title: 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 (68)Ge/(68)Ga 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 (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.
authors.affiliation: Division of Nuclear Medicine, The Russell H. Morgan Department of Radiology and Radiological Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Martin A
authors.initials: MA
authors.lastname: Lodge
authors.affiliation: Division of Nuclear Medicine, The Russell H. Morgan Department of Radiology and Radiological Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Holt
authors.affiliation: Department of Radiology, University of Washington, Seattle, WA, USA.
authors.forename: Paul E
authors.initials: PE
authors.lastname: Kinahan
authors.affiliation: Division of Nuclear Medicine, The Russell H. Morgan Department of Radiology and Radiological Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Dean F
authors.initials: DF
authors.lastname: Wong
authors.affiliation: Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA.
authors.forename: Richard L
authors.initials: RL
authors.lastname: Wahl
keywords: Calibration; Efficiency; Gamma counter; Positron emission tomography; Sample volume; Standardization; Well counter