The International Atomic Energy Agency (IAEA) recently initiated a Cooperative Research Project (CRP) entitled “Development of Quantitative Nuclear Medicine Imaging for Patient Specific Dosimetry” as part of a larger program aimed at enhancing the practice of nuclear medicine physics in its Member States.
One of the early goals of the CRP is to assess the global state of the accuracy and consistency of Single Photon Emission Tomography (SPECT) image quantification, as evidenced by the performance of the institutions participating in the project during a series of comparison exercises.
Based on information provided by the participants regarding the type of instrumentation (i.e., activity calibrators and scanners) that they have available and the types of medical procedures that are performed in their centers, it was decided that 131I should be used as the test radionuclide for the comparison because of its widespread use around amongst the Member States. As the quantity of interest is the accuracy with which the participants could determine the activity concentration of the radioactive sources using SPECT imaging, it was necessary for the activity content of the sources to be traceable to a single standard. The relatively short half-life (T1/2=8.0233(19) d)  of 131I and the large distances between the participating laboratories made it logistically impossible to prepare and calibrate phantoms of 131I and have them shipped to the participants with reasonable activity levels. For that reason, 133Ba was chosen as a long-lived surrogate because of its long half-life of 10.540(6) a  and the fact that the most abundant γ-ray in the decay of 133Ba at 356 keV is similar in energy and emission probability to the most abundant γ-ray in the decay of 131I at 364 keV.