Variability in PET image quality and quantification measured with a permanently filled (68)Ge-phantom: a multi-center study

BACKGROUND: This study evaluated, as a snapshot, the variability in quantification and image quality (IQ) of the clinically utilized PET [(18)F]FDG whole-body protocols in Finland using a NEMA/IEC IQ phantom permanently filled with (68)Ge. METHODS: The phantom was imaged on 14 PET-CT scanners, including a variety of models from two major vendors. The variability of the recovery coefficients (RC(max), RC(mean) and RC(peak)) of the hot spheres as well as percent background variability (PBV), coefficient of variation of the background (COV(BG)) and accuracy of corrections (AOC) were studied using images from clinical and standardized protocols with 20 repeated measurements. The ranges of the RCs were also compared to the limits of the EARL (18)F standards 2 accreditation (EARL2). The impact of image noise on these parameters was studied using averaged images (AVIs). RESULTS: The largest variability in RC values of the routine protocols was found for the RC(max) with a range of 68% and with 10% intra-scanner variability, decreasing to 36% when excluding protocols with suspected cross-calibration failure or without point-spread-function (PSF) correction. The RC ranges of individual hot spheres in routine or standardized protocols or AVIs fulfilled the EARL2 ranges with two minor exceptions, but fulfilling the exact EARL2 limits for all hot spheres was variable. RC(peak) was less dependent on averaging and reconstruction parameters than RC(max) and RC(mean). The PBV, COV(BG) and AOC varied between 2.3–11.8%, 9.6–17.8% and 4.8–32.0%, respectively, for the routine protocols. The RC ranges, PBV and COV(BG) were decreased when using AVIs. With AOC, when excluding routine protocols without PSF correction, the maximum value dropped to 15.5%. CONCLUSION: The maximum variability of the RC values for the [(18)F]FDG whole-body protocols was about 60%. The RC ranges of properly cross-calibrated scanners with PSF correction fitted to the EARL2 RC ranges for individual sphere sizes, but fulfilling the exact RC limits would have needed further optimization. RC(peak) was the most robust RC measure. Besides COV(BG), also RCs and PVB were sensitive to image noise.