For radiotherapy treatment planning, it is essential to perform the MRI/CT exams in treatment position. For this purpose, thermoplastic immobilization masks are used for brain and head-and-neck radiotherapy. However, since standard immobilization masks do not fit in the diagnostic MR head/neck coils, suboptimal surface-loop coils leading to poor image quality are used in clinical practice. Here, we present a new immobilization setup. This setup has several advantages compared to state-of-the-art setups: it fits in the diagnostic head/neck MR coils, it allows diagnostic image quality in treatment position, high SNR, homogenous signal, restricted motion (about 1 mm) and accurate inter-fraction repositioning.