Evaluation of the Stability and System Characteristics of Digital Photon Counting PET/CT

Objectives PET detector technology moving from the conventional photomultiplier tube (PMT) detector to the next generation solid state digital photon counting (DPC) detector is a technology leap. This study evaluates the stability and characteristics of the solid state digital photon counting (DPC) detector PET/CT system within a 1 yr time window following its installation.

Methods PET acceptance testing was performed on a next generation time-of-flight (TOF) digital PET/CT system (Vereos) in September 2014 (Testing 1) and 12-month post its installation (Testing 2). System testing was performed and evaluated with NEMA NU2 2012 criteria. System characteristics and stability were assessed and compared at both time points in spatial resolution, sensitivity, noise equivalent counting rate (~30mCi F18 with ~12hours in count loss) as well as image quality. Timing resolution, energy resolution, digital detector temperature and humidity were monitored across the 1 yr time period with system built-in procedures. Measurements of sensitivity, timing, energy and spatial resolution were characterized across the clinically relevant count rate range.

Results Spatial resolution (in mm FWHM) from NEMA Testing 1 (from 4.11 & 3.96 at 1 cm to 5.79 & 6.20 at 20 cm in transverse and axial, respectively) to NEMA Testing 2 (from 4.10 & 3.95 at 1 cm to 5.79 & 6.21 at 20 cm) was achieved. 324 ps timing resolution and 11.1 % energy resolution were obtained at Testing 1 compared to 323ps and 10.9% at Testing 2. A 5.7 kcps/MBq system sensitivity and 24.1 kcps/MBq effective sensitivity with TOF gain were obtained at Testing 1, versus 5.7kcps/MBq and 24.2 kcps/MBq respectively at Testing 2. 171 kcps peak NECR and > 680 kcps peak true rate were obtained at 50 kBq/mL, with 30 % scatter fraction at Testing 1, with <3% variances at Testing-2. NEMA image quality at Testing 1 and Testing 2 demonstrated hot sphere contrast ranging from ~62%±2% (10 mm) to ~88%±2% (22 mm), cold sphere contrasts of ~86%±2% (28 mm) and ~89%±3% (37 mm) and excellent image uniformity. Monitoring the 1 yr changes of system characteristics, it revealed <2% change in timing, ±0.4% change energy resolution, as well as <10% variations in temperature and humidity detector environment.

Conclusions Characteristics and stability of the next generation digital photon counting PET detector system over a 1 yr time window were tested, monitored and evaluated in this study. It demonstrated very stable and robust system performance in relevant aspects such as spatial resolution, sensitivity, timing and energy resolution, count rate and image quality.