Radioisotopes often emit one or more types of radiation, including alpha particles, beta particles, and gamma rays. The distance the radiation travels through matter before being absorbed increases in order from alpha to gamma. Which statement describes the best radioisotope that can be used for radioisotope imaging and for radionuclide therapy?

For imaging, radiation must escape the body and be detected, so gamma rays are useful because they travel far through matter. For therapy, beta particles are useful because they deposit energy in nearby tissue and can damage targeted cells. A radioisotope that emits both beta particles and gamma rays can therefore serve both purposes; the best choice is “It emits beta particles and gamma rays.”

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