*2.3. 18F-FDG PET/MR Scan*

After PET/CT acquisition, all patients underwent a neck PET/MR (3.0 T, signa TOF-PET/MR, GE Healthcare, Milwaukee, WI, USA) scan around 120 min after 18F-FDG injection. Head and neck coil and field of view (FOV) were used for PET/MR scanning. The scanning sequence included axial fast spin echo T1 weighted imaging (FSE T1WI), axial, sagittal, and coronal FSE T2WI, and finally axial DWI. PET scanning used 3D acquisition, with about 15 min per bed. The scanning parameters were T1WI (turn angle = 142◦, echo time [TE]/repeat time [TR] = 13.2/640 ms, bandwidth = 41.67 KHz, FOV = 24 cm × 24 cm, matrix = 256 × 192), T2WI (turn angle = 142◦, echo time [TE]/repeat time [TR] = 161/3054 ms, bandwidth = 50 KHz, FOV = 22 cm × 22 cm, matrix = 288 × 288), DWI (echo time [TE]/repeat time [TR] = 191/2500 ms, bandwidth = 250 KHz, FOV = 24 cm × 24 cm, matrix = 96 × 96, b = 600). The time of flight (TOF) technique and OSEM algorithm were used to reconstruct the PET data with the following parameters: FOV = 30 cm × 30 cm, matrix = 192 × 192, filter cutoff = 3.0 mm, subset = 28, iteration = 3. PET attenuation correction was based on atlas MR attenuation correction combined with the Dixon water–fat separation method.
