How to identify COVID-19 on a chest CT scan

7th Jan 2021

When it comes to COVID-19 infection, non-standard and vague terminology has been used for describing the changes on chest radiographs such as airspace disease, pneumonia, infiltrates, patchy opacities, and hazy opacities. This makes it difficult to conclude a typical picture of the disease on these films. 

Simply put, the primary finding of COVID-19 on imaging, particularly chest radiographs, is that of non-specific pneumonia often involving the peripheral and basal lung areas.

Lungs with shading on right middle border and left lower segments. Illustration.

Figure 1. The primary finding of COVID-19 on chest radiographs is non-specific pneumonia in the peripheral and basal lung areas. 

 

Chest CT use in COVID-19

Computer tomography of the chest, or chest CT, is a sensitive tool used to diagnose COVID-19 infection. It can also be used to track the progression of the disease, guide disease management, and potentially predict clinical outcomes or disease severity. 

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Common chest CT findings in COVID-19

There are three predominant chest CT abnormalities seen in COVID-19:

  1. Ground-glass opacity
  2. Consolidation
  3. Crazy-paving patterns

Ground-glass opacity 

Bilateral, peripheral and basal predominant ground-glass opacity is commonly found in chest CT imaging of COVID-19 patients. Ground-glass opacity is a non-specific CT finding associated with a hazy opacity that doesn’t obscure the underlying pulmonary vessels or bronchial structures. Its presence suggests a partial filling of the air spaces in the lungs by inflammatory exudates.

Chest CT showing bilateral lungs with ground-glass opacity.

Figure 2. Bilateral, peripheral and basal ground-glass opacity is the predominant finding on chest CT in patients with COVID-19 pneumonia.

Lung consolidation

Bilateral, peripheral and basal lung consolidations are also often seen, indicating a complete filling of the air spaces by the inflammatory exudates. Opacities often have an extensive geographic distribution. This means they aren’t round like nodules or masses, they are geographic—more like the outline of a country on a map. However, at times they can be round and nodular.

Chest CT cross section with lung consolidation. CT image.

Figure 3. Bilateral, peripheral and basal lung consolidations with a geographic distribution are often seen in chest CT images of COVID-19 pneumonia. (Image courtesy of Song F, Shi, N, Shan, F, et al. 2020.)

The presence of both focal and multifocal ground-glass opacities and consolidations may be seen in patients with COVID-19. Ground-glass opacities are the main findings, but they can co-exist with consolidations or evolve to consolidations. Sometimes, a COVID-19-positive patient may show only consolidations on chest CT. 

Chest CT cross section with multiple areas of ground-glass opacity and consolidation. CT image

Figure 4. Multiple discrete areas of ground-glass opacity, consolidation, or both may occur in patients with COVID-19. (Image courtesy of Song F, Shi, N, Shan, F, et al. 2020.)

Crazy-paving patterns

Along with the ground-glass opacities and consolidations, there is another characteristic finding that’s commonly seen on CT of patients with COVID-19 pneumonia called the crazy-paving appearance. Named for its resemblance to paving stones, this finding indicates the presence of both airspace and interstitial lung disease. Studies found that the crazy-paving appearance was associated with peak lung involvement—correlating with the worst clinical and radiological picture.

In a typical example of the crazy-paving appearance we see interlobular and intralobular septal thickening on a background of ground-glass opacity in the subpleural lung zones bilaterally. 

The presence of the crazy paving patterns in the lungs on chest CT indicates airspace and interstitial lung disease in patients with COVID-19. (Image courtesy of Shi, H, Han, X, Jiang, N, et al, 2020.) 

Figure 5. The presence of the crazy-paving patterns in the lungs on chest CT indicates airspace and interstitial lung disease in patients with COVID-19. (Image courtesy of Song F, Shi, N, Shan, F, et al 2020.)

The predominant CT imaging findings of COVID-19 include a geographic distribution of peripheral focal or multifocal ground-glass opacities affecting both lungs, with consolidations and crazy paving sometimes present later in disease progression.

 

Other findings on chest CT in COVID-19

But what else might you see?

Some other reported imaging findings include lung nodules and cystic changes, mostly representing pneumatocele.

Linear opacities, called subpleural lines, may also be seen in some patients. When seen after the resolution of the other findings, they’ve been suggested to represent fibrotic changes. But given the short period of time since the emergence of this pandemic, it’s still too early to determine whether they represent irreversible fibrosis, or whether they might clear with more time after recovery.

Chest CT images showing lung nodules, cystic changes and linear opacities.

Figure 6. Other chest CT findings in patients with COVID-19 include lung nodules, cystic changes and linear opacities. (Images courtesy of Li, X, Zeng, X, Liu, B, 2020 and Kong, W and Ararwal, PP, 2020.)

The reversed halo sign, or atoll sign, is another finding reported in some patients with COVID-19. The reversed halo sign reflects a peripheral dense consolidation surrounding a less dense ground-glass opacity in the center of the same lesion. 

This sign was mostly described in patients with organizing pneumonia. But interestingly, in a study of over 400 cases—half of them with COVID-19, and the other half with other viral pneumonias—the reversed halo sign was found to be one of the differentiating signs between the two groups of pneumonias. It was seen mostly in patients with COVID-19 pneumonia as compared to those with other viral pneumonias.

Chest CT image showing reversed halo sign.

Figure 7. The reversed halo sign, or atoll sign, is seen more commonly in patients with COVID-19 when compared to other viral pneumonias. It indicates a peripheral dense consolidation surrounding a less dense core of ground-glass appearance in the same lesion. (Image courtesy of Wu, Y, Xie, Y, and Want, X, 2020.)

 

Uncommon chest CT findings in COVID-19

Pleural effusion is considered to be characteristically absent in COVID-19 and its presence may indicate a co-existing bacterial pneumonia. There are some reported cases of COVID-19 with pleural effusion although this is still uncommon. In a study that compared COVID-19 with other viral pneumonias, pleural effusion was more common in non-COVID-19 viral pneumonia. But if found in a patient with COVID-19, pleural effusion may be a poor prognostic indicator, according to several preliminary studies. 

Lungs with bacterial pneumonia. Illustration.

Figure 8. Pleural effusions are not commonly seen in patients with COVID-19 and if found in these patients may indicate a co-existing bacterial pneumonia. (Image courtesy of Song, F, Shi, F, et al., 2020.)

 

Absent chest CT findings in COVID-19

COVID-19 patients rarely develop lymphadenopathy, tree-in-bud sign, masses, cavitations, or calcifications, which often suggest bacterial or chronic infections of the lung.

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References

  1. Song, F, Shi, N, Shan, F, et al. 2020 Emerging 2019 novel coronavirus (2019-nCoV) pneumonia. Radiology295: 210–217. PMID: 32027573
  2. Shi, H, Han, X, Jiang, N, et al. 2020. Radiological findings from 81 patients with COVID-19 pneumonia in Wuhan, China: A descriptive study. Lancet Infect Dis20: 425–434. PMID: 32105637
  3. Li, X, Zeng, X, Liu, B, et al. 2020. COVID-19 infection presenting with CT halo sign. Radiol Cardiothorac Imaging. 2: e200026. PMCID: PMC7194018
  4. Kong, W and Agarwal, PP. 2020. Chest imaging appearance of COVID-19 infection. Radiol Cardiothorac Imaging. 2: e200028. PMCID: PMC7233424
  5. Wu, Y, Xie, Y, and Wang, X. 2020. Longitudinal CT findings in COVID-19 pneumonia: case presenting organizing pneumonia pattern. Radiol Cardiothorac Imaging. 2: e200031. PMCID: PMC7233389