On the use of solid ¹³³Ba sources as surrogate for liquid ¹³¹I 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 Ba-133 as a surrogate for I-131 imaging.Materials and methods: Two sets of four traceable Ba-133 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 I-131 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 Ba-133 sources and liquid I-131.Results: As anticipated, the I-131 pseudo-image calibration factors (cps/MBq) were higher than those for Ba-133 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 Ba-133 and I-131 for all cylinder volumes, which highlights the potential use of Ba-133 sources to calculate recovery coefficients for partial volume correction.Conclusion: This comparison exercise demonstrated that traceable solid Ba-133 sources can be used as surrogate for liquid I-131 imaging. The use of solid surrogate sources could solve the radiation protection problem inherent in the preparation of phantoms with I-131 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.