Monte Carlo calculated calibration settings for commercially available nuclear medicine ionization chambers

Abstract

Purpose A method for calculating nuclear medicine ionization chamber (NMIC) calibration settings with a Monte Carlo model is presented and validated against physical measurements. This work provides Monte Carlo–calculated calibration settings for select isotopes with no current manufacturer recommendations and a method by which NMIC manufacturers or standards laboratories can utilize highly detailed specifications to calculate comprehensive lists of calibration settings for general isotopes. Methods A Monte Carlo model of a Capintec PET series NMIC was developed and used to calculate the chamber response to relevant radioactive decay products over an energy range relevant to nuclear medicine. The photon detection efficiency (PDE) of a high purity germanium (HPGe) detector was modeled and physically validated to facilitate measurements of NMIC calibration settings with HPGe detector spectroscopy. Modeled NMIC response to various isotopes was compared against spectroscopic measurements and National Institute of Standards and Technology (NIST)‐validated calibration settings to validate the Monte Carlo–calculated NMIC calibration settings. Results HPGe detector PDE was validated against the physical measurements to within 3.3%$3.3\%$ at 95%$95\%$ confidence and used to measure calibration settings, which produced activity readings 0.7%$0.7\%$, 1.6%$1.6\%$, 0.8%$0.8\%$, and 1.0%$1.0\%$ different than those validated by NIST for 11$^{11}$C, 18$^{18}$F, 68$^{68}$Ga, and 64$^{64}$Cu respectively. The Monte Carlo model of the NMIC reproduced measured calibration settings to within 7%$7\%$ at 95%$95\%$ confidence for isotopes with a sufficiently small yield of low energy photons. Conclusions A method of calculating NMIC calibration settings with Monte Carlo modeling has been developed and validated against HPGe detector spectroscopy. NMIC manufacturers or standards laboratories can use more detailed specifications of the chamber geometries to extend the applicability of this method to a wider range of isotopes.