Technetium 99^m

Nuclear medicine is the branch of medicine that uses radioisotopes for research diagnosis and treatment of disease.

Since its humble beginnings in 1958 (see here for details of how it was discovered), technetium-99m has become the most widely used radioisotope for diagnosing diseased organs.

All isotopes of technetium are radioactive.Technetium-99m is a metastable isotope of the element technetium. It's short half life means that it is not found in any abundance naturally and therefore it has to be produced in a nuclear reactor. It is classed as a man-made or artificial isotope rather than naturally occurring. The name originates from the Greek word 'technetos' meaning artificial!

The common isotope of technetium is Tc-98, but it is the metastable isotope of Tc-99 that is most sought after - Tc-99^m.

The 'm' in its name stands for metastable. That means it has an excited nucleus. In order for that nucleus go to a more stable state it has to give out the excess energy, it does this by emitting a gamma ray.

Tc-99 ^m is ideal as a medical tracer because the gamma radiation it emits allows the medical practitioner to image internal body organs causing hardly any radiation damage to the patient.

Technetium-99m is used in over 20 million diagnostic nuclear medical procedures every year, half of which are bone scans, and the other half are roughly divided between kidney, heart and lung scans. Approximately 85% of diagnostic imaging procedures in nuclear medicine use this isotope.

Tc-99 ^m is an ideal marker radionuclide for imaging as it is:-

• a pure gamma-emitter,
• has a physical half-life of only 6 hours and
• emits g-rays of energy that it ideal for detection with a g-camera (140 keV).

Technetium 99^m is a very useful radionuclide in gamma imaging because it only emits gamma rays and these are of an energy that are easily detected by a gamma camera (140keV). It has an ideal half life (six hours) which is long enough for diagnostic procedures to take place but short enough for the patient not to be inconvenienced unduly by remaining ‘radioactive’ for too long a period after the investigation. It is suitable not only for use alone but also for attachment to a wide range of compounds for tracer experiments. It can easily be produced ‘in situ’ using a ‘cow’ as it is the daughter nucleus of the decay of Molybdenum 99 and is easily separated from the parent (more hazardous b-emitter) by a saline flush. Molybdenum-99 has a half-life of 66 hours, allowing it to be transported over fairly long distances. A fresh supply is brought to the hospital fortnightly and then the ‘cow’ is ‘milked’ as and when required.

When Technetium 99m is used as a radioactive tracer a 'butterfly' can be used to ensure that an injection of radioactive material is inserted into the correct site.

The radionuclide in the syringe is shielded to protect the person giving the injection.

The gamma ray output is then studied with a gamma camera. The operator has to take into account not only its 6 hour half life but also the rate at which it will be flushed biologically out of the body. This will depend upon the chemical compound to which it is attached and the way an individual’s metabolism copes with that compound. (See effective halflife).

Example of the use of Technetium 99^m

A Technetium-99^m - antimony sulphide colloid can be used as a radioactive marker to examine the function of the lymph nodes. It is injected into the drainage areas to be visualised. Cancerous cells divide more frequently than non-cancerous ones and therefore a 'hot-spot' of high activity results from cancerous growth.

• See the page on the Gamma Camera to learn how the gamma rays from the tracer are detected and used to diagnose medical conditions.
• See the page on Effective Half Life
• See the page on the development of the first Tc99^m generator