To investigate whether phenylephrine and ephedrine causes different alterations in microcirculation and oxygenation, as measured with MRI and PET, in anesthetized patients with brain tumors. Using MRI and PET, the study will assess whether there is a difference in deoxyhemoglobin concentration (Bold signal), CTTH, cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) between ephedrine and phenylephrine