A radioactive tracer is a substance that can be easily detected with special instruments. It is used in medicine and research to see how things move or function inside the body. Full definition
Measuring the activity of transporter proteins in a living organism has been a challenge for scientists, because the methods are difficult, often require the use of radioactive tracers, and are hard to use in intact tissues and organs. (sciencedaily.com)
In the 1980s, positron emission tomography (PET) scans, which detect neural metabolism with radioactive tracers, snapped the first pictures of the brain in midthought. (discovermagazine.com)