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Message: <blockquote data-quote="freyar" data-source="post: 6868682" data-attributes="member: 40227">As in PET scans, the positrons (anti-electrons) we'd normally encounter are produced as a product of nuclear decay; large particle accelerator experiments also produce them a lot, though not necessarily from photon-photon interactions.  The electron/positron annihilation into 2 gamma rays is precisely the reverse process of two photons hitting each other to form an electron/positron pair.I agree with you and your medical professional that you don't want a PET scan if you don't medically need one.  Compared to an X-ray, you get a high dose of radiation from them. But I do think it's worth noting that the scary estimates of how likely that dose of radiation is to cause a future medical problem, like cancer, are based on extrapolation from extremely high doses of radiation (like people who were exposed to nuclear accidents, etc).  There's no direct evidence that PET or CT scans cause cancers --- in fact, one study I saw on the BBC news page a while back concluded that CT scans don't increase the risk of cancer at all. So, while I think best medical practice is to avoid unnecessary testing and unnecessary risks, I wouldn't be scared of getting a PET scan if the doctors think it would be helpful for treatment.</blockquote>