The new study was led by Dr. Robert Stern of Boston University, which thus far has the largest collection of donated brains from former pro football players. He led a coalition of investigators at multiple centers who took brain images from 26 former pro players, aged 40 to 69, who had a variety of memory, mood and mental problems associated with C.T.E.

Those images showed marked elevation of tau proteins in the areas of the brain that display the tau signature when diagnosed post-mortem. The players’ tau signal in those areas was higher, on average, than the tau signal from a control group of men who had not played.

“We found, as well, that the amount of abnormal tau detected in these PET scans was associated with the number of years playing football,” Dr. Stern said.

His collaborators included brain scientists from the Mayo Clinic Arizona, Brigham and Women’s Hospital and Avid Radiopharmaceuticals. Avid makes a molecule, called a ligand, that binds to proteins, in this case in the brain. Avid’s ligand is the most studied of the so-called tau detectors, and the company helped finance the study.

Experts said the findings were encouraging, because any reliable marker for abnormal tau accumulation would allow doctors not only to identify people with C.T.E., but also to monitor progress from potential drug treatments.

But, these experts said, much more work is required to develop a reliable test for a disorder that is still not well understood. As in tests for people suffering from Alzheimer’s and other diseases that affect the brain, researchers have spent years trying to precisely refine the ligands that are ingested by patients before they receive PET scans and other imaging tests.

There are also many open questions about the tau protein that is a signature of C.T.E. Researchers are trying to determine whether the protein, which occurs naturally in the brain, accumulates faster in people who have received repeated head trauma, and how those accumulating levels are related to behaviors associated with C.T.E., which include not only memory deficits but also impulse control issues and symptoms of depression.