Radioligand

A radioligand is a radioactive biochemical substance (in particular, a ligand that is radiolabeled) that is used for diagnosis or for research-oriented study of the receptor systems of the body.
In a neuroimaging application the radioligand is injected into the pertinent tissue, or infused into the bloodstream. It binds to its receptor. When the radioactive isotope in the ligand decays it can be measured by positron emission tomography (PET) or single-photon emission computed tomography (SPECT). In "in vivo" systems it is often used to quantify the binding of a test molecule to the binding site of a radioligand. The higher the affinity of the molecule the more radioligand is displaced from the binding site and the increasing radioactive decay can be measured by scintillography. This assay is commonly used to calculate the binding constant of molecules to receptors.
The transport of the radioligand is described by receptor kinetics.
History.
Radioligands are credited with making possible the study of biomolecular behaviour, a previously mysterious area of research that had evaded researchers. With this capacity radioligand techniques enabled researchers to identify receptor devices within cells.
Radioactive isotopes commonly used.
In PET the isotopes fluorine-18, carbon-11, and copper-64 are often used in molecular imaging.
List of radioligands.
Radioligands may be constructed to bind selectively to a particular neuroreceptor or a particular neurotransmitter transporter. 
Examples of radioligands include: 