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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 an inconvenient half-life of only 6 hours for transport) is extracted and used for a variety of nuclear medicine diagnostic procedures, where its short half-life is very useful.

Mechanism

The half-life of the mother 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 every 6 hours or, at most, twice daily 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.

The pertechnetate may then be added to an appropriate concentration to the organ-specific pharmaceutical to be used, or 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.

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). The target is irradiated with neutrons to form ⁹⁹Mo as a fission product [1]. Molybendum-99 is then separated from other fission products in a hot cell.[2]

External links

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

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• This page was last modified on 9 September 2008, at 05:15.

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