Luciano Medrano, Facebook asked:
In PET scans, you get these positrons as we heard are from radioactive decays. How do you make sure that they annihilate the electrons in your body and not in the stuff that's around your body and therefore, create false signals?”
We posed this question to Professor Andy Parker from Cambridge University ...
Andy - Okay, well they're created at very low energy because they come out of a nuclear source with not a lot of energy intrinsically, and so they stop rather quickly in the body and they annihilate very close to the site where they are made. There’s a lot of matter in their way and they're not moving fast enough to get through it