[super 2]H, [super 15]N-Substituted nitroxides a sensitive probes for electron paramagnetic resonance imaging

3-Carboxy-2,2,5,5-tetra([super 2][H.sub.3])methyl-1-(3,4,4-[super 2][H.sub.3],1-[super 15]N) pyrrolidinyloxyl is prepared for increasing the sensitivity of [O.sub.2] measurement by EPR imaging. The new isotopically substituted nitroxide has displayed increased sensitivity to changes in [O.sub.2] concentration, which has enabled accurate [O.sub.2] measurement in tissues by using electron paramagnetic resonance imaging.