OXYGEN IMAGING IN PERFUSED HEARTS BY DYNAMIC NUCLEAR-POLARIZATION

The feasibility of localized oximetry by dynamic nuclear polarization (DNP) imaging is demonstrated on perfused sheep hearts. DNPI is a magn etic, double resonance technique, in which the electron paramagnetic r esonance (EPR) of a free radical dissolved in a perfusion medium is sa turated, while the nuclear magnetic resonance (NMR) of water protons i s used to perform conventional NMR imaging. The presence of oxygen red uces the enhancement of the NMR signal induced by DNP. The oxygen cont ent of sheep heart tissues was detected by the subtraction of the DNP image of the heart, perfused with a nitrogen-equilibrated solution, fr om an image obtained when the heart was perfused with an oxygen-equili brated solution. This result was obtained with extreme oxygen partial pressure, and the discussion presents physical and chemical means for improving the DNP imaging method. Physical means include field cycling , electron paramagnetic rotary saturation, and the use of a 180-degree s NMR pulse before EPR irradiation. The chemical means discussed are d euterium substitution in nitroxides and the potential use of solid, fr ee radical probes. It is suggested to use the perfused heart model for comparing the numerous methods available to measure the oxygen conten t of tissues.