Well, I'm not a doctor, but I used Google. Apparently a TRACER is a substance, such as a dye or a radioactive isotope, that is introduced into and followed through a biological or chemical process, by virtue of its radioactive signature, color, or other distinguishing physical property, thus providing information on the course of the process or on the components or events involved. A RADIOTRACER is when you use a radioactive isotope as a tracer.
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