In general, radiopharmacy is the approach to prepare compounds for direct application of the Tracer Principle on the in-vivo level both in experimental research and clinical diagnostics.
Due to the short half-lives, compounds labeled with PET radionuclides (“tracers”) have very low mass such as 10-6 g or even less as it is administered in a whole body dose. Therefore, those tracers allow to “trace” the biochemical processes of interest directly with organism without any alteration.