Accurate and efficient testing is a powerful weapon in the fight against COVID-19, helping to direct medical resources and minimize community spread. Polymerase chain reaction (PCR) tests using a nasal/throat swab are presently the recommended method for detecting an active coronavirus infection. However, what happens when these tests are not widely available or are inconclusive?
In the case of COVID-19, imaging tests, such as computerized tomography (CT) scans, have proven to be helpful tools—with a few important caveats.
"There is controversy right now about how imaging should be used, CT imaging in particular, for diagnosing COVID-19," says Brent Little, MD, a thoracic radiologist at Massachusetts General Hospital Imaging. Dr. Little has been doing research on the use of imaging in diagnosing the virus.
The Pros and Cons of CT Scans in Diagnosing COVID-19
CT scans generate high quality, detailed images of the patient's bones and organs. With these images, radiologists can look for either opacities or bright spots on the patient's lungs. In diagnosing COVID-19, CT scans of the patient's lungs suspected to be infected can reveal hazy abnormalities that tend to cluster in the outer parts of the lungs, more commonly known as ground-glass opacities. With these findings, physicians can diagnose a COVID-19 infection that has traveled deeper into the lungs and may have been missed by a swab test.
According to Dr. Little, the benefits of CT scans in diagnosing COVID-19 include that they are:
- Readily available
- Fast examinations with immediate results
- More sensitive than some PCR tests
- May stay positive as the infection evolves
However, one significant disadvantage of CT scans in diagnosing COVID-19 is that they can result in false negative results, especially early in the infection. In turn, this can give people false reassurance, cause them to refrain from proper isolation and ultimately increase the risk of infecting other people in the community. False positive findings can also occur, because other conditions can mimic the findings of COVID-19 on CTs.
Imaging for Viral Testing Historically and Worldwide
There is a long history of using imaging for diagnosing acute viral infections, such as influenza and pneumonia. Most recently, imaging techniques were used in China and Italy to test for COVID-19—in fact, for a few weeks in China, the criteria for health organizations even broadened the standard definition to allow for CT scans as the primary diagnostic tool for COVID-19 pneumonia, a condition that can occur when COVID-19 spreads to the lungs.
For both countries, there was a high prevalence of COVID-19 in the community combined with an acute shortage of PCR tests. Due to these circumstances, researchers suggested that chest CT scans could supplement, or even presumptively diagnose, COVID-19 pneumonia.
"Almost overnight, it increased the number of cases that were being reported as positive. The barrier to testing prior to CT scans was the lack of availability of the PCR laboratory testing," Dr. Little says. However, CT scans as a primary method of diagnosing COVID-19 are not being performed widely in the United States today due to a few key flaws as well as availability of PCR tests.
When using CT scans as a diagnostic tool, almost any abnormal result can be interpreted to indicate COVID-19 infection, despite many other conditions appearing similarly on image as COVID-19.
"The bottom line is that when PCR testing is available, imaging is not recommended as a primary method of diagnosing COVID-19," says Dr. Little. "But it is a valuable tool for troubleshooting."
When CT Scans for COVID-19 Make Sense
According to Dr. Little, even when PCR tests are available, there are circumstances when imaging tests are helpful for COVID-19:
- CT can be a problem-solving tool for detecting suspected cases that do not show up on PCR tests. At Mass General, some symptomatic patients who tested negative for COVID-19 via first swab tests have had the infection revealed in a CT scan
- Imaging tests can further the understanding of complications in known cases of COVID-19, such as revealing the severity of illness, checking for other infections, revealing blood clots in the lungs (pulmonary emboli) and demonstrating the effects of heart failure
"We are still doing CT scans as part of our standard treatment plans for non-COVID-19 patients who have other illnesses," says Dr. Little. "We have seen several cases of unsuspected COVID-19 with abnormal imaging that suggested further testing and later turned out to be positive for the coronavirus infection."
Implications for the Future
Radiology researchers are actively investigating the possible long-term effects of COVID-19, such as possible scarring in the lungs, and imaging research can help identify how COVID-19 affects the blood vessels in the lungs—ultimately explaining why patients are low on oxygen.
"We are realizing that COVID-19 is a multi-system disease that can affect many different parts of the body, including the heart, blood vessels, the brain and other organs such as the liver and the kidneys," says Dr. Little.
Studies on CT scans could help the medical community understand and fight the potential complications that manifest in those who were infected.