What are the Chemists called who are synthesizing radioactive isotopes for PET scans for Cancer Patients?

Given the extremely short half life of most of these isotopes they must be produced as needed in close proximity to the testing facilities. Traditionally this is done by nuclear pharmacists. I think you are thinking about the chemists who investigate medical applications for different radioisotopes, the subtype of chemistry involved would be nuclear chemistry, but they would almost certainly work in conjunction with other scientists that specialize in biology, oncology, etc.