In this study, the invesigators look at how a new MRI technique (called amide proton transfer weighted (APTw) chemical exchange saturation transfer (CEST)) can improve treatment for brain tumors through early detection of tumor progression after radiotherapy and/or chemotherapy treatment. An issue with the current treatment for patients with a brain tumor, is the inability to detect tumor progression early. An abnormality is seen on MRI scans taken shortly after treatment with radiotherapy in about 30% of patients. This abnormality may be a sign that radiotherapy treatment has worked well and will disappear on its own after a while. However, an abnormality can also be a sign of active tumor tissue. Then it shows that the treatment has not worked well enough. When there is active tumor tissue, this is called 'tumor progression'. When there is an abnormality that disappears on its own after a while, there is no active tumor tissue. This is called 'pseudoprogression'. Currently, there are two options to determine whether tumor progression or pseudoprogression has taken place: Do another brain surgery to see if the abnormality contains active tumor tissue or wait and have regular MRI scans until it is clear whether the abnormality goes away on its own. Amide proton transfer weighted chemical exchange saturation transfer (APTw-CEST) imaging is a new MRI technique in which investigators can produce images that show the accumulation of protein. APTw-CEST has previously been shown to be able to distinguish tumor progression from pseudoprogression earlier than current standard MRI scans. However, these previous studies have drawbacks: they were either done with a small group of patients in 1 hospital, or with a 7 Tesla MRI scanner, a rare type of scanner not standard in hospitals, or done in animal models. In this research project the investigators now want to prepare APTw-CEST for standard patient care for patients with glioblastoma in the Netherlands so that in the future APTw-CEST can be included as a standard scan during treatment. The investigators are doing this by introducing APTw-CEST MRI on the clinical MRI scanners of four different hospitals. Ultimately, the investigators want to use this to create national guidelines for measuring and viewing APTw-CEST MRI images for early detection of tumor progression.