Dave Duerson and Junior Seau are former NFL players who committed suicide.
After their deaths autopsies revealed that they suffered from chronic traumatic encephalopathy.
CTE can lead to dementia, memory loss and depression.
Both men reportedly struggled in later life with symptoms so severe they took their own lives.
Until now CTE was only diagnosed through autopsy and that may one day change.
Researchers at UCLA and the NorthShore Neurological Institute in Evanston recruited five NFL players aged 45 to 73 for a study.
Each player had suffered at least one concussion and some were experiencing cognitive or mood symptoms.
Former NFL player Wayne Clark participated in the study and UCLA provided his comments.
"When I first saw the scan I thought 'whoa', that looks pretty extensive," Clark said.
Repeated hits to the head may cause impairment including CTE. It's a condition associated with the buildup of abnormal tau proteins in the brain.
In this preliminary study, those proteins were spotted in the brains of living former professional football players.
"This is the holy grail of this scientific effort; to be able to make a diagnosis while someone is still alive and perhaps intervene," the Director of NorthShore Neurological Institute Dr. Julian Bailes said.
Researchers used a "pet scan" coupled with a special injected marker that binds to these defective proteins, which are also associated with Alzheimer's Disease.
Early detection may help physicians understand what is happening sooner in the brain ultimately leading to treatments, according to Bailes.
"This is the tip of the iceberg," Bailes said. "We are getting for the first time, a glimpse that maybe we can label it and identify it in live people but many answers are yet to come."
For Clark, it's a mixed result.
His scans show the troublesome protein but researchers said he still has normal cognitive function.
"I go through stages where I think, 'Oooh how come I can't remember that? And I'm always wondering are these age related or concussion related,'" Clark said.