Radionuclide is appropriate when several radioactive elements are involved.Most clinical diagnostics procedures used photons of some type usually gamma rays. Gamma rays with energies above 100 keV can penetrate many centimeters in the tissue.

Metastable Radionuclides :It is means half stable ,like 99m Tc which is one of man –made radionuclides that emits type of radiation not emitted by natural radioactive substance .A metastable radionuclide decays by emitting gamma ray only and the daughter nucleus differs from its parent only in having less energy. For example 99m Tc decays to form 99 Tc by emitting gamma ray of 140 keV .This is a very useful energy for nuclear medicine applications since it is penetrating enough to get out of the body easily and it is easy to shield with a few mm of lead. When a metastable radionuclide is used internally the absence of beta rays greatly reduces the radiation dose to the patient.

Types of disintegrations in man –made radionuclide :

1.The emission of positive beta (+β) or positron: this will lead to annihilation radiation ,It is accrue when the emitted positron annihilate with an electron ,energy equivalent to their masses (511keV each) is usually emitted this annihilation radiation is useful for mapping the distribution of positron emitters in the body.

2. Electron capture: The nucleus can capture one of its own electrons (usually K) which then complain with proton in the nucleus to form another neutron and thus reduces the charge on the nucleus for example the decay of 125I ,a characteristic (kα or kβ) x-ray is always emitted after k capture.
After ¹²⁵I   has swallowed one of its own electron to become 125Xe the nucleus has 35 keV  of energy which is some times emitted as 35 keV gamma ray ,or the nucleus transfers this energy directly to the remaining K electron which then uses the energy to escape leaving another vacancy in the K shell, This is called internal conversion or isomeric transition(IT).