may need to be assigned as an alternative. If the CT is uninterpretable then it is CO-RADS 0, and if there is a confirmed positive RT-PCR test then it is CO-RADS 6.
The first study investigating the use of CO-RADS found a reasonable level of interobserver variation, with a Fleiss' kappa score of 0.47 (cf. 0.24 for PI-RADS and 0.67 for Lung-RADS).
COVID-RADS
In April 2020, American radiologists based at the University of Southern California proposed the COVID-19 imaging reporting and data system (COVID-RADS), which has a confusingly similar name to CO-RADS (see above)
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