Radiologist Spotlight: Dr. McComb Reveals What Radiologists Should Know About Diagnosing COVID-19 with Medical Imaging
Written by Barbara McComb, MD, FACR
If we had a rapid test for COVID-19 that could provide 100% sensitivity and specificity, we might be able to successfully identify all people infected with the virus and triage them correctly and quickly. That would be a perfect world. Instead, we must judiciously consider how to leverage available tests to create the best possible scenario. So, what can we say about imaging and COVID-19?
Radiographic and CT Features of COVID-19
Imaging features of COVID-19 are generally nonspecific and overlap with features of several other diseases, such as influenza and other viral pneumonias, atypical bacterial pneumonias, noninfectious organizing and eosinophilic pneumonias, drug toxicity, certain connective tissue diseases, and pulmonary edema.
At the time of this writing, the CDC does not recommend the use of chest radiographs or CT to diagnose COVID-19. The CDC recommends viral testing, even if an imaging result suggests a new diagnosis. That said, chest radiography and CT are being utilized sparingly as discriminatory elements in COVID-19.
Lower sensitivity has been reported for chest radiographs compared to RT-PCR, although radiographs may be abnormal in patients with negative RT-PCR testing results. Radiographs are most likely to be normal in mild or early disease.
When abnormal, chest radiographs frequently demonstrate subtle or conspicuous ill-defined areas of opacification. Opacities are often peripheral and primarily lower zonal, and become more widespread in the second week in more severe cases. Radiographs may also reveal findings uncommon for COVID-19, such as pleural disease, cavities, cystic lung disease, or discrete nodules. Some findings may suggest severe, alternative or concomitant disease.
CT may be normal in patients with positive RT-PCR test results, particularly early in the course of COVID-19 infection. CT may also be abnormal in patients imaged before the onset of symptoms and with negative RT-PCR test results. Findings in critically ill patients with other respiratory pathologies may resemble COVID-19. CT findings may also suggest an alternative diagnosis in some patients. The Society of Thoracic Radiology (STR) and American Society of Emergency Radiology (ASER) currently recommend restricting the use of CT to patients who test positive and who are suspected of complications, such as abscess or empyema.
Certain CT findings are considered highly suggestive or typical of COVID-19 pneumonia. These include bilateral peripheral confluent or multifocal ground-glass opacities with or without consolidation or septal thickening (crazy paving). Opacities are often geographic, but may also have a rounded or reverse halo appearance. Confidence in diagnosing COVID-19 is considered indeterminate when typical findings are lacking and the distribution of opacities is unilateral, central, or diffuse, or if very few small opacities are seen. Confidence is low when findings are atypical, such as focal lobar or segmental consolidation, tree-in-bud or centrilobular nodularity, cavitation, or smooth septal thickening with pleural effusion. Lymphadenopathy is also uncommon in COVID-19.
Various authors have described the temporal evolution of COVID-19 infection on CT. Pan et al. reported ground glass opacification to be the main finding in early stage disease (0-4 days after symptom onset). Early CT studies were also sometimes normal. During the progressive stage (5-8 days), increased bilateral ground glass, consolidation, and crazy paving were seen, followed by worsening of these findings during peak stage (9-13 days). As consolidation improved during recovery, residual ground glass was seen in the absorption stage (> 14 days). The development of subpleural parenchymal bands was also demonstrated in later stage disease. Continued deterioration of imaging findings despite medical treatment for COVID-19 has been associated with poor prognosis.
Advice Going Forward
We are all learning about COVID-19 together. We must continue to read and reach out to one another as needed. Viral testing is key, and imaging studies are being used sparingly. CT is a limited resource. Portable radiography is less sensitive, but has certain benefits like mobility and relative ease of disinfection. Not addressed here, point-of-care ultrasound is also being used for bedside thoracic evaluation in suspected COVID-19, although it requires close contact and special training. There is no one-stop shop, but as we gather information, we improve our decision-making abilities, better utilize our local resources, and become better team members during this pandemic.
Wonderful online resources for radiologists are becoming available every day to help us best interpret imaging studies and navigate the handling of patients safely. Among them, the ACR, RSNA, and ARRS each now provide robust collections of free information. We can turn to a recently published RSNA Expert Consensus Document by Simpson et al. for guidance on reporting CT findings, an article by Wong et al. for a scoring system for chest radiographs, and an article by Mossa-Basha et al. for information on department preparedness.
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