Technetium generator | Everything you need to know

Hi My name is Paulien, I’m a  Belgian nuclear medicine resident and this video is about the  production of technetium. This is not an easy topic, but I’ll  try to explain it as simple as I can. First, I will tell you why we need a generator  to produce technetium, then I will tell you what a generator is and finally, I’ll go through  the different steps of the production process.

Technetium is a radioisotope that has a  half-life of 6 hours. This means that half of the radioisotope has decayed after 6 hours. After  another 6 hours, the activity of technetium is 25% of the original activity or half of  the value after the first 6 hours.

After approximately 40 hours (or 7 half-lives), there is very little activity remaining  compared with the initial activity. Technetium is often the agent of choice in  nuclear medicine. Its short half-life, ease of incorporation into a variety of carrier molecules,  low energy gamma emission, and rapid excretion make it ideal for obtaining images of the major  internal organs and skeleton of the human body.

However, the time available  for processing, transportation, storage and dispensing is very limited  because of their fast decay. The use of a generator solves the problem associated with  the transportation of short-lived radioisotopes. Generators are devices that produce a  useful short-lived radioisotope (also known as ‘the daughter’) from the radioactive  transformation of a long-lived radioisotope (also known as ‘the parent’).

Molybdenum, the parent, has a half-life of 66 hours and can easily  be transported over long distances without serious loss of activity. Its  short half-life decay product, technetium, also called ‘the daughter’, can be extracted  and used at a remote facility. In other words, By having a supply of the parent at a facility,  the daughter can be continually generated on-site.

The generator is often referred to as a cow.  The word cow alludes to the fact that the device “milks” technetium from molybdenum, then waits  for a day and milks it again, just like a cow. The decay of Molybdenum is  shown by the upper curve.

The generator enables the separation of  the daughter radioisotope from the parent. This separation process is called an  elution (also referred to as a "milking"). Once separation occurs, the generator starts  generating more daughter that can be again separated in sufficient quantity at a later time.

Because the half-life of Mo-99 is 66 hours or slightly less than 3 days, the supply of parent  product will deplete to insufficient levels in roughly one week and must be  replaced with a fresh system. The generator system is a simple  apparatus composed of a column of alumina (Al2O3) on which the molybdenum (MoO42-)  is absorbed due to its very high affinity for alumina. Molybdenum decays to form  pertechnetate (TcO4-), which is less tightly bound to the column  because of its single charge.

Next, a normal saline solution is passed  through the column of the immobilized molybdenum and pertechnetate is eluted,.  This radioisotope is ready for injection or for the preparation  of other radiopharmaceuticals. After flushing, the concentration of  Tc-99m is largely depleted from the column but immediately starts to increase again. 

This increase of daughter activity continues but ultimately begins to slow. Eventually,  the daughter activity is produced at a rate that nearly equals that at which it decays,  the system is said to be in equilibrium. It takes about 4 daughter half-lives (or 24  hours) to reach equilibrium with the parent.

Once equilibrium has been achieved between the  parent and daughter, the time is ripe to remove technetium from the column. The peak activity  of technetium decreases as molybdenum decays, this is why the generator has  to be replaced every week. Please remember that: The Use of a generator avoids the challenge of distributing short-lived  radioisotopes from the original production site (typically a nuclear reactor) to individual users;  the loss of activity due to decay in transit can result in too little being supplied or the need  for much larger initial quantities to be sent out.

The most common radioisotope generator is  the 99Mo-99mTc generator in which a stored quantity of 99Mo decays with a half-life of 66  hours into the 6-hour half-life product 99mTc. These generators are produced and delivered  weekly and provide all of the 99mTc used daily.