What is radionuclidic purity?
Radionuclidic purity refers to the proportion of the total radioactivity present in a preparation that comes from the desired radionuclide. It describes whether unwanted radioactive isotopes are present.
In a Tc-99m generator eluate, radionuclidic purity means that nearly all measured activity should be due to Tc-99m, with minimal contamination from other radionuclides such as molybdenum-99.
Radionuclidic purity refers to the proportion of total activity arising from the intended radionuclide, ensuring safety and accurate imaging.
High radionuclidic purity is essential for patient safety and accurate imaging.
Understanding the physics
When a radionuclide is produced, especially in reactors or cyclotrons, multiple radioactive species may be created. Even after chemical separation, small amounts of unwanted radionuclides may remain.
Radionuclidic purity concerns the identity of the radioactive atoms themselves, not their chemical form.
For example, in a Tc-99m generator:
The desired radionuclide is Tc-99m.
The parent Mo-99 should remain bound to the alumina column.
However, trace amounts of Mo-99 may occasionally be present in the eluate. This is known as molybdenum breakthrough.
If Mo-99 contaminates the eluate, it introduces:
Unwanted beta radiation
Higher patient radiation dose
Additional gamma emissions that degrade image quality
Radionuclidic purity is therefore assessed by measuring whether other radioactive species are present above permitted limits.
It is important to distinguish radionuclidic purity from:
Radiochemical purity (correct chemical form of the radionuclide)
Chemical purity (presence of non-radioactive contaminants)
Each represents a different type of quality control.
Where this matters clinically
Poor radionuclidic purity can increase patient radiation dose and degrade image quality. Regulatory standards specify acceptable limits for contaminants such as Mo-99 in Tc-99m preparations.
Routine quality control ensures radionuclidic purity before clinical administration.