Researchers may be closing in on a way to check athletes while they're alive for signs of a degenerative brain disease that's been linked to frequent head blows. Experimental scans found higher levels of an abnormal protein tied to the disease in a study of former National Football League players who were having mood and thinking problems.

It's the first time a major study has tested these scans for detecting chronic traumatic encephalopathy, or CTE, which is only diagnosed now after death, with brain autopsies.

Doctors are searching for a way to tell when players, veterans or others with conc...