Technetium-99m
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Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer. 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 6.01 hours (meaning that about fifteen sixteenths (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 Tc-99 (a less excited state of the same isotope) by rearrangement of nucleons in its nucleus. Technetium-99 is an isotope which emits soft beta rays but no gamma rays.
Due to its short half-life, technetium-99m for nuclear medicine purposes is usually extracted from technetium-99m generators which contain 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 from only four reactors around the world: NRU, Canada; BR2, Belgium; SAFARI-1, South Africa; and HFR, the Netherlands. Production from LEU is possible, and is proposed at the new OPAL reactor, Australia, as well as other sites. Activation of Mo-98 is another, currently smaller, route of production.[1]
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[edit] Technetium-99m in Nuclear Medicine
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 Tc-99m is tagged (or bound to) a pharmaceutical that transports the Tc-99m to its required location. For example, when Tc-99m is chemically bound to Exametazime, the drug is able to be cross the blood brain barrier and flow through the vessels in the brain to see cerebral blood flow (it is also used for labeling white blood cells to visualize sites of infection. Tc-99m Sestamibi is used for myocardial perfusion imaging (which shows how well the blood flows through the heart). Measurements of renal function and imaging is undertaken by tagged to Mercapto Acetyl Tri Glycine, known as a MAG3 scan.
Technetium-99m is made 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 Tc-99m, a negative beta, and an antineutrino (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 betas produced are easily absorbed, and Mo-99 generators are only minor radiation hazards, mostly due to secondary X-rays produced by the betas (also known as bremsstrahlung).
- 99Mo (Negative Beta Decay) → 99mTc + β- + ν-
Where β- = a negative beta particle (electron), and ν- = an antineutrino.
99mTc will then undergo an isomeric transition to yield 99Tc and a monoenergetic gamma emission.
- 99mTc → 99Tc + γ
When a hospital receives molybdenum-99 generator, the technetium-99m from within can be easily chemically extracted. That same molybdenum-99 generator (holding only a few micrograms) can potentially diagnose ten thousand patients because it will be producing technetium-99m, strongly for over a week. The radioisotope is perfect for medicinal purposes. The short half life of the isotope allows for scanning procedures which 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 and substantially reduce the chance of ionization.
[edit] Technetium-99m in SPECT
Single photon emission computed tomography known as 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 calculates 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. Although Tc-99m is used for diagnostic nuclear medicine imaging procedures, it is not used for any therapeutic procedures, such as treating carcinomas.
