One of the most striking things for many clinicians treating their first few Covid-19 patients are the spectacular chest x-ray changes seen in those presenting in extremis. However, there is a spectrum of appearances on plain radiography associated with Covid-19 which may range from subtle ill-defined patchy ground-glass opacities to the widespread non-cardiogenic pulmonary oedema of ARDS.
In a study of 64 Chinese Covid-19 PCR positive patients, up to 70% of those admitted to hospital had an abnormal chest x-ray on admission and a further 10% will developed changes whilst an inpatient. The worst chest x-ray appearances occurred at 10-12 days post-symptom onset. The most common features were:
- Consolidation (47%)- radio-opaque densities, obscuring normal markings, that may contain air bronchograms
- Ground-glass opacities (33%) – hazy ill-defined increased lung opacities that do not obscure pulmonary vasculature or bronchial architecture
The distribution was most commonly:
- Peripheral (41%)
- Lower zone (50%)
- Bilateral (50%)
Differential diagnosis of bilateral patchy infiltrates on CXR:
- Other causes of viral pneumonia e.g. influenza, CMV, HSV
- Atypical bacterial pneumonia e.g. mycoplasma
- Other causes of pneumonia in immunosuppressed e.g. PCP
- Pulmonary oedema
- Organising pneumonia
Acute Respiratory distress syndrome
ARDS in Covid-19 infection may occur rapidly and the findings on chest x-ray include extensive, usually symmetrical bilateral patchy alveolar infiltrates involving all regions of the lungs that then coalesce and progress to more dense consolidation. Finally, as the ARDS resolves, patients may be left with a range of residual radiographic abnormalities; most frequently a reticular pattern of shadowing, which is indicative of post-ARDS pulmonary fibrosis.
Differential diagnosis of ARDS on CXR:
- Acute cardiogenic pulmonary oedema- More likely to have effusions, Kerley B lines, cardiomegaly. Less likely to have air bronchograms
- Multilobar bacterial pneumonia- May differ in clinical presentation. More focal, defined consolidation.
- Pulmonary haemorrhage- May differ in clinical presentation. Commonly focal, rarely diffuse.
Similar to plain radiography, changes on CT are primarily bilateral, peripheral and basal-predominant and peak at around 10 days post-symptom onset. Typical stages on CT as the disease progresses are:
- Ground-glass opacities
- ‘Crazy paving’- ground glass changes with thickened interlobular septae and intralobular lines
Intralesional bronchvascular thickening is also observed.
Interestingly, over half of Covid-19 positive asymptomatic patients have changes on CT.
When to CT?
CT is not indicated as part of the routine work-up for a suspected Covid patient but can be performed to rule out alternative diagnoses.
Radiopaedia: An in-depth overview of imaging in Covid-19 from the ever-excellent online radiology reference site. Packed with scrollable CTs from real cases and links to related articles.
The Radiology Assistant: Another great online resource (albeit not as slick as Radiopaedia) which neatly breaks down the characteristic findings on CT. It also has a handy Covid CT reporting template.
Osmosis: imaging features of Covid: A succinct 7 minute animated summary of everything the general medic needs to know about imaging in the Covid pandemic.
Imaging findings of Covid-19: More than just ground glass opacities: From Dr Henry Guo of Stanford University Medical Centre, this Youtube video provides a 25 minute deep dive into the key radiological findings in Covid.
Radiology preparedness for Covid-19: An expert panel from the RSNA discuss how to adapt the policies, protocols and structure of your radiology department in response to Covid.
British Institute of Radiology: Imaging in Covid-19: Although free to view, you have to register to access this 3 hour webinar from leading UK radiologists which also provided 3 CPD points.