Two separate groups of patients will be recruited to each phase of the study. Each phase of the study will involve MR-ICPs. The MR-ICP is an extra 5-10 minute sequence at the end of a MRI that assesses the pressure in the head. The MRI will take scans of the brain during different phases of the cardiac cycle and will assess the volume changes while the patient is lying flat. The pressure in the head will be estimated based on these calculations. This estimate will be compared to the pressure in the head as determined by the ICP monitor.