We describe polarizing agents for use in enhancing NMR and MRI signals via dynamic nuclear polarization (DNP). The polarizing agents include two or more paramagnetic centers, preferably two paramagnetic centers. In a preferred embodiment, the polarizing agent comprises two nitroxide radicals tethered by a polyethylene glycol chain of variable length. Signal enhancements of up to 175 have been achieved in comparison with factors of .about.45 at similar concentrations of monomeric radical such as TEMPO.