Technetium-99m generator

Five Technetium-99m-Generators

A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the metastable isotope ^99mTc of technetium from a source of decaying molybdenum-99. ⁹⁹Mo has a half-life of 66 hours^[1] and can be easily transported over long distances to hospitals where its decay product technetium-99m (with a half-life of only 6 hours, inconvenient for transport) is extracted and used for a variety of nuclear medicine diagnostic procedures, where its short half-life is very useful.

Parent isotope source

⁹⁹Mo can be obtained by the neutron activation (n,γ reaction) of ⁹⁸Mo in a high neutron flux reactor. However, the most frequently used method requires a uranium target with high enriched uranium-235 (up to 90% ²³⁵U) or low enriched uranium (less than 20% ²³⁵U).^[2] The target is irradiated with neutrons to form ⁹⁹Mo as a fission product [1]. Molybdenum-99 is then separated from other fission products in a hot cell.[2]

Generator function and mechanism

The half-life of the parent nuclide (⁹⁹Mo) is much longer than that of the daughter nuclide (^99mTc). 50% of equilibrium activity is reached within one daughter half-life, 75% within two daughter half-lives. Hence, removing the daughter nuclide (elution process) from the generator ("milking" the cow) is reasonably done as often as every 6 hours in a ⁹⁹Mo/^99mTc generator.

Most commercial ⁹⁹Mo/^99mTc generators use column chromatography, in which ⁹⁹Mo in the form of molybdate, MoO₄^2- is adsorbed onto acid alumina (Al₂O₃). When the Mo-99 decays it forms pertechnetate TcO₄^-, which because of its single charge is less tightly bound to the alumina. Pulling normal saline solution through the column of immobilized ⁹⁹Mo elutes the soluble ^99mTc, resulting in a saline solution containing the ^99mTc as the pertechnetate, with sodium as the counterbalancing cation.

The solution of sodium pertechnetate may then be added in an appropriate concentration to the organ-specific pharmaceutical to be used, or sodium pertechnetate can be used directly without pharmaceutical tagging for specific procedures requiring only the ^99mTcO₄^- as the primary radiopharmaceutical. A large percentage of the ^99mTc generated by a ⁹⁹Mo/^99mTc generator is produced in the first 3 parent half-lives, or approximately one week. Hence, clinical nuclear medicine units purchase at least one such generator per week or order several in a staggered fashion.

References

1. ^ R. Nave. "Technetium-99m". HyperPhysics (Georgia State University). http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/technetium.html. 
2. ^ Frank N. Von Hippel, Laura H. Kahn (December 2006). "Feasibility of Eliminating the Use of Highly Enriched Uranium in the Production of Medical Radioisotopes". Science & Global Security 14 (2 & 3): 151–162. doi:10.1080/08929880600993071. http://www.informaworld.com/smpp/content~content=a769414426~db=all. Retrieved 2010-03-26. 

External links

• Brookhaven National Laboratory site on the history of the technetium cow.