There are many! See the attached link to the Wikipedia article on radiopharmacology. Out of them all, I'd say my favorite pharmacological radioisotopes are 131I, 18F and 99-mTe, which are probably...
To use a radioactive isotope as a tracer, you simply introduce a compound containing it into the system you're looking at at the chosen starting point, then look for its radioactivity at the test...