Each and every organ in our body acts differently. Certain tissues show a special attraction to specific chemical substances- Eg: Thyroid: Iodine, Brain: Glucose.
With this knowledge, it is possible to attach various radioisotopes to biologically active substances. Once a radioactive form of one of these substances enters the body, it is incorporated into the normal biological processes and excreted in the usual ways. A detector that traces the movement of this chemical through the human body can be used to obtain valuable information regarding the human body and itís functioning.