Investigation of noise equivalent count rate in positron imaging using a dual head gamma camera

In positron imaging, image quality depends on scatter, random and coincidence rate. It is known that noise equivalent count (NEC) rate is a good indicator of image quality and that it helps optimizing acquisition parameters. We measured the NEC curve with a SophyCamera DST (SMV) using a 20 cm cylinder phantom filled with ¹⁸F (59 MBq). The field of view of the DST is determined by two rectangular parallel detectors (400*300 mm) separated by 720 mm and each scintillation detector is composed of a ⅜ of an inch thick NaI:Tl crystal. The coincidence time window was set to 15 ns and the scatter fraction (sf) was measured using a PET NEMA protocol. We measured random and coincident count rates using tomographic acquisition with two 20% energy windows centered on 511 KeV (photopeak) and 417 KeV (Compton) over 6 half-lives of ¹⁸F. We investigated the random coincidence rate by delaying signals from one of the coincident detectors (45 ns) and measured system sensitivity as the slope at the origin of the NEC curve. The NEC rate peaked at 199 cps for photopeak (with a 0.20 sf) and at 252 cps for photopeak+Compton (with 0.28 sf) coincidences. These maxima were attained over activity concentration values ranging from 1.33 kBq/cc to 1.85 kBq/cc. Our results show that image quality improves when using photopeak+Compton coincidences with a 30% greater sensitivity and an increased NEC rate