99Mo → 99mTc radionuclide principles
The molybdenum-technetium generator system is based on the principle that the shorter-lived radioactive daughter nuclide (99mTc) can be easily and repeatedly isolated from its longer-lived parent radionuclide (99Mo). This is possible because of the physical differences resulting from the chemistry of these two elements.
The first requirement is to obtain the parent radionuclide in a pure form from some reliable source, preferably at a low cost. Fortunately there are a number of methods to produce the radionuclide and a number of suppliers are available. The two major production methods are from neutron activation of cold molybdenum and by separation of the material from the fission products of uranium. Link to Radionuclide production).
To prepare the generator system the longer half-life 99Mo (67 hours) in the form of molybdate ion, MoO42- is bound to alumina on a column. The shorter-lived 99mTc is continually produced by decay of the parent and is formed as the pertechnetate ion TcO4-. This chemical form of technetium is not bound by the alumina and can be washed off (eluted from) the column with physiological saline (0.9% NaCl). Through this process the daughter radionuclides isolated free of contamination from the parent.
Further discussion of the relationship between these two radionuclides in the generator and a discussion of the elution process can be found in Decay equilibrium. The generator system provides us with a way to obtain 99mTc regularly and as a purified radionuclide separated from the parent 99Mo. The ability to remove 99mTc at regular intervals is very useful in an operating radiopharmacy. A single generator can have a useful life of 2 weeks or more although the activity of the parent continues to decrease due to its decay and consequently less of the daughter radionuclide is available from the elution process. This table shows the activity of the parent99Mo and the daughter 99mTc from a typical generator that is prepared with a 2 Ci (74 GBq) initial calibrated activity of parent. The daughter activities represent the theoretical activity available if the generator is at equilibrium, but if the generator has been eluted within the previous 24 hours this activity will not be fully available. This relationship is described and illustrated in more detail in Decay equilibrium.
We can see how important this separation is if we understand the medical uses of 99mTc and what physical properties are desirable for these uses. The shorter half life of 6 hours for 99mTc and its decay by isomeric transition make it highly useful for medical imaging applications.
More details of the 99Mo → 99mTc process and radionuclide principals are provided under the following headings:
|Figure 1: A typical bone scan produced by medical imaging of a patient following the injection of the 99mTc-labeled compound called methylenediphosphonate or MDP. The compound causes the activity to be selectively taken up in the bone while being eliminated from the rest of the body. Scans can help the physician diagnose diseases which impact the bone matrix such as osteoporosis, cancer and Paget’s disease as well as being able to detect bone injury or infection.|