system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't they diagnose CTE in NFL players until AFTER they're dead?

CTE has been in the news alot over the last few years. There are several high profile football players who exhibited erratic and impulsive behavior only to find out after they've passed they had holes in their brain (Aaron Hernandez, Junior Seau, Vincent Jackson and it goes on). Some are saying the outburst by Anonio Brown this weekend is likely due to CTE...In looking at pictures of healthy brains vs ones w CTE via Google the differences are so stark it seems like something you should be able to diagnose when they are alive. So what gives?
gpt
The "pictures" you are looking at are slices of the brain stained with a contrast enhancing agent.  You can't slice someone's brain while they are alive, because the slicing itself would cut all the neural connections needed to keep the person thinking.  Tiny cuts in the brain were used in lobotomies, with severe consequences for the patient.