EPR Imaging: Progress and Prospects

In this brief article we provide an assessment of the state-of-the-art in EPR imaging, and suggest some of the future directions. Several reviews, including two by Ohno (1,2) and two by us (3,4) comprehensively discuss the literature. Here we provide an overview of published work (1-51), and unpublished results presented at the 10th International EPR Symposium in August 1987 (52-64). Only a very few papers had appeared by the time of the first symposium on EPR imaging, in August 1984. At that time experiments, some unpublished, had been done by Lauterbur, Berliner, Freed, and Eaton in the US, Hoch in South Africa, Ohno in Japan, Lebedev in the USSR, and Karthe and Ewert in East Germany. Now there are many more research groups involved in EPR imaging, including one researcher who used to say that there were "only two problems with EPR imaging: there is nothing to image and there is no way to do it." Most reports of EPR imaging have emphasized making pictures of objects. There has been an implied concept that the dimensions of interest were the three cartesian dimensions of the laboratory coordinate system. The time-dependence of an image has been used to study chemical kinetics and diffusion. Recent developments have encompassed a wider range of dimensions including g-values, hyperfine splitting, relaxation times Ti and T2i and the microwave magnetic field distribution. The EPR imaging literature is summarized in Table I.