Clinical Role of Positron Emission Tomography/Computed Tomography Imaging in Head and Neck Squamous Cell Carcinoma PET Imaging of Oral Cavity and Oropharyngeal Cancers PET/Computed Tomography: Laryngeal and Hypopharyngeal Cancers 18F-fluorodeoxyglucose PET/Computed Tomography: Head and Neck Salivary Gland Tumors Positron Emission Tomography/Computed Tomography in Thyroid Cancer PET/CT: Nasopharyngeal Cancers PET/CT: Radiation Therapy Planning in Head and Neck Cancer 2-Deoxy-2-[18F] Fluoro-D-Glucose PET/Computed Tomography: Therapy Response Assessment in Head and Neck Cancer PET/Computed Tomography: Post-therapy Follow-up in Head and Neck Cancer