This is because these tumors disrupt the normal functioning of the BBB and lead to an increase in its permeability. More recently, advancements have been made to increase the utility of MRI in providing physiological data that can help to inform diagnosis and prognosis. Perfusion Weighted Imaging (PWI) and Diffusion Weighted Imaging (DWI) are two MRI techniques that reviews have been shown to be useful in classifying tumors by grade, which was not previously viable using only structural imaging. However, these techniques cannot alone diagnose high- versus low-grade gliomas, and thus the definitive diagnosis of brain tumor should only be confirmed by histological examination of tumor tissue samples obtained either by means of brain biopsy or open surgery. The histological examination is essential for determining the appropriate treatment and the correct prognosis. This examination, performed by a pathologist, typically has three stages: interoperative examination of fresh tissue, preliminary microscopic examination of prepared tissues, and follow-up examination of prepared tissues after immunohistochemical staining or genetic analysis.