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

The international reference system for beta-particle emitting radionuclides: Validation through the pilot study CCRI(II)-P1.Co-60.

The Bureau International des Poids et Mesures (BIPM) is developing a new transfer instrument to extend its centralized services for assessing the international equivalence of radioactive standards to new radionuclides. A liquid scintillation counter using the triple/double coincidence ratio method is being studied and tested in the CCRI(II)-P1.Co-60 pilot study. The pilot study, involving 13 participating laboratories with primary calibration capabilities, validated the approach against the original international reference system based on ionization chambers, which has been in operation since 1976. The results are in agreement and an accuracy suitable for purpose, below 5×10-4, is achieved. The pilot study also reveals an issue when impurities emitting low-energy electrons are present in the standard solution, which have a different impact on liquid scintillation counting compared to other primary measurement methods.

A bilateral comparison between LNHB and PTB to determine the activity concentration of the same 125I solution.

A bilateral comparison to determine the activity concentration of the same 125I solution was organized. As electron-capture radionuclide with a rather high atomic number, 125I must be regarded as difficult to measure. The situation is partly exacerbated by the fact that some established standardization methods, like photon-photon coincidence counting, can no longer be applied due to the unavailability of appropriate equipment and expertise. One aim of this work is to compare modern liquid scintillation counting methods for the standardization of 125I. Both participating metrology institutes have used their custom-built triple-to-double-coincidence ratio (TDCR) counters and the determined activity concentrations are in excellent agreement even though the ways to analyze the data and to compute counting efficiencies were widely independent. The results also agree with the outcome of 4π-γ counting that was carried out at LNHB. In both laboratories, the measurements were complemented by measurements with several secondary standardization methods which even allow to establish a link to the CCRI(II)-K2.I-125(2) comparison started in 2004. A good agreement between the TDCR results and the key comparison reference value of the 2004/2005 comparison was obtained.

Analysis of gamma-ray spectra with spectral unmixing - Part I: Determination of the characteristic limits (decision threshold and statistical uncertainty) for measurements of environmental aerosol filters.

Poisson-statistics based spectral unmixing has been shown to be an efficient analysis tool for the radionuclides activity estimation from gamma-ray spectrometry measurements. However, the calculation of the corresponding characteristic limits has not been investigated so far. In this paper, we present the quantification of the decision threshold and the limits of the coverage interval for the metrological use of such spectral unmixing algorithms. The proposed approach is evaluated and validated with simulated spectra of HPGe and NaI measurements by comparing the results to characteristic limits calculated from Monte-Carlo simulations. We focus particularly on the validation of the method for the metrological analysis of environmental measurements, for which the low-level activity quantification requires an accurate characteristic limits determination. Along with the instrument calibration, we establish a metrological analysis tool by using the spectral unmixing algorithm for environmental aerosol filters measured by gamma-ray spectrometry.

Analysis of gamma-ray spectra with spectral unmixing, Part II: Recalibration for the quantitative analysis of HPGe measurements.

In the context of radioactivity measurements, the quantitative analysis of a gamma-ray spectrum depends on the analysis algorithm. To that end, we recently introduced a Poisson statistics-based spectral unmixing approach. However, it also relies on a proper instrument recalibration as well as on an uncertainty estimation, for which no solution has been proposed so far. The goal of this article is twofold: i) we first present a novel method to correct for the instrument calibration of an HPGe detection system, which is tailored to spectral unmixing algorithms, and ii) we apply this new approach to the quantitative analysis of real data as well as on the evaluation of the uncertainty. Along with the characteristic limits determination investigated, this paper introduces the first full metrological analysis sequence of aerosol filter measurements based on spectral unmixing, which allows to quantify both the radionuclides' activities and their associated uncertainties.

Half-life determination and comparison of activity standards of 231Pa.

The results of an international comparison of activity measurements of a solution of 231Pa are reported and analysed. Prior to this, no known standardisation of 231Pa by activity measurement had been carried out. The comparison was run in 2017-2018 involving eight laboratories, and returned results with no identifiable inconsistencies between methods or laboratories. The results, including one mass determination, gave a231Pa activity concentration of 41.461(48) kBq g-1 and a231Pa atom concentration of 61.48(23) × 1015 atoms g-1, from which a half-life value of 32 570(130) years was derived.

The half-life of 129I.

The radionuclide 129I is a long-lived fission product that decays to 129Xe by beta-particle emission. It is an important tracer in geological and biological processes and is considered one of the most important radionuclides to be assessed in studies of global circulation. It is also one of the major contributors to radiation dose from nuclear waste in a deep geological repository. Its half-life has been obtained by a combination of activity and mass concentration measurements in the frame of a cooperation of 6 European metrology institutes. The value obtained for the half-life of 129I is 16.14 (12) × 106 a, in good agreement with recommended data but with a significant improvement in the uncertainty.

Determination of X- and gamma-ray emission intensities in the decay of (131)I.

The activity per unit mass of an iodine-131 solution was absolutely standardized by both the 4πß-γ coincidence method and the 4πγ counting technique. The calibrated solution was used to prepare point sources after a preliminary deposit of AgNO3 to prevent the loss of volatile iodine. Relative and absolute photon emission intensities of 15 sgamma-rays and those of the two K X-rays of xenon were determined by gamma-ray spectrometry, with relative uncertainties of 0.8% for the three main emissions.