This is a single center study on healthy volunteers. The subjects will be seen by the study coordinator in the SCMRC to obtain written informed consent and to assess the co-variable data (vitals, medication, history, contraindications for MRI). The study nurse will schedule two visits for endothelial function assessment that are at least one but not more than three weeks apart. Both visits are to be at the same time of day. The initial volunteer will be assigned an endothelial function assessment with MRI first followed by FMD in the endothelial function lab. Subsequent volunteers will alternate assignment to FMD or MRI first to rule out pre-conditioning effects. Simultaneous to FMD assessment, plethysmographic pulse volume will be evaluated with finger probes on the index finger of each hand. Subjects will be studied in a temperature-controlled room with minimal background distraction and in a fasting state for the previous 4 hours. Subjects will also refrain from ingesting caffeine or nicotine in these preceding 4 hours. Each study day volunteers will undergo two endothelial function assessments. The first baseline assessment takes place after a 10 minute accommodation period. The volunteer is then subjected to a 15 minute upper arm tourniquet. The second assessment takes place 15 minutes after the release of the tourniquet. This so called 'ischemia reperfusion' is known to markedly impair endothelial function for approximately one hour without affecting the early bloodflow responses to reactive hyperemia. This effect is reversible and will help to determine the actual endothelial component of measured BOLD signal changes.