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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.
To minimize logistical problems associated with shipping liquid radioactive sources internationally, the test objects for the comparison were a set of 4 cylindrical phantom inserts, each containing a calibrated amount of 133Ba in epoxy. The activity concentration in the three lowest volumes was to be about 200 kBq•g-1 (denoted “high-level”) and that of the 23 mL sources was to be 50 kBq•g-1 (denoted “low-level”). The success that we achieved in developing the methodology to calibrate similar sources containing 68Ge [3, 4, 5] suggested that a similar approach could also be used to calibrate these 133Ba sources.
The sources for this study consisted of poly(methyl methacrylate) (PMMA) cylinders having a wall thickness of 1 mm and bottom thickness of 2 mm. The active length of the epoxy was nominally 3.8 cm and the inside diameters were varied to give approximate volumes of 2 mL, 4 mL, 6 mL, and 23 mL. Each cylinder was fitted with a PMMA screw cap having a total length of 1 cm into which a 5 mm diameter threaded hole is drilled. The threaded hole accommodates a Plexiglas rod that is used to mount the cylinder to the bottom of a standard Jaczszak phantom. A set of the cylinder sources can be seen in the photograph in the right column.
Calibration factors for the NIST-maintained Vinten 671 ionization chamber (VIC) and efficiency factors for the NIST high-purity germanium (HPGe) photon detectors were determined from sources prepared by accurate filling of a set of three cylinders of each volume with a calibrated solution of 133Ba. Correction factors to account for differences in photon attenuation between the solution sources and the 133Ba epoxy sources were determined from Monte Carlo simulations.
The preparation of the 133Ba epoxy and filling of the epoxy-based cylinder sources were performed by an outside company. The cylinders were weighed before and after filling to accurately determine the mass of added 133Ba epoxy. The cylinders were then shipped to NIST for calibration.
The average activity concentration results for the cylinders of each volume for both of the measurement methods are given in the table below. While each technique taken on its own is able to provide a value for the 133Ba activity concentration in a single source with a relative combined uncertainty of about 1 %, a systematic difference of up to 1 % between the results obtained with the two methods is observed. By considering the two data sets together and assuming that the overall uncertainty consists of a within-method uncertainty as well as a between-method uncertainty, we used the method of Vangel and Rukhin  to estimate the magnitude of the between-method component of uncertainty to 0.6 % and 1.0 % for the “high” and “low” level sources, respectively. Combining this uncertainty with the average value of the combined standard uncertainties for the activity concentration determination from the two methods, we were able to assign average 133Ba activity concentration values to the “high” and “low” level 133Ba epoxies studied, with relative combined uncertainties of 1.4 % and 1.7 %, respectively.
The comparison has been completed and final results are being compiled for publication.
Table: Average activity concentrations for the 133Ba epoxy-filled cylinders sources of a given volume, along with the percent difference between the averages, as determined from HPGe and VIC measurements. The uncertainties here are the combined uncertainties of the average standard uncertainty for each source combined with the standard deviation on the sources within each set.
Photograph on right: A set of 133Ba epoxy sources prepared for and calibrated in this study. The sources appear dark due to a dye that was added to the epoxy during manufacturing as a visual indicator of uniform mixing. (Photograph by B. E. Zimmerman.)