Impact of Recent Change in the National Institute of Standards and Technology Standard for ¹⁸F on the Relative Response of ⁶⁸Ge-Based Mock Syringe Dose Calibrator Standards

Abstract

As a result of a recent change in the National Institute of Standards and Technology (NIST) activity standard for 18 F, we have determined new relative response ratios for a 68 Ge solid epoxy mock syringe source used in activity calibrators as a long-lived substitute for 18 F. New standardized solutions of each radionuclide were used to determine the response ratios while maintaining traceability to national standards. This work updates our previously published data from 2010. Methods: Following our previously published methodology, solution-filled mock syringe sources, identical in geometry to the solid 68 Ge epoxy calibration source currently on the market, were prepared using NIST-calibrated solutions of 68 GeCl 4 and 18 F-FDG and directly compared in several models of activity calibrators to determine empirically the relative response ratios for these 2 radionuclides. Results: The new relative response ratios measured in this study reflect the change in 18 F activity measurements that arise from the recent −4% change in the NIST activity standard. The results allow the 68 Ge activity of the mock syringe source to be expressed in terms of equivalent 18 F activity, with a relative combined standard uncertainty of about 0.8% for the activity calibrators used in this study. Conclusion: This work revises our previously derived relative response ratios for 18 F and 68 Ge by −3.7%, allowing users of the commercial mock syringe surrogate source to calibrate their activity calibrators in a way that is consistent with the recent change in the NIST 18 F standard. In 2 recent publications (1,2), the National Institute of Standards and Technology (NIST) reported on a change of 24% in its radioactivity standard for the positron emitter 18 F. In addition to the obvious change that this brings in directly measuring this radionuclide in activity calibrators, it has an influence on the relative response ratio that allows 68 Ge-based solid mock syringe sources to be used as long-lived calibration artifacts for 18 F in those instruments.In 2010, we published a procedure for calibrating a specific commercially available mock syringe source for 68 Ge content (3). At the same time, we provided data on the relative response between that source and the equivalent activity of 18 F when measured in several models of activity calibrators. Although it would have been possible to merely take the magnitude of the published change in the 18 F standard and recalculate a new relative response ratio, a rigorous approach involving new direct measurements of sources containing calibrated 68 Ge and 18 F solutions gives more confidence in the results. Direct measurement of the ratios has 2 distinct advantages: direct traceability back to the NIST primary standards for the 2 radionuclides is maintained, and a proper uncertainty assessment for the new ratios is possible. We have undertaken a new series of measurements to experimentally determine the new 18 F-to-68 Ge relative response ratios for the 6-mL (filling volume nominally 3 mL) epoxy-based...

Measuring temporal stability of positron emission tomography standardized uptake value bias using long-lived sources in a multicenter network

“…The dose calibrator standard was assayed as if it were a patient dose. A conversion factor supplied by NIST allowed either 68 Ge or 18 F calibration settings to be used 18,19 for the measurement, and sites used 18 F settings to best represent clinical bias. A background radiation reading for the dose calibrator was recorded and subtracted from source assay.…”

Evaluation of Cross-Calibrated 68Ge/68Ga Phantoms for Assessing PET/CT Measurement Bias in Oncology Imaging for Single- and Multicenter Trials

“…The X-Cal independent cross-calibration kits were developed based on the traceable dose calibrator standards from RadQual, LLC., and the NIST (22, 23). In a previous study, using a larger prototype phantom with a 6 cm diameter, we found that bias in SUV measurements varied over time and between scanner sites (21).…”

“…Each source’s activity is known to within ±2.5% with a 95% confidence level (22, 23). The dose calibrator reference sources are directly traceable to National Institute of Standards and Technology (NIST) standards.…”

“…The assay was performed by placing the source (Figure 1, Right) in the syringe holder of the dose calibrator and assaying it as if it were a patient dose using the calibration setting for FDG. The expected reading is computed using a relative response factor that converts the manufacturer’s stated activity of 68 Ge to equivalent 18 F activity (22, 23). …”

“…A subtle point is that we do not call the latter the “true” value, as the NIST-traceable standards are stated with a known measurement uncertainty of ±2.5% with a 95% confidence level (22, 23). Where needed, a similar ratio measurement was used for the dose calibrators.…”