Cyclen is a macrocyclic aza analogue of the crown ether 12-crown-4. Cyclen compounds are capable of selectively binding cations and are used as a ligand with chemicals used in MRI contrast (as well as other imaging) agents.;Cyclen is a macrocyclic aza analogue of the crown ether 12-crown-4. Cyclen compounds are capable of selectively binding cations and are used as a ligand with chemicals used in MRI contrast (as well as other imaging) agents.A macrocyclic aza analogue of the crown ether 12-crown-4