 PET scanning protocol On the morning of the study, patients will arrive at the Mayo Clinic PET Imaging Center in a fasting state. The use of medications will be ascertained and recorded. An intravenous cannula will be placed in each arm. The subject will then be positioned in the PET scanner . After optimal positioning of the left ventricle within the field of view, a transmission scan will be performed with either a germanium-68 or CT source for subsequent attenuation correction. The PET scanning sequence is outlined below. Because 11C-HED uptake is dependent on flow characteristics, a flow study will be performed using N-13 ammonia. N-13 ammonia 10 to 20 mCi will be injected over 20 seconds, and dynamic acquisition will be performed for 20 minutes with the following sequence:16 frames at 3 seconds, 10 frames at 12 seconds, and 2 frames at 240 seconds. Following a 50-minute period of N-13 ammonia decay, 11C-HED 20mCi will be injected over 30 seconds, and a dynamic acquisition will be performed with the following sequence: 6 frames at 30 seconds, 2 frames at 60 seconds, 2 frames at 150 seconds, 2 frames at 300 seconds, 2 frames at 600 seconds, and 1 frames at 1,200 seconds. To correct for 11C-metabolites in the blood activity, venous samples will be drawn at 0, 1, 5, 10, 20, 40, and 60 minutes total 25 ml of blood after the injection of C-11 HED. Heart rate, systemic blood pressure, and a 12-lead electrocardiogram will be obtained noninvasively with each peak isotope activity. 