The purpose of this research study is to evaluate the use of PET scanning (Positron Emission Tomography) to localize a neuroendocrine tumor (such as carcinoid, pheochromocytoma, medullary carcinoma of the thyroid, or other types) with a radiotracer.  A neuroendocrine tumor is a type of tumor that produces and secretes chemical substances that can make you feel sick. The neuroendocrine tumor may cause symptoms such as diarrhea and/or redness of the skin (flushing) or high blood pressure in some patients with pheochromocytoma, or other symptoms according to the type of neuroendocrine tumor.  A radiotracer is a radioactive chemical that is injected into the bloodstream in very small amounts and will serve as an indicator of the presence of the tumor.