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The COVID-19 pandemic has brought radiologists' penchant for descriptive terms front-and-center, with frequent references to one feature in particular: ground-glass opacities.

The term refers to the hazy, white-flecked pattern seen on lung CT scans, indicative of increased density. It's not quite as dense as the "" pattern, which looks like a mosaic or pavers, and less confounding than the "," a juxtaposition of three or more densities present in the same lung.

Essentially, a ground-glass opacity describes the "" in between a normal lung scan and one from an extremely diseased lung that shows up nearly all white because it's full of puss or fluid, said Henry Guo, MD, PhD, of Stanford University in Palo Alto, California.

The term has its origins in the way old movies shot their flashback scenes, through a ground-glass lens that gave the film a hazy appearance, Guo said. Chest radiologists adopted it in the 1980s, with a first appearance in the in 1984.

"We see [ground-glass opacities] so often in chest imaging," Guo told 乌鸦传媒. They come in different shapes, sizes, quantities, and locations, and they can indicate many different underlying pathologies -- including other viral infections, chronic lung disease, fibrosis, other inflammatory conditions, and cancers.

"Clinical context matters for interpreting these findings," said Javad Azadi, MD, of Johns Hopkins University in Baltimore.

Ground-glass opacities aren't likely to be found in healthy lungs, though, and wouldn't result from exposures like air pollution or smoking, radiologists said.

"It's safe to say that if you are a healthy person, you shouldn't have ground-glass opacities," said Paras Lakhani, MD, of Thomas Jefferson University in Philadelphia, speaking on behalf of the Radiological Society of North America (RSNA).

Lakhani noted that a found ground-glass opacities were present in 15% -- but these are patients "who have or are more likely to have cancer so this data isn't reflective of the prevalence in the general population."

Given how common ground-glass opacities can be in various diseases, researchers have taken a hard look at their clinical features in COVID-19.

Adam Bernheim, MD, of the Mount Sinai system in New York City, authored . He and his colleagues studied images captured from 121 patients at four centers in China mostly in late January.

"There are a lot of diseases that can cause ground-glass opacities, but in COVID-19, there's a distinct distribution, a preference for certain parts of the lung," chiefly in the lower lobes and periphery, and it appears multifocally and bilaterally, Bernheim said.

COVID-related ground-glass opacities also have a very round shape that's "really unusual compared with other ground-glass opacities," he said.

"Influenza or other pneumonias don't often have that pattern," he said. "Patients with flu can have ground-glass opacities, but they won't be in that distribution. It would be more in the middle parts of the lungs. And flu doesn't have these round, circular shapes."

Soon after their study published, New York's coronavirus case count started to rise, and Bernheim's team saw those features borne out in their own practice.

"Before long, we had cases here in our hospital in New York, and it turned out to be the same pattern," he said. "It substantiated and confirmed what we were seeing in the Chinese patients."

They also had many cases where COVID-19 wasn't initially suspected but was detected incidentally. For instance, patients who had gastrointestinal issues but no respiratory symptoms were sent for abdominal CT, which catches the bottom of the lungs. "In many cases we saw ground-glass opacities" and those patients were subsequently diagnosed with COVID-19, he said.

Yet chest CT has not panned out as an official diagnostic tool for COVID-19.

Matthew Cham, MD, of the University of Washington in Seattle, said there was initial hope as one study determined that CT can show ground-glass opacities during the first few days of COVID infection, when PCR testing may be especially susceptible to false negatives.

"This initial observation led to substantial excitement around the potential usefulness of chest CTs for the early detection of COVID-19," Cham told 乌鸦传媒.

Subsequent studies, however, showed some COVID patients have normal chest CTs with no ground-glass opacities during the first few days of the infection, Cham said.

No radiological society has recommended diagnostic chest CT for COVID-19. Instead, RSNA, the American College of Radiology, and the Society of Thoracic Radiology to help radiologists recognize COVID-19 pneumonia and communicate their findings to others.

Chest CT is still useful in detecting additional bacterial infections that appear on top of COVID-19, and will probably play a more important role in managing patients who have residual scarring after recovering from COVID-19, Bernheim said.

"The role of chest CT in COVID-19 is evolving," he said, "and it can be useful in providing a lot of helpful information for diagnosing and managing patients."

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