The aim of this study is to (i) Determine if tumor hypoxia can be accurately visualised with [18F] HX4 PET imaging in head and neck tumors (ii) correlate the [18F] HX4 PET images with blood and tissue markers and (iii) investigate the quality and optimal timing of [18F] HX4 PET imaging (iv) compare [18F] HX4 PET uptake with [18F] FDG PET uptake before and after treatment.