13] In Vivo electron paramagnetic resonance imaging of skin

Abstract

Electron paramagnetic resonance imaging (EPRI) is a unique method for visualizing the spatial distribution of paramagnetic centers, for example, free radicals. The natural free radical concentration in biological systems is usually too low to give signal intensity sufficient for detection by EPRI. The paramagnetic species observed to date are nitroxide free radicals, but new labels with narrow linewidths and trapping capabilities might be used to image the generation of free radicals. One of the driving forces for the development of EPRI is the potential use of nitroxide free radicals. Nitroxides are sensitive to motion, polarity, structural order and fluidity, oxygen tension, redox processes, bioenergetic factors, and the chemical reactivity of their ultimate biophysical surroundings. In the biomedical field, interest has been directed toward in vivo measurements. Application of the EPRI technique to intact animals and humans is limited by two factors. The first is that the conventional EPR frequency of 9 GHz does not permit the use of large aqueous samples, because of the high dielectric loss. Use of low microwave frequencies (0.5–3.0 GHz) solves this problem. The second limiting factor is that stable concentrations of nitroxide radicals are difficult to achieve in tissues, because of nitroxide metabolism and diffusion of the probes. Using highly biostable nitroxides or encapsulating nitroxides in liposomes helps overcome this difficulty.