In December 2019, a new form of coronavirus, SARS-CoV-2, infected humans in Wuhan in the Chinese province of Hubei. The respiratory disease named COVID-19 has become a pandemic that is now spreading very rapidly. Imaging procedures are crucial in the COVID-19 pandemic when it comes to assessing suspected cases and the course of the disease. On the basis of the latest scientific evidence we consider the role of X-ray imaging in the current situation.
Photos: Image courtesy of Medizinisches Versorgungszentrum Prof. Dr. Uhlenbrock & Partner, Dortmund, Germany
The primary test for diagnosing infection with SARS-CoV-2 is a real-time polymerase chain reaction (RT-PCR) assay of throat swabs or sputum [1]. However, since incubation periods of up to 14 days are possible [2], in the early stage the possibility of negative RT-PCR findings must be taken into account. In the first few days after the onset of symptoms, computer tomography (CT) can confirm suspected cases and facilitate the prognosis of severe cases [3, 4]. CT captures changes in the lungs of COVID-19 patients at a high rate. This sensitivity can reach 97 percent [5].
What can X-ray imaging do?
Studies of chest X-ray examinations found a lower sensitivity for COVID-19-related lung shadowing of 25 to 69 percent [6, 7]. On the other hand, the ability to identify the disease correctly – the specificity – can be 90 percent [6]. In the studies, all cases of COVID-19 were confirmed with RT-PCR. The low number of participants (17 in [6] and 64 in [7]) might have contributed to the discrepancies in sensitivity.
An important factor in the reliability of X-ray findings could be the time elapsing between the appearance of initial symptoms and the imaging procedure. While no signs of the disease were yet visible in the X-rays within the first three days after the onset of coughing and fever, they were most obvious after 10 to 12 days [7]. An Italian study with 72 symptomatic patients published in mid-April 2020 seems to confirm this time factor. At the time the imaging procedure was carried out, all patients were already under quarantine at home, and came to hospital because their symptoms worsened. The sensitivity of the chest X-ray was 69 percent (no details of specificity were provided) [8].
In a webinar published by Siemens Healthineers on 13 April, Stuart Cohen, MD, a radiologist at Northwell Health (New York City, USA), pointed out that X-ray findings for the lungs should also be evaluated in conjunction with the local prevalence of SARS-CoV-2 and the patient’s probable risk of exposure.
Characteristic signs
Even though the numbers of cases covered by individual COVID-19 X-ray studies are comparatively low, a characteristic set of findings is coming together [6, 8, 9]:
- The most common changes in the lung include
- consolidation, in other words accumulations of fluid and/or tissue in pulmonary alveoli preventing gas exchange,
- ground glass opacity, and
- nodular shadowing.
- They primarily affect peripheral and lower areas of the lungs.
X-ray imaging in current recommendations
Medical societies and expert bodies are trying to provide guidance in light of the volatile data situation. Given this data situation, they primarily address CT. In an expert consensus statement, the Radiological Society of North America (RSNA) emphasizes that a screening CT to diagnose or exclude COVID-19 is currently not recommended [10]. In its statement, the Fleischner Society confirms that chest X-rays are insensitive in the early stages of the disease. However, if quarantined patients whose symptoms are already advanced are examined, X-ray imaging often reveals changes in the lungs. In the Fleischner Society’s view, chest X-rays might be appropriate for patients already receiving inpatient care to assess the course of the disease and evaluate pneumonia due to other causes [11]. In addition to this, the European Society of Radiology (ESR) and the European Society of Thoracic Imaging (ESTI) recommend the use of X-ray imaging primarily for COVID-19 patients in intensive care who are not stable enough to be taken for a CT scan [12].
There are already also consensus recommendations for imaging for children with COVID-19 [13]. Under these recommendations, X-ray examinations can be considered if a child suspected with COVID-19 has moderate to severe symptoms of acute respiratory disease. If the initial chest X-ray yields concrete signs of COVID-19, repeated X-ray examinations might be appropriate to monitor the course of the disease. According to the recommendations, that would also be justified if the patient’s state of health deteriorates.
Using X-ray imaging
Contamination of equipment must be systematically avoided, especially in the current situation. X-ray systems are easier to disinfect than CT equipment. In patients with pronounced symptoms, images can be assessed in 2D for triaging those with COVID-19. This would free up resources for CT scans. Since a chest X-ray can be done with the patient either standing or lying down, it could be used to examine them in bed in a room specifically assigned for that purpose.
Mobile X-ray equipment has the additional advantage that it can be taken directly to the patient’s bedside. As the Fleischner Society summarizes, the risk of transmitting COVID-19 that exists while a patient is on the way to CT is thereby effectively ruled out [11]. If possible, mobile systems are recommended, particularly to reduce the challenges of decontaminating equipment [14].
About the Author
Matthias Manych, a biologist based in Berlin, works as a freelance scientific journalist, editor, and author specializing in medicine. His texts appear primarily in specialized journals, but also in newspapers and online.