- Some radioactive isotopes emit antimatter particles called positrons as they decay. When a positron meets an electron, they annihilate each other and their mass is converted to energy in the form of electromagnetic radiation, i.e., gamma rays. It follows from the law of conservation of momentum that these gamma rays must travel in opposite directions. This feature is very useful because it means that by rotating a scanner around the body, a doctor can locate and follow the radioactively tagged molecules inside the body.
- Radioactive isotopes remain radioactive and continue to decay at the same rate regardless of their physical or chemical states, so doctors can tag molecules with radioactive isotopes and see what happens to them inside the body. This information can help map metabolic activity. Glucose tagged with fluorine-18, for example, is taken up by the brain and crosses the blood/brain barrier. Therefore, a doctor or scientist can use it to monitor metabolic activity in different parts of the brain or in other areas of the body.
- Because tumors tend to have very high levels of metabolic activity, PET scans offer a way to image them. By tracking areas of high glucose uptake, doctors can monitor tumor shrinkage or growth. Labeled glucose is the most common way to do so, although other molecules such as labeled estrogen can be used for certain kinds of tumors. Doctors can also track metabolic activity in specific regions of the brain and correlate this with possible symptoms of mental disorders.
- Myocardial infusion imaging uses radioactive tracers to monitor heart health. Tracers are very useful for this purpose because they enable the doctor to track blood flow in heart muscle from various angles. Unlike PET, SPECT directly measures gamma radiation emitted by the decaying isotope. PET has higher resolution but SPECT can work with longer-lived isotopes. In both SPECT and PET, the tracers have a relatively short half-life, ensuring the patient is exposed to as little radiation as possible.
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