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...
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...