Technetium-99m

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.

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. 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 beta, and a neutrino (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.

99Mo →    99mTc +  β +  ν

When a hospital receives a bottle of molybdenum-99, the technetium-99m from within can be easily chemically extracted. That same bottle of molybdenum-99 (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. If the patient weighs 80 kilograms the absorbed dose is: 140,000 eV   x    (1.6 x 10^ -16) =  2.24 x 10^ -11 J (2.24 x 10^ -11) / (80 kg) =  2.8 x 10^ -13 Gy For this absorbed dose, the dose equivalent is: 2.8 x 10^ -13   x    1(quality factor) = 2.8 x 10^ -13 Sv These equations prove that the level of radiation the patient is exposed to poses minimal threat and risk.

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. If necessary the same radioisotope can be used in larger amounts for treating tumors and cancers as well.

99mTc →  99Tc  +  γ

This is the equation which shows the radioactive decay of technetium-99m into technetium-99 by the gamma decay process.