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

In positron imaging, Noise Equivalent Count (NEC) rate curves provide a good link between image quality and scatter, random and true coincidence rates. The NEC curve has been measured on a SophyCamera DST (SMV) with a 20 cm cylinder phantom filled with ¹⁸F (59 MBq). On that system, the field of view is determined by two rectangular parallel detectors (400*300 mm) separated by 720 mm. Each scintillation detector is composed of a ⅜ inch thick NaI(Tl) crystal. The coincidence time window was set to 15 ns. The scatter fraction (sf) was measured using PET NEMA protocol. Random and coincident count rates were measured using tomographic acquisition with two 20% energy windows centered on 511 keV (photopeak) and 417 keV (Compton) over 6 half-lives of ¹⁸F. Delaying signals from one of the coincident detectors (45 ns) allowed to investigate random coincidence rate. System sensitivity was measured as the slope at the origin of the NEC curve. The NEC rate peaked at 199 cps and 252 cps for photopeak (with a 0.20 sf) and photopeak+Compton (with 0.28 sf) coincidences respectively. These maxima were reached over activity concentration values ranging from 1.33 kBq/cc to 1.85 kBq/cc. With a sensitivity 1.3 times better, a NEC rate always greater, image quality improves when using photopeak+Compton coincidences