%0 Journal Article %A Martin Lodge %A Magnus Dahlbom %A Richard Wahl %T Cross-calibration of gamma counters and PET scanners using Ge-68/Ga-68 %D 2011 %J Journal of Nuclear Medicine %P 2065-2065 %V 52 %N supplement 1 %X 2065 Objectives Accurate cross-calibration between NaI gamma counters and PET scanners is required for many tracer kinetic modeling studies. This calibration is isotope-dependent because the different physical characteristics of PET isotopes (positron fraction, positron energy, non-511 keV gammas) give rise to different gamma counter sensitivities. We present a method for determining isotope-specific cross calibration factors by scaling measurements made with a long-lived Ge-68/Ga-68 source. Methods Gamma counter-to-PET cross calibration factors (fi) were measured for different PET isotopes (fC11, fF18 ) using aliquots taken from a fillable 20 cm diameter cylinder. Measurements were made on a 1282 CompuGamma gamma counter and were related to image data obtained with a clinical PET scanner (separately calibrated using an NIST traceable positron source). A 20 cm diameter Ge-68/Ga-68 cylinder with well counter compatible aliquots, were also used to measure fGa68. Gamma counter measurements were made using both a narrow photopeak window (450-550 keV) and a wide window (450-2000 keV) that included both the 511 keV peak and the sum peak. Results fC11 / fGa68 and fF18 / fGa68 were 0.87 and 0.88 respectively for the narrow window and 0.99 and 1.00 for the wide window. For the narrow window, the relationship between fi and fGa68 was dominated by the difference in positron fractions. For the wide window, the effect of different positron fractions was offset by the inclusion of single photon emissions (1.08 MeV and others) from Ga-68. Compared to the narrow window, the sensitivity of the wide window was greater by factors of 2.07 (C-11), 2.07 (F-18) and 2.35 (Ga-68), implying a substantial reduction in statistical noise. Conclusions The relationship between gamma counter measurements of a long-lived Ge-68/Ga-68 source and other positron-emitting isotopes has been characterized. This relationship provides a mechanism for routinely updating calibration factors in order to reflect drifts in system performance, without the need for short-lived isotopes %U