Answer :
Naturally occurring radionuclides are not used for nuclear medicine imaging because imaging because their typically long half lives or less than ideal physical or chemical characteristics.
What is radionuclides?
Radionuclide is an unstable form of a chemical element that releases radiation as it breaks down and becomes more stable. Radionuclides may occur in nature or be made in a laboratory. It has excess of nuclear energy, which makes it unstable. This excess energy can be used in three ways,
(a) Emitted from the nucleus as gamma radiation,
(b) Transferred to one of its electrons to release it as a conversion electron,
(c) Used to create and emit a new particle from the nucleus.
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