Commentary: Electron spin resonance imaging studies of biological systems

Electron spin resonance (ESR) was first demonstrated in 1945, the same year that the first NMR experiments were carried out. The earliest publication on twodimensional ESR imaging (ESRI) appeared in 1979, around the same time as the first good quality, whole-body NMR images were presented. Fundamentally, ESR and NMR differ only in the fact that one method involves a magnetic resonance experiment on the unpaired electron, while the other uses the atomic nucleus. Why, then, have the rates of progress in the two fields diverged so significantly? Many thousands of MRI machines are now installed worldwide, and there are several hundred active research groups developing new hardware, methodologies and applications. In contrast, there are less than 20 research groups worldwide working on ESRI for biological and medical applications. Is it a lost cause, or is its day still to come? This article will summarise the current status of ESRI and related techniques, and will indicate how, in the author's opinion, the field is likely to develop over the next decade.