Radionuclidic purity is defined as "the ratio, expressed as a percentage, of the radioactivity of the desired radionuclide to the total radioactivity of the source". Radionuclidic purity is important in radiopharmacy since any radionuclidic impurities may increase the radiation dose received by the patient and may also degrade the quality of any imaging procedure performed. No radionuclide sample is 100% pure and all contain some contaminants arising from the production process or the decay of the primary radioisotope. In order to control the effects of these contaminants on the radiation dose received by the patient, limits must be set on the maximum levels of contamination allowed. These limits are defined in pharmacopoeia monographs and vary depending upon the radionuclide concerned and the physical decay characteristics of the likely contaminants. Measurement of radionuclidic purity is normally performed using gamma-ray spectroscopy. Positron emitting radionuclides also use half-life determination.
Measurement of the radionuclidic purity of technetium-99m is particularly important since
- this is the most widely used isotope in Nuclear Medicine and impurities in 99mTc samples can significantly influence the radiation dose received by the patient population
- the radionuclide is produced in-house from a 99Mo/99mTc generator and consequently it is less easy to control the level of impurities than at a central manufacturing site.
The radionuclidic purity of sodium [99mTc] pertechnetate depends on the source of the parent molybdenum-99: When the Mo-99 is produced by neutron bombardment of Mo-98 not more than 0.1% of the total radioactivity may be present in the form of Mo-99 and not more than 0.01% in the form of any other radionuclidic impurity. When the Mo-99 is produced from uranium fission the radionuclidic purity specification is more stringent:
Levels of the daughter radionuclide Tc-99 are not controlled by the pharmacopoeias but high levels of this radionuclide can affect labelling efficiency during radiopharmaceutical preparation.