This research group has developed an acoustic coupling fluid intended for use in the resection cavity instead of saline. Tests in laboratory measurements have shown that the fluid reduces artifacts and has the potential to enhance ultrasound image quality in brain tumor surgery. Three different concentrations of the acoustic coupling fluid have been tested in a phase 1 study that included 15 patients with glioblastoma. The concentration that provided the optimal ultrasound images, from qualitative and quantitative inspection, is used in the current phase II study. This study is a randomized controlled trial aiming to include 82 patients with glial brain tumours. Its purpose is to test the fluid during surgery of glial brain tumours to further investigate safety and efficacy.