 In this study, PET imaging studies of two structurally related 18F-hydroxyphenethylguanidines, 4-18Ffluoro-meta-hydroxyphenethylguanidine 18F4F-MHPG and 3-18Ffluoro-para-hydroxyphenethylguanidine 18F3F-PHPG will be performed. All studies will be performed in normal healthy volunteers. 