Technetium-99m generator
A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the metastable isotope
Parent isotope source
99Mo can be obtained by the
Generator invention and history
99mTc remained a scientific curiosity until the 1950s when Powell Richards realized the potential of technetium-99m as a medical radiotracer and promoted its use among the medical community.
Generator function and mechanism
Technetium-99m's short half-life of 6 hours makes long-term storage impossible. Transport of 99mTc from the limited number of production sites to
Production
Generators provide radiation shielding for transport and to minimize the extraction work done at the medical facility. A typical dose rate at 1 metre from 99mTc generator is 20–50
These generators' output declines with time and must be replaced weekly, since the half-life of 99Mo is still only 66 hours. Since 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.[16]
Separation
Most commercial 99Mo/99mTc generators use column chromatography, in which 99Mo in the form of molybdate, MoO42− 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. Pouring normal saline solution through the column of immobilized 99Mo elutes the soluble 99mTc, resulting in a saline solution containing the 99mTc as pertechnetate, with sodium as the counterion.
The solution of sodium pertechnetate may then be added in an appropriate concentration to the pharmaceutical kit 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.[17]
Isomeric ratio
When the generator is left unused, 99Mo decays to 99mTc, which in turn decays to 99Tc. The half-life of 99Tc is far longer than its metastable isomer, so the ratio of 99Tc to 99mTc increases over time. Both isomers are carried out by the elution process and react equally well with the ligand, but the 99Tc is an impurity useless to imaging (and cannot be separated).
The generator is washed of 99Tc and 99mTc at the end of the manufacturing process of the generator, but the ratio of 99Tc to 99mTc then builds up again during transport or any other period when the generator is left unused. The first few elutions will have reduced effectiveness because of this high ratio.[18]
References
- ^ R. Nave. "Technetium-99m". HyperPhysics. Georgia State University.
- ^ The National Research Council. Medical Isotope Production Without Highly Enriched Uranium (Report). Retrieved 20 November 2012.
- ^ "Archived copy" (PDF). Archived from the original (PDF) on 24 May 2009. Retrieved 2 August 2008.
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: CS1 maint: archived copy as title (link) - OSTI 146775.
- ^ Gasparini, Allison (24 October 2018). "Celebrating the 60th Anniversary of Technetium-99m". Brookhaven National Laboratory.
- ^ "Brookhaven Graphite Research Reactor". bnl.gov. Archived from the original on 2 April 2013. Retrieved 3 May 2012.
- OSTI 5612212.
- ^ Tucker, W.D.; Greene, M.W.; Weiss, A.J.; Murrenhoff, A. (1958). "Methods of preparation of some carrier-free radioisotopes involving sorption on alumina". Transactions American Nuclear Society. 1: 160–161.
- ^ Richards, Powell (1960). "A survey of the production at Brookhaven National Laboratory of radioisotopes for medical research". VII Rassegna Internazionale Elettronica e Nucleare Roma: 223–244.
- ^ "The Technetium-99m Generator". Bnl.gov. Archived from the original on 2 April 2013.
- PMID 6759417.
- ISSN 0096-6207.
- doi:10.1039/a827043z.
- S2CID 21121327.
- ^ Shaw, Ken B. (Spring 1985). "Worker Exposures: How Much in the UK?" (PDF). IAEA Bulletin. Archived from the original (PDF) on 5 September 2011. Retrieved 19 May 2012.
- ISBN 9781451171396.
- ISBN 9783540006909.
- PMID 6100549. Retrieved 11 May 2012.