Characteristics of positron emitters:
We can see from the above table that radioisotopes used in tomography have particularly short half-lives. Some are so brief that the products must be used in the same place as their manufacture, with only fluorine 18 lasting long enough for transportation to be worthwhile. The gamma photons which form the image are created after the emitted positrons travel a few millimetres in the body. The shorter the distance travelled, the more accurate the final image.
IN2P3