In vivo EPR: when, how and why?

This special issue is aimed at providing the readers of this journal with an indication of the exciting and important areas in which in vivo electron paramagnetic resonance (EPR) [or equivalently electron spin resonance (ESR)] is making contributions to experimental progress and to provide perspectives on future developments, including the potential for in vivo EPR to be an important new clinical tool. There also are many situations where the combination of in vivo EPR with NMR may be very synergistic. EPR (ESR) is a magnetic resonance-based technique that detects species with unpaired electrons. The technique has become a major tool in diverse fields ranging from biology and chemistry to solid-state physics. In the last few years, many publications have demonstrated that EPR measurements in living animals (in vivo EPR) can provide very significant new insights to physiology, pathophysiology and pharmacology. The most successful applications of in vivo EPR have been non-invasive measurements of oxygen, nitric oxide, bioradicals, pH and redox state, with applications in oncology, cardiology, neuroscience and toxicology. EPR also appears to be the method of choice for measuring radiation dose retrospectively, including the potential to do this in vivo in human subjects. While far from comprehensive, the reviews, original contributions and viewpoints provided in this issue by several leaders in the field of in vivo EPR should provide the readers with confirmation that in vivo EPR is an exciting field that is likely to provide very valuable complementary information for many NMR-based studies in experimental animals and, probably, also for clinical studies.