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.

UNLABELLED: 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)GeCl4 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.

Current status and future needs for standards of radionuclides used in positron emission tomography.

Positron Emission Tomography (PET) is being increasingly used as a quantitative technique for detecting disease and monitoring patient progress during treatment. To ensure the validity of the quantitative information derived from the imaging data, it is imperative that all radioactivity measurements that are part of the imaging procedure be traceable to national or international standards. This paper reviews the current status of standards for positron emitting radionuclides (e.g., (18)F, (68)Ge/(68)Ga, and (124)I) and suggests needs for future work.

Quantifying and reducing the effect of calibration error on variability of PET/CT standardized uptake value measurements.

UNLABELLED: The purpose of this study was to measure the errors introduced by regular calibration of PET/CT scanners and to minimize the effect of calibration error on standardized uptake value measurements. METHODS: Global calibration factors from 2 PET/CT scanners were recorded for 3.5 and 1.8 y, comparing manufacturer-recommended protocols with modified protocols to evaluate error contributions due to operator-influenced procedures. Dose calibrator measurements were evaluated using National Institute of Standards and Technology-traceable sources. RESULTS: Dose calibrator variability was less than 1%, although there was a consistent bias. Global scaling variability was reduced from 6% to 4% for scanner 1 and from 11% to 4% for scanner 2 when quality assurance and quality control procedures were applied to the calibration protocol. When calibrations were done using a (68)Ge/(68)Ga phantom, the variability for both scanners was reduced to approximately 3%. CONCLUSION: Applying quality assurance and quality control procedures to scanner calibration reduces variability, but there is a still a residual longitudinal scanner variability of 3%-4%. The procedures proposed here reduce the impact of operator error on scanner calibration and thereby minimize longitudinal variability in standardized uptake value measurements.

Standardization of 68(Ge+Ga).

The radionuclide 68Ga is mainly a positron emitter (89.2%), with a half-life of 67.7 min. It is used in nuclear medicine, being chemically extracted from the mixture 68(Ge+Ga); its precursor, 68Ge, disintegrates 100% by electron capture, with a half-life of 270.8d (Table of radionuclides, comments and Evaluation). A 4pibeta-gamma coincidence method was used for standardization, with a 4pi proportional beta-detector and a NaI(Tl) gamma detector. Registration of the capture radiations was avoided using foil absorption and a high beta threshold. Using supplementary foils for positron absorption, extrapolation graphs were obtained, with a mean slope of -4.4%. Care was taken to compensate for the loss of 68Ge during the preparation of solid sources for measurement. A combined uncertainty of 1.1% was estimated.