## 1 Introduction  
Gliomas are a type of brain tumor that arises from neuroepithelial tissue and make up about 28% of all brain tumors [1]. Depending on the originating cell type, gliomas are grouped as either astrocytomas or oligodendrogliomas. Furthermore, the World Health Organization (WHO) suggested a subdivision of tumor grades, namely grade I for the lowest and IV for the highest grade. To be specific, grades I and II are denoted as low-grade gliomas (LGGs), and grades III and IV are high-grade gliomas (HGGs). This classification has been in place since the last update in 2016 [2]. However, despite these categorizations, gliomas are heterogeneous regarding their histopathology, mutation type, and methylation. In particular, diffuse gliomas (grades II-IV) are currently subdivided based on their isocitrate dehydrogenase (IDH) 1 and 2 gene expression, methylation, i.e., O-6-Methylguanin-DNA-Methyltransferase (MGMT), and further parameters such as telomerase, tumor suppressor genes, and others [2]. Next to the tumor mass itself, edema is likely to surround the lesion, which results from the leakage of plasma that comes across the vessel wall into the parenchyma after disruption of the blood-brain barrier. Due to the accumulation of liquid inside the fixed system of the brain, which is mainly limited by the skull, such edema may result in increased intracranial pressure (ICP) and potentially lead to permanent neurological damage by compression of the brain tissue [3, 4]. For the diagnosis of gliomas and the surrounding edema, the gold standard method is stereotactic biopsy and/or sample collection during neurosurgery. Due to its invasiveness, however, such methods might not be able to be performed in all patients and bear various risks, such as infections. In order to non-invasively evaluate tumors and their progression, radiological and nuclear medicine imaging methods are available and clinically used. These include methods using ionizing radiation, such as positron emission tomography (PET) or contrast-enhanced computed tomography (CT). While the latter is generally not seen as a standard method, the combination of PET and CT is commonly used for glioma imaging. PET uses radioactive tracers that bind to specific targets (e.g., tumor cells) to visualize them [5; 6; 7]. This method, however, involves ionizing radiation from the tracer and the CT scanner as well as bears a risk from the injection of the radionuclide. Another method is Magnetic Resonance Imaging (MRI), which is generally considered the most important modality and not only includes the initial diagnosis but also the therapy planning and follow-up imaging [8]. Sometimes MRI is also used in combination with PET-CT and shows improved specificity (66-95.5%), sensitivity (81-90.05%), and accuracy (70.9-74.9%) [9]. Most often, structural imaging with T1 and T2 (FLAIR) image contrast is performed, commonly with the use of a contrast agent, to visualize the hypervascularized tumor tissue. Such structural methods are, however, limited, and therefore, advanced techniques are commonly considered to be added to current protocols for a more comprehensive analysis. These include but are not limited to diffusion-weighted imaging (DWI), dynamic susceptibility contrast imaging (DSC), and susceptibility-weighted imaging (SWI) as already described in the literature [10]. However, there is still a need to improve the available biomarkers that could allow for a safer diagnosis and could lead to the reduction of biopsies, which would be beneficial for patients due to its invasiveness. A promising method in this regard is Arterial Spin Labeling (ASL), which is a method that allows for non-contrast-enhanced perfusion imaging. ASL can be used for the diagnosis of various pathologies that alter brain perfusion, including strokes, arterio-venous malformations, and tumors [11; 12].