Armed with that definition and higher-resolution scans, Gandy was better able to tentatively diagnose the retired NFL player who had suffered 22 concussions, and who also showed neuropsychological testing deficits and diffuse brain damage through separate MRI scans. (The player discussed Gandy’s findings with The Atlantic, but requested anonymity because of the stigma associated with brain injury.) “With the better resolution in the [PET] scans, we could see the localization of the tau in exactly that spot, the bottom of the walls of the wrinkles,” Gandy says. “It looked very similar to what you would expect from a postmortem [CTE] brain.”

Since then, Gandy’s team has examined and scanned almost 30 combat veterans and former football players. Ten of those patients have clinical symptoms of CTE; of that group, eight have what Gandy describes as “clearly positive scans” for abnormal tau buildup, similar to that of the NFL retiree. The other two “may not have accumulated enough tau for us to see it yet,” Gandy says.

Before PET scans can be used to definitively diagnose CTE—or simply be used with a high degree of diagnostic confidence—more work needs to be done. A handful of cases are suggestive, but not conclusive; many more patients need to be scanned, evaluated by doctors, tracked over time, and eventually autopsied after death to validate the bright-red regions Gandy and his team are seeing on their computer screens. “That’s the gold standard,” Gandy says. “It will take a while to accumulate enough data to feel confident that a certain amount of signal represents a certain amount of tau.”

A number of tau tracers have been developed in recent years, and researchers are still figuring out which ones are the most useful for identifying various neurodegenerative diseases. Two years ago, researchers at the University of California, Los Angeles, published a brain-imaging study of 14 former football players using a tracer called FDDNP to identify what appeared to be CTE. But that same tracer also binds to amyloid, making it difficult to differentiate the disease from Alzheimer’s.

Gandy uses a compound called Flortaucipir, which binds much more readily to tau than to amyloid. But he cautions that it may not prove to be the best available imaging agent. “Most people who get the scans want a definitive result,” he says. “But we always tell them this is experimental, and we could find out something tomorrow that changes things. ”

Better answers may come from a recently announced seven-year, $16 million study funded by the National Institutes of Health and NINDS that’s aimed at diagnosing CTE—a project the NFL was also slated to fund before backing out amid controversy. Headed by Stern, the project is the largest and most thorough study of the disease ever conducted, and will put former football players through a series of tests including an MRI; two PET scans; blood, saliva, and spinal-fluid collection; genetic evaluations; neuropsychological testing; and clinical examinations and histories.