1. Radioimmune & Inorganic Chemistry Section, Radiation Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-1088, Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-1088, and Imaging Physics, Laboratory Animal Sciences Program, SAIC-Frederick, Inc., NCI-Frederick, Frederick, Maryland 21702