It is an object of the invention to address the above mentioned issues related to image quality and patient positioning. This object is achieved by a mock-up antenna configured to be used during radiation treatment delivery, wherein the radiation treatment is delivered based on a radiation treatment plan and wherein the radiation treatment plan is at least partly based on a planning magnetic resonance image. The mock-up antenna is substantially transparent to radiation and comprises connection means configured to allow a connection between the mock up antenna and a fixation means, which fixation means is configured to fixate a position of the mock-up antenna during radiation treatment and. The mock-up antenna further comprises an inner surface configured to be positioned towards a patient in a way such that is affects a position and/or orientation of the patient during radiation treatment delivery, wherein the inner surface has a shape substantially similar to a shape of a working magnetic resonance imaging antenna used during an acquisition of the planning magnetic resonance image.