The most important example in nuclear medicine is the molybdenum-99 / technetium-99m generator, which provides on-site access to Tc-99m for SPECT imaging.

Generators are essential when the daughter radionuclide has a short half-life and cannot be easily transported from a production facility.

A radionuclide generator works on the principle of parent–daughter radioactive decay.

A long-lived parent radionuclide decays into a short-lived daughter radionuclide. If the parent remains physically fixed within a column, the daughter can accumulate over time. Because the parent and daughter are chemically different elements, they can be separated.

In a Tc-99m generator:

• The parent radionuclide is molybdenum-99 (half-life ~66 hours).

• Mo-99 decays to technetium-99m (half-life ~6 hours).

• The molybdenum is chemically bound to an alumina column.

• As Mo-99 decays, Tc-99m forms and accumulates on the column.

When saline is passed through the column (a process called elution), technetium-99m is washed off in solution while molybdenum remains bound to the column.

Because the parent has a longer half-life than the daughter, the generator can be eluted repeatedly. After elution, Tc-99m gradually builds up again as more Mo-99 decays.

This system allows hospitals to obtain fresh Tc-99m daily without needing an on-site reactor or cyclotron.