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Design principles of the 99Mo → 99mTc radionuclide generator

The diagram illustrates the typical components found in a 99Mo → 99mTc radionuclide generator. The design of the individual components will vary by manufacturer but will always allow for the separation and elution of the daughter radionuclide 99mTc from the parent radionuclide 99Mo. The elution will result in a product that is sterile and free of impurities thus making it immediately suitable for human injection. The components in the typical generator are:

The column, usually glass, containing a bed of aluminium oxide (alumina) as a support for the parent radionuclide.99Mo (molybdate) will bind strongly to this support media and is not washed off during the subsequent elution of the daughter radionuclide 99mTc (pertechnetate).

A system of tubing that will allow the column to be washed with a sterile saline solution. The tubing will be medically approved since the liquids that contact this material may be injected into humans. The inlet tube will generally be accessible to the person who will be collecting the 99mTc from the generator, usually via a needle onto which a vial of saline can be attached as needed. Other designs provide an internal reservoir of saline for this purpose. The outlet tube is also accessible and generally terminates as a needle onto which an empty vial can be attached. Most generators use an evacuated vial for collection so that saline from the inlet side is drawn through the column and into the outlet vial.

Filters will be found in generators in the form of porous frits, which serve to contain the alumina within the column, and usually a 0.22 µm filter which serves to remove any small particles from the eluted sample and to act as a safety device to ensure a sterile product.

Lead shielding is required for operator safety. Both the parent and the daughter radionuclides emit radiation that must be absorbed. The glass column acts as a partial shield but can not stop the penetrating gamma radiation. All generators will provide a lead shield around the column and outlet tubing. Additional shielding is used during the collection process and of course the eluted 99mTc must be shielded once it is collected from the generator.

Generators are contained in a plastic housing and have some method (handles, straps) to allow manual or mechanical lifting and positioning. A generator with its lead shielding will weigh more than 10 kg depending on the manufacturer's design. There is a cover to protect the inlet and outlet needles and appropriate labelling on ther generator housing.

The generator described above is the one in general use and available from commercial suppliers. There have been various attempts to design other systems to produce 99mTc generators but technical and product issues have prevented their widespread application. (Link to Alternative 99Mo → 99mTc generator designs).