Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer, i.e., it does not change into another element (transmute) upon its "decay". It is a gamma ray emitting isotope used in radioactive isotope medical tests, for example as a radioactive tracer that medical equipment can detect in the body. It is well suited to the role because it emits readily detectable 140 keV gamma rays (these are about the same wavelength emitted by conventional X-ray diagnostic equipment), and its half-life for gamma emission is 6.0058 hours (meaning that 93.7% of it decays to 99Tc in 24 hours). The short half life of the isotope allows for scanning procedures which collect data rapidly, but keep total patient radiation exposure low. For a full discussion of its uses in nuclear medicine, see the article on technetium.
Technetium-99m decays to technetium-99 (Tc-99, the ground state of the same isotope) by rearrangement of nucleons in its nucleus. Technetium-99 emits soft beta rays (electrons) but no gamma rays (photons).
Due to its short half life, technetium-99m for nuclear medicine purposes is usually extracted from technetium-99m generators which contain molybdenum-99 (Mo-99), which is the usual parent nuclide for this isotope. The majority of Mo-99 produced for Tc-99m medical use comes from fission of HEU (highly enriched uranium) from only five reactors around the world: NRU, Canada; BR2, Belgium; SAFARI-1, South Africa; HFR (Petten), the Netherlands; and the OSIRIS reactor in Saclay, France. [1] Production from LEU is possible, and is produced at the new OPAL reactor, Australia, as well as other sites. Activation of Mo-98 is another, currently smaller, route of production.[2]
A global shortage of technetium-99m is emerging because two aging nuclear reactors (NRU and HFR) that provide about two-thirds of the world’s supply of Mo-99, are currently shut down for repairs. [3]
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Uses
Nuclear medicine
Technetium-99m or 99mTc ("m" indicates that this is a metastable nuclear isomer) is used in radioactive isotope medical tests: for example, as a radioactive tracer that medical equipment can detect in the human body.[4] It is well-suited to the role because it emits readily detectable 140 keV gamma rays, and its half-life is only about six hours. It dissolves in aqua regia, nitric acid, and concentrated sulfuric acid, but it is not soluble in hydrochloric acid of any strength.[5] Claus Schwochau's book Technetium lists 31 radiopharmaceuticals based on 99mTc for imaging and functional studies of the brain, myocardium, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood, and tumors.[6]
Technetium-99m is used in 20 million diagnostic nuclear medical procedures every year. Approximately 85 percent of diagnostic imaging procedures in nuclear medicine use this isotope. Depending on the type of nuclear medicine procedure, the 99mTc is tagged (or bound to) a pharmaceutical that transports the Tc-99m to its required location. For example, when 99mTc is chemically bound to exametazime, the drug is able to cross the blood-brain barrier and flow through the vessels in the brain for cerebral blood flow imaging. This combination is also used for labeling white blood cells to visualize sites of infection. 99mTc Sestamibi is used for myocardial perfusion imaging, which shows how well the blood flows through the heart. Imaging to measure renal function is done by attaching 99mTc to Mercapto Acetyl Tri Glycine; this procedure is known as a MAG3 scan.
Technetium-99m is produced from the synthetic substance molybdenum-99 which is a by-product of nuclear fission. It is because of its parent nuclide that technetium-99m is so suitable to modern medicine. Molybdenum-99 has a half-life of approximately 66 hours, and decays to 99mTc through beta decay, emitting an electron and an antineutrino in the process (see equation below). This is a useful life since, once this product (molybdenum-99) is created, it can be transported to any hospital in the world and would still be producing technetium-99m for the next week. The electrons produced are easily absorbed, and Mo-99 generators are only minor radiation hazards, mostly due to secondary X-rays produced by the electrons (also known as bremsstrahlung).
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 99Mo 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 dissolved sodium salt of the pertechnetate.
The decay process that produces 99mTc:
- 99Mo → 99mTc + e− + νe
can also be written as
- 99Mo → 99mTc + β− + νe
where e− (or β−) denotes the electron (beta particle) emitted from the nucleus, and νe denotes the emitted antineutrino (or more specifically, an electron antineutrino).
99mTc will then undergo an isomeric transition to yield 99Tc and a monoenergetic gamma emission:
- 99mTc → 99Tc + γ
When a hospital receives a technetium-99m generator containing molybdenum-99, the technetium-99m that forms through 99Mo decay can be chemically extracted easily. One technetium-99m generator generator, holding only a few micrograms of 99Mo, can potentially diagnose ten thousand patients because it will be producing 99mTc strongly for over a week. The short half life of the isotope allows for scanning procedures that collect data rapidly. The isotope is also of a very low energy level for a gamma emitter. Its ~140 keV of energy make its use very safe because of the substantially-reduced chance of ionization.
Immunoscintigraphy incorporates 99mTc into a monoclonal antibody, an immune system protein, capable of binding to cancer cells. A few hours after injection, medical equipment is used to detect the gamma rays emitted by the 99mTc; higher concentrations indicate where the tumor is. This technique is particularly useful for detecting hard-to-find cancers, such as those affecting the intestine. These modified antibodies are sold by the German company Hoechst (now part of Sanofi-Aventis) under the name "Scintium".[4]
When 99mTc is combined with a tin compound it binds to red blood cells and can therefore be used to map circulatory system disorders. It is commonly used to detect gastrointestinal bleeding sites. A pyrophosphate ion with 99mTc adheres to calcium deposits in damaged heart muscle, making it useful to gauge damage after a heart attack.[7] The sulfur colloid of 99mTc is scavenged by the spleen, making it possible to image the structure of the spleen.[5]
Contamination and elimination
Radiation exposure due to diagnostic treatment involving 99mTc can be kept low. Because 99mTc has a short half-life and emits primarily a gamma ray, allowing small amounts to be easily detected, its quick decay into the far-less radioactive 99Tc results in a relatively low total radiation dose to the patient per unit of initial activity after administration. In the form (usually pertechnetate) administered in medical tests, both isotopes are quickly eliminated from the body, generally within a few days.[7] Technetium used in nuclear medicine is usually extracted from technetium-99m generators, because of its short 6 hour half-life.[8]
SPECT
Single photon emission computed tomography (SPECT) is a nuclear medicine imaging technique using gamma rays. In the use of technetium-99m, the radioisotope is administered to the patient and the escaping gamma rays are incident upon a gamma camera which computes and processes the image. To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3-6 degrees. In most cases, a full 360 degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15–20 seconds is typical. This gives a total scan time of 15–20 minutes. The technetium-99m radioisotope is used predominantly in both bone and brain scans to check for any irregularities.[clarification needed] Although Tc-99m is used for diagnostic nuclear medicine imaging procedures, it is not used for any therapeutic procedures.
References
- ^ [1]
- ^ Our Work: Nuclear Fuel Cycle and Materials Section
- ^ http://www.nytimes.com/2009/07/24/science/24isotope.html
- ^ a b John Emsley (2001). Nature's Building Blocks: An A-Z Guide to the Elements. New York: Oxford University Press. ISBN 0-19-850340-7.
- ^ a b S. J. Rimshaw (1968). Cifford A. Hampel. ed. The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corporation. pp. 689-693.
- ^ Schwochau, p.414
- ^ a b Joseph F. Smith. "[http://www.chclibrary.org/micromed/00067370.html Technetium heart scan]". http://www.chclibrary.org/micromed/00067370.html. Retrieved 2009-05-05.
- ^ Dilworth, Jonathan R.; Parrott, Suzanne J. (1998). "The biomedical chemistry of technetium and rhenium". Chemical Society Reviews 27: 43–55. doi:.
- Schwochau, Klaus (2000). Technetium. Wiley-VCH. ISBN 3-527-29496-1.
See also
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