Discovery st 16

The PET/CT system Discovery ST16 (GE Medical Systems, Powell, TN) was used. This system combines a high-speed ultra 16-detector-row (912 detectors per row) CT unit and a PET scanner with 10,080 bismuth germanate crystals in 24 rings with a 128 × 128 matrix. Crystal size 6.2 × 6.2 × 30 mm, coincidence window 11.7 nanoseconds, system sensitivity 9.3 cps/kBq in 3D mode, dispersion fraction 44%, maximum count rate in cps at 50% dead time 63 kcps @ 12 kBq/mL (3D), axial full width at half maximum (FWHM) 1 cm radius 5.2 mm in 3D mode, and axial field of view 157 mm.
Before and after FDG injection, hydratation (500 mL of iv NaCl) 0.9%) to reduce the pooling of the radiotracer in the kidneys was performed. All subjects had normal serum glucose level and fasted for at least 5 hours before the ¹⁸F FDG injection.^{12 (link)}

Whole body FDG-PET/CT was performed on a Discovery ST16 PET/CT scanner (GE Healthcare, Little Chalfont, Great Britain). The patients underwent a low-dose non-contrast-enhanced CT scan (120 kV, auto mA 20–100 mA, noise index 28, Pitch 1,375:1) with 3,75 mm slice thickness, matching that of the PET emission. After 4 hours of fasting the patients received an intravenous bolus injection of mean 185 MBq FDG (2 MBq/kg). Image acquisition began one hour after injection. A CT scan for attenuation correction was followed by PET emission, 3 minutes per bed position, from head to toe. Attenuation-corrected emission images were reconstructed with an iterative algorithm and the reconstructed images were recalculated to standardized uptake value (SUV) images, corrected for patient body weight and amount of injected FDG. The software that was used for ROI definition and SUVmax read out in this study was Carestream Vue PACS version 11.3.1.1 (Carestream Health, NY, USA). The tumors were delineated with circular regions of interest (ROIs) in the PET section measuring the SUV_max^{50 (link)}. The assessment of the PET examinations was performed by professor AS who has 27 years of experience. All measurements of SUVmax in both injected and not injected metastases were repeated three times and the results were similar.

In our hospital, PET/CT would be recommended if patients were diagnosed with N2-3 NPC by MRI or patients asked for this examination. Patients fasted for at least 6 h before the PET/CT scanning; patients with fasting plasma glucose > 2 g/L were excluded from this study. The PET/CT scanner (Discovery ST 16; GE Healthcare, Little Chalfont, United Kingdom) was employed to obtain PET/CT images according to the guidelines for tumor imaging with PET/CT [18 (link)]. PET/CT scans were initiated 45–60 min after injection of 5.55 MBq/kg ¹⁸F-FDG. Two doctors of nuclear medicine (X.Z. and X.P.L.) with more than 10-year experience at our center separately evaluated all small lymph nodes based on standardized uptake value (SUV) without reference to the MRI findings, and any disagreements were resolved by consensus. Usually, small lymph nodes with abnormal uptake of ¹⁸F-FDG were regard as metastatic lymph nodes. Notably, the diagnostic criteria of SUV for metastatic lymph nodes is about 2.5 [19 (link)].