On the use of solid 133 Ba sources as surrogate for liquid 131 I in SPECT/CT calibration: a European multi-centre evaluation

Introduction Commissioning, calibration, and quality control procedures for nuclear medicine imaging systems are typically performed using hollow containers filled with radionuclide solutions. This leads to multiple sources of uncertainty, many of which can be overcome by using traceable, sealed, long-lived surrogate sources containing a radionuclide of comparable energies and emission probabilities. This study presents the results of a quantitative SPECT/CT imaging comparison exercise performed within the MRTDosimetry consortium to assess the feasibility of using 133 Ba as a surrogate for 131 I imaging. Materials and methods Two sets of four traceable 133 Ba sources were produced at two National Metrology Institutes and encapsulated in 3D-printed cylinders (volume range 1.68–107.4 mL). Corresponding hollow cylinders to be filled with liquid 131 I and a mounting baseplate for repeatable positioning within a Jaszczak phantom were also produced. A quantitative SPECT/CT imaging comparison exercise was conducted between seven members of the consortium (eight SPECT/CT systems from two major vendors) based on a standardised protocol. Each site had to perform three measurements with the two sets of 133 Ba sources and liquid 131 I. Results As anticipated, the 131 I pseudo-image calibration factors (cps/MBq) were higher than those for 133 Ba for all reconstructions and systems. A site-specific cross-calibration reduced the performance differences between both radionuclides with respect to a cross-calibration based on the ratio of emission probabilities from a median of 12–1.5%. The site-specific cross-calibration method also showed agreement between 133 Ba and 131 I for all cylinder volumes, which highlights the potential use of 133 Ba sources to calculate recovery coefficients for partial volume correction. Conclusion This comparison exercise demonstrated that traceable solid 133 Ba sources can be used as surrogate for liquid 131 I imaging. The use of solid surrogate sources could solve the radiation protection problem inherent in the preparation of phantoms with 131 I liquid activity solutions as well as reduce the measurement uncertainties in the activity. This is particularly relevant for stability measurements, which have to be carried out at regular intervals.