A noise equivalent counts approach for optimizing I-124 imaging performance with a small animal PET scanner

Due to its long radiological half life, ¹²⁴I can be used for investigation of lengthy biological processes. However, it is not an ideal PET isotope because it has poor positron yield and half of the positrons are emitted in cascade with 603 keV gammas. This decay spectrum results in coincidences between gammas and annihilation photons (dirty coincidences) leading to both inaccurate quantitation and overall reduction in the image quality. It has been shown that noise equivalent counts (NEC) have a direct link to signal to noise ratio of the reconstructed images. The purpose of this work was to investigate the effect of varying the energy window of the digitizer on the NEC measured from the non-pure positron emitter ¹²⁴I and hence, on the image quality. Simulations were performed using the GATE software for a 1.32 cm radius cylinder filled with 300 muCi of ¹²⁴I placed at the center of field of view. We report that by windowing in on the upper energy window of the data acquisition from a symmetric 50% (255-766 keV) energy window, to an asymmetric 255-600 keV one, it is possible to preferentially reduce the dirty coincidences with minor sensitivity changes. These findings will be verified with experimental data, and will lead to optimizing the data acquisition chain for ¹²⁴I imaging performance