Technetium-99m

Overview

Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as ^99mTc. The "m" indicates that this is a metastable nuclear isomer.

Historical Perspective

• In 1938 Emilio Segrè and Glenn T. Seaborg isolated for the first time the metastable isotope technetium-99m, after bombarding natural molybdenum with 8 MeV deuterons in the 37 inches (940 mm) cyclotron of Ernest Orlando Lawrence's Radiation laboratory.
• in 1940, Emilio Segrè and Chien-Shiung Wu published the experimental results of the analysis of fission products of U-235, among which Mo99 and detected the 6 h activity of element 43, later labelled as Tc-99m.
• In 1950s when Powell Richards realized in the potential of technetium-99m as a medical radiotracer and promoted its use among the medical community.

Nuclear Properties

• It is an isotope that emit gamma ray. It is used in medical tests that involves radioactive isotope. Example: As a radioactive tracer that medical equipment (gamma camera) can detect in the body.
• 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.
• Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies.

Rationale for its use

• 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).
• 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.
  • 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.
• 99mTc, when chemically bound to exametazime, 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.

Benefits over other isotopes

• Technetium-99m has several features that make it safer than other possible isotopes.
• Its gamma decay mode can be easily detected by a camera, allowing the use of smaller quantities.

• Due to a short half-life, it decays quickly into the far less radioactive technetium-99 resulting in relatively low total radiation dose to the patient per unit of initial activity after administration, as compared to other radioisotopes.
• In the form administered in these medical tests (usually pertechnetate), technetium-99m and technetium-99 are eliminated from the body within a few days.

Technetium-99m in Nuclear Medicine

• Each year, more than 20 million Americans are benefited from nuclear medicine procedures, which are used to diagnose and treat a wide variety of diseases. 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.

Procedure

• 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.

Mechanism

The following equation shows the radioactive decay of technetium-99m into technetium-99 by the gamma decay process.

^99mTc → ⁹⁹Tc + γ

Radiation side-effects

• Diagnostic treatment involving technetium-99m will result in radiation exposure to technicians, patients, and passers-by.
• Typical quantities of technetium administered for immunoscintigraphy tests, such as SPECT tests, range from 400 to 1,100 MBq (11 to 30 mCi) for adults.
  • These doses result in radiation exposures to the patient around 10 mSv, (1000 mrem,) the equivalent of about 500 chest X-ray exposures.
  • This level of radiation exposure carries a 1 in 1000 lifetime risk of developing a solid cancer or leukemia in the patient.
  • The risk is higher in younger patients, and lower in older ones.
  • Unlike a chest x-ray, the radiation source is inside the patient and will be carried around for a few days, exposing others to second-hand radiation.
  • A spouse who stays constantly by the side of the patient through this time might receive one thousandth of patient's radiation dose this way.

See also

• Isotopes of technetium