MRI and PET measurements are performed after induction of general anesthesia with propofol and remifentanil. The MRI or PET sequence is then performed before administration of study-drug and again after the infusion of either phenylephrine or ephedrine results in MAP > 60mmHg. Depth of anesthesia is monitored with BIS in both studies to ensure equal level of anesthetic depth in the two groups. NIRS is monitored in both studies to compare measurements of brain oxygenation determined by PET/CT. NIRS measurements are not possible during MR scan and will be measured either before or after the scan. BIS and NIRS are registered in each patients case report form (CRF). After the MRI/PET study the patient is transported to the operating theatre and the craniotomy is performed. After removal of the bone-flap, subdural ICP is measured and recorded as previously described.