Indeed.. a PET scan is a normal procedure that can be used outside of cancer diagnoses as well. They use glucose for exactly the reason you mention, it's the preferred energy source of the brain in particular, and it shows all areas of a brain that are currently active.

It's useful in brain cancer scans because you can highlight and find areas of unusual activity.

Just like.. your bones and tissues show up on an X-Ray. Then we find things that don't look right in the picture, compared to a normal picture, and then say, there is your problem right there! PETs are no different in this context.