Radioactive decay is the change of instable atoms to a more stable state. This change to a different nuclide by the spontaneous emission of radiation such as alpha or beta particles, gamma rays, or by electron capture follows an element-specific decay chain. Each step in the decay chain has a definite half-life.
Sometimes also the reduction of excitation energy of the nucleus by e.g. internal conversion is mentioned as radioactive decay.

See also Decay Chain, Radioisotope.

[Radioactive Isotope] Radioisotopes are atoms with an unstable nucleus, which try to reach a more stable state by emitting radiation. Through one or more steps down its decay chain a more stable number of protons and neutrons is reached. Approximately 3,800 natural and artificial (fission, generator) radioisotopes exist. Radioisotopes are commonly used in science, industry, and nuclear medicine.

See also Isotope, Decay and Decay Chain.

(ROI) Keyboard controlled identification of a given area of an image for numerical analysis and the area of anatomy being scanned that is of particular importance in the image. This area can also be a volume, than called (VOI) volume of interest.
A ROI can be used for purposes such as calculation of pixel statistics. ROI measurements should be made with the smallest area and away from boarders and edges to avoid partial volume averaging.

The second is the SI unit of time. Definition: The length of time taken for 9192631770 periods of vibration of the caesium-133 atom.