Despite of availability of a variety of clinical imaging modalities, nuclear medicine techniques (such as positron-emission tomography – PET, and single-photon emission tomography – SPECT) only could offer in vivo imaging at molecular level in human being (figure below). All nuclear medicine procedures begin with a radiopharmaceutical, radioisotope attached to targeting biomolecule.

Nuclear medicine becomes a vehicle for extending medicine from the cellular to molecular domain.
      Henry N. Wagner, Jr
No technique has revolutionized modern medicine more than the ability to see inside the human body and inside its cells.
     Chemical Engineering, July 25, 2005, 83(30), 25-34.