Evaluation of performance of the CPET scanner using standardized measurement techniques

Abstract

We report on our evaluation of the CPET (ADAC/UGM) and compare the different results obtained according to the IEC (61675-1, 1998), NEMA (NU-2, 1994) and updated NEMA (NEMA 2000+) protocols. The latter uses phantoms more clinically relevant for whole-body studies. The CPET has six curved NaI(Tl) detectors (2.54 cm thick) and no septa. The ring diameter is 90 cm with a patient port of 56 cm, and the axial FOV is 25.6 cm. The energy resolution is 11% and the lower energy threshold is set at 435 keV. The spatial resolution for a point (line) source in the transaxial direction is 4.6 (4.7) mm (FWHM) on the scanner axis, and the axial resolution is 5.7 mm. Following the NU-2 analysis, the scatter fraction is 25%, the sensitivity is 12.7 cps/Bq/ml, and the peak noise equivalent countrate (NEC) is 47.4 kcps at an activity concentration of 12.3 kBq/ml. For the IEC protocol, the scatter fraction is 31.6%, the sensitivity is 11.6 cps/Bq/ml, and the peak NEC is 39.5 kcps at 12.3 kBq/ml. For the NEMA 2000+ protocol, the scatter fraction is 35.0%, the sensitivity is 3.0 cps/kBq (note the change in units), and the peak NEC is 14 kcps at 3.8 kBq/ml. Compared to the previous Penn-PET scanners, upon which the CPET is based, the use of curved detectors and improvements in the pulse-shaping, integration deadtime, and triggering lead to improved countrate capability, spatial resolution and image quality of the scanner. The current NEMA and IEC measurements (which use 19-cm long phantoms) characterize the best possible performance for PET imaging, while the new proposed measurements (using longer phantoms and point instead of line sources) better characterize the performance under clinical conditions, especially 3D whole-body scanning.