Evaluation of low energy threshold settings for PVI PET systems

The authors studied the effects of adjusting the low level energy discriminator (LLD) for positron volume imaging (PVI) mode acquisitions using a GE Advance PET system. NEMA scatter fraction and count loss measurements were performed for a 20 cm right circular cylinder at LLD between 300 keV and 450 keV. From these data, noise equivalent count rate (NECR) curves were calculated to estimate effects of LLD on scans of head sized objects. To evaluate the effects of LLD on whole-body image quality, contrast-to-noise ratio (CNR) values were obtained from images processed without attenuation or scatter correction. CNR data were acquired for 1.6 cm and 1.0 cm spherical lesions within the liver region of a Data Spectrum torso phantom scanned with arms, and activity outside the field of view. The results indicate that the NECR for a head-sized object is fairly insensitive to LLD differences between 300 keV and 425 keV for activity concentrations up to 0.4 μCi/cc. The NECR improved slightly for higher concentrations with 375 keV < LLD < 425 keV. Torso phantom images processed without scatter or attenuation correction show some CNR improvement with increasing LLD up to 425 keV for clinically relevant activity concentrations.