Performance assessment of a NaI(Tl) gamma counter for PET applications with methods for improved quantitative accuracy and greater standardization
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 68 Ge/ 68 Ga sources for standardizing system calibration. Methods: A NaI(Tl) gamma counter was characterized with
... t 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 18 F and compared with corresponding data obtained using a long-lived 68 Ge/ 68 Ga source that was implicitly traceable to a national standard. Results: One percent count loss was measured at 450 × 10 3 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 11 C, 18 F, 125 I, 99m Tc, and 51 Cr, respectively. The efficiency of a traceable 68 Ge/ 68 Ga source was 30.1% ± 0.07% and was found to be in close agreement with the efficiency for 18 F after consideration of the different positron fractions. Conclusions: Long-lived 68 Ge/ 68 Ga reference sources, implicitly traceable to a national metrology institute, can aid standardization of gamma counter calibration for 18 F. 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.