EPR Imaging of Free Radicals in Biological Systems

Abstract

After a decade of development and experimentation, the fields of electron paramagnetic resonance (EPR) spectroscopy and imaging (EPRI) have advanced to the point of enabling useful physiological and biochemical information to be obtained from living tissues (Ohno, 1987; Berliner, 1992; Eaton et al., 1991). The development of low-frequency EPR instrumentation at L-band, 1-2 GHz, or lower frequencies and lumped circuit resonators has made it possible to perform EPR measurements on these lossy samples. Previous in vivo or ex vivo EPR spectroscopy studies have focused on global measurements of free radical metabolism and measurements of tissue oxygenation. Studies that require measurements of the spatial distribution of free radicals within the sample (EPR imaging) can be performed utilizing magnetic field gradients in a manner similar to that of NMR imaging. EPR imaging, however, is faced with a number of technical problems, which make this technique more difficult to achieve in practice than that of NMR. The linewidths associated with EPR signals are three orders of magnitude larger compared to that of NMR signals, and hence EPR imaging requires 100 to 1000 times more powerful gradients. Further, the paramagnetic centers to be studied are present in submillimolar concentrations compared to more than 100 molar concentrations of water protons utilized in NMR imaging. In addition, the EPR absorption function of most stable paramagnetic labels contains multiple lines due to hyperfine splitting.