• Compounds are generally chemically equivalent or close analogs to naturally occurring compounds.
  
• They provide functional images of the human body.
  
• Many PET radiopharmaceuticals are radiolabeled versions of substances commonly present in the body, e.g., ¹³N-ammonia, ¹⁵O-water, ¹¹C-acetate, ¹¹C-methionine, ¹⁸F-fluoride.