Performance assessment of a NaI(Tl) gamma counter for PET applications with methods for improved quantitative accuracy and greater standardization

Abstract

Background Although NaI(Tl) gamma counters play an important role in many quantitative positron emission tomography (PET) protocols, their calibration for positron-emitting samples has not been standardized across imaging sites. In this study, we characterized the operational range of a gamma counter specifically for positron-emitting radionuclides, and we assessed the role of traceable 68Ge/68Ga sources for standardizing system calibration. Methods A NaI(Tl) gamma counter was characterized with respect to count rate performance, adequacy of detector shielding, system stability, and sample volume effects using positron-emitting radionuclides (409- to 613-keV energy window). System efficiency was measured using 18F and compared with corresponding data obtained using a long-lived 68Ge/68Ga source that was implicitly traceable to a national standard. Results One percent count loss was measured at 450 × 103 counts per minute. Penetration of the detector shielding by 511-keV photons gave rise to a negligible background count rate. System stability tests showed a coefficient of variation of 0.13% over 100 days. For a sample volume of 4 mL, the efficiencies relative to those at 0.1 mL were 0.96, 0.94, 0.91, 0.78, and 0.72 for 11C, 18F, 125I, 99mTc, and 51Cr, respectively. The efficiency of a traceable 68Ge/68Ga source was 30.1% ± 0.07% and was found to be in close agreement with the efficiency for 18F after consideration of the different positron fractions. Conclusions Long-lived 68Ge/68Ga reference sources, implicitly traceable to a national metrology institute, can aid standardization of gamma counter calibration for 18F. A characteristic feature of positron emitters meant that accurate calibration could be maintained over a wide range of sample volumes by using a narrow energy window centered on the 511-keV peak.

Development of a non-radiometric method for measuring the arterial input function of a 11C-labeled PET radiotracer

“…Clearly, these surrogate isotopes have decay modes that are very different from those of 11 C (β + , ~ 99.8%). Moreover, the accuracy of radioactivity measured with an ionization chamber or γ-counter is well known to be influenced by sample volume and geometry effects 14,24,25 . Studies seeking to measure positron-emitters in ionization chambers have been conducted with fluorine-18 (t 1/2 = 109.8 min) 24,25 , but none have used shorter-lived carbon-11.…”

“…Whereas the dose of a radiotracer administered to a human subject (~ 750 MBq) is measured with an ionization chamber, the low levels of radioactivity found in plasma samples (of the order of kBq) are typically measured with a sensitive γ-counter. The accuracy of radioactivity measured with an ionization chamber or γ-counter depends, among other factors, on the surrogate radioisotope(s) used to calibrate the instruments 13 , 14 . Notably, no alternative method has ever been available to validate AIFs measured with the conventional radiometric technique.…”

“…the instruments 13,14 . Notably, no alternative method has ever been available to validate AIFs measured with the conventional radiometric technique.…”

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

“…The scanner and well counter sources are implicitly NIST-traceable, that is, they are made following the same procedures but are not certified by NIST. Results from the well counter sources are separately reported (24). The present study uses only the scanner source and dose calibrator source from each kit.…”

Performance Assessment of NaI detector Spectrometer for Safeguard Measurements

“…In the case of isolation of a radioactive source as well as scintillation detector from all encircling materials, the detected pulse-amplitude spectrum in terms of shape and magnitude would depend solely on the gamma-ray energy, the physically inherent characteristics of both source and detector in addition to the geometrical connection between both of them. Practically, several variables linked to the experimental setting would influence the detected pulse-amplitude distribution pattern (Heath, 1997;Lodge et al, 2015).…”