On regular (diagnostic) MRI images brain tumors can show "contrast enhancement": uptake of an intravenously administered contrast agent can cause an enhancement pattern that is seen as a white area on a frequently used MRI protocol ("T1 weighted imaging"). High grade gliomas are a common brain tumor that share this enhancement pattern. The goal of surgery is to resect this contrast enhancing part without causing additional neurological damage. Intraoperative MRI (iMRI) is a helpful tool in achieving this goal, because it can provide updated images during resection and correct for deformations that occur in the brain during surgery. These deformations make preoperative images that are used for standard neuronavigation systems less reliable. However, due to manipulations during surgery, the contrast uptake during surgery may differ from contrast uptake in diagnostic MRI. This study aims to relate contrast enhancement on iMRI and tumor characteristics on tissue samples from the tumor. When the neurosurgeon considers the resection of the high grade glioma to be complete, an iMRI scan will be made, and tissue sampling will be performed on the borderzones of the tumor or tumor resection cavity respectively. This will provide insight in the relation between contrast enhancement on iMRI and the presence of tumor tissue. Such knowledge is important to improve effectiveness and safety of iMRI guided brain tumor resection.