An observational study which entails: medical history; physical examination; blood tests; MRI; microcirculation tests (two noninvasive tests: 1) a procedure that measures the resting blood flow of the skin and 2) a technique that introduces acetylcholine and sodium nitroprusside in the skin and measures the ability of the skin's vessels to dilate and increase blood flow); and macrocirculation tests (noninvasive ultrasound of arm). This study also involves 2 Visits to the Beth Israel Deaconess Medical Center in Boston, MA. (Joslin-Beth Israel Deaconess Foot Center, Lowry Outpatient Laboratory, MRI Suite).