Why is technetium-99m a good tag for medical imaging?
Technetium-99m is a good tag because it's a pure gamma emitter with a short half-life. This means that it doesn't produce more damaging alpha and beta particles and it's radioactivity disappears after a few hours.
What does the m mean?
M stands for metastable. In other words technetium-99m is more stable than you'd expect it to be.
Normally a gamma ray is emitted at almost exactly the same time as an alpha or beta particle. This means most substances aren't pure gamma emitters. However with technetium-99m there's a delay between it being created and giving out its gamma ray.
The technetium-99m decay chain
A molybdemum-99 nucleus decays into a technetium-99m nucleus by beta emission. After a period of a few hours or so the technetium-99m emits a gamma ray and changes into technetium-99.
You need a nuclear reactor to have technetium-99m
You need to keep getting technetium-99m from somewhere because it keeps decaying into technetium-99 regardless of whether you're using it.
Molybdenum-99 (the parent), like almost all useful radionuclides, is synthesized in a nuclear reactor. A thin sheet of uranium-235 is bombarded with neutrons. The uranium-235 nuclei split and form different isotopes. The sheet is processed chemically to separate out any molybdenum-99 present.
The molybdenum-99 needs to be packaged so hospitals can use it
Molybdenum tends to cling to the surfaces of powdered alumina so a cartridge of these two chemicals is made up for easy handling. The cartridge is placed in a lead-lined ‘technetium generator’ which is what the hospital receives. The whole process is carried out remotely in a special radiation-proof room called a ‘hot cell’.
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The molybdenum has a half-life of about 3 days so there’s enough time to transport the generator to the hospital.
The molybdenum-99 starts decaying into technetium-99m as soon as it’s produced and that in turn starts decaying into technetium-99. So after a while the cartridge will contain a mixture of molybdenum-99, technetium-99m and technetium-99. The technetium-99 is fairly stable so we don’t need to worry about injecting it into the patient.
When a doctor needs some technetium-99m he needs to separate it from the molybdenum. He sticks a vial onto the needle. The vial has had some of the air sucked out of it. Salt solution rushes in to equalize the pressure, picking up technetium as it passes. The molybdenum gets left behind on the alumina.
So the technetium ‘generator’ would be better called a ‘separator’.
The technetium-99m is then used to label whatever pharmaceutical is going to be injected into the patient. Each time some technetium is drawn off you need to wait a while for the molybdenum to decay into enough technetium to use again.