*2.2. 18F-FDG PET/CT Acquisition and Interpretation*

18F-FDG PET/CT scans were acquired after at least 6 h of fasting and with blood glucose levels below 150 mg/dL. An activity of 3.5–4.5 MBq/kg of 18F-FDG was intravenously administrated to the patients 1 hour before images acquisition. Images were acquired from the base of the skull to the mid-thigh. All of the patients were instructed to void before the PET/CT acquisition and no type of oral or intravenous contrast agents were given for the execution of the scan. Similarly, none of the patients had performed any intestinal preparation.

In our study, we made use of 2 different PET/CT tomographs: the first (scanner 1) was a Discovery 690 PET/CT (General Electric Company-Milwaukee, WI, USA) while the second (scanner 2) was a Discovery STE PET/CT (General Electric Company, Milwaukee, WI, USA). On both of them standard acquisition parameters (CT: 80 mA, 120 Kv without contrast; 2.5–4 min per bed- PET-step, axial width 15 cm) and standard reconstruction parameters were used (256 × 256 matrix and 60 cm field of view).

Furthermore, scanner 1 was characterized by the presence of LYSO (cerium-doped lutetium yttrium oxyorthosilicate) scintillator crystals with a decay time of 45 ns, while scanner 2 had BGO (bismuth germanate) scintillator crystals with a decay time of 300 ns. Scanners were not harmonized with a cross-calibration program and all PET/CT scans were acquired at free-breath, instructing the patients to have regular breathing. For both scanners, a low dose CT at free breathing and without contrast agent was acquired in order to perform attenuation correction and for anatomical correlation. In particular, CT acquisition parameters for scanner 1 were: 120 kV, fixed tube current ≈ 60 mAs (40–100 mAs), 64 slices × 3.75 mm and 3.27 mm interval, pitch 0.984:1, tube rotation 0.5 s. CT acquisition parameters for scanner 2 were: 120 kV, fixed tube current ≈ 73 mAs (40–160 mAs), 4 slices × 3.75 mm and 3.27 mm interval, pitch 1.5:1, tube rotation 0.8 s. Furthermore, on scanner 1 time of flight (TOF) and point spread function (PSF) algorithm were used for the reconstruction of images, with filter cut-off 5 mm, 18 subsets and 3 iterations. Again, on scanner 2, an ordered subset expectation maximization (OSEM) algorithm with filter cut-off 5 mm, 21 subsets and 2 iterations were used.

PET images were visually and semiquantitatively analyzed by a nuclear physician with at least 10 years of experience, measuring parameters of TIs: the maximum standardized uptake value corrected for body weight (SUVmax), mean SUV corrected for body weight (SUVmean), maximum standardized uptake value lean body mass (SUVlbm), maximum standardized uptake value body surface area (SUVbsa), MTV and TLG. SUVrelated parameters were measured on a Xeleris 3.1 GE workstation. MTV was calculated by drawing a volume of interest (VOI) on TIs on 18F-FDG PET/CT images corrected for attenuation, using a SUV-based automated contouring program (Advantage Workstation 4.6, GE HealthCare) with an isocounter threshold method based on 41% of the SUVmax, as previously recommended by the European Association of Nuclear Medicine (EANM) because of its high inter-observer reproducibility [25]. TLG values were calculated as the product of the MTV of the VOI for its SUVmean.
