Random fun fact: They don't want you to think too hard while waiting for your PET scan in case it makes your brain glow.
(Some technical liberties may have been taken with this explanation.)
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I never really understood why: signal strength of brain compared to everything else is not used diagnostically. I don't think PET is so close to the acceptable SNR boundary that the amount of "shiny glucose" this takes away from everywhere else matters (or am I wrong?). Or does this introduce variations _within_ the brain that might be confusing?
Also, _good luck_ getting some people not to think too hard :)
@robryk No idea, it's just what I was told! My guess is it's harder to spot weird stuff in an active brain vs. a chilled one? (Checking the brain in this scan wasn't a primary goal, just a nice extra.)