Electron spin resonance imaging: a functional imaging tool for biomedical science

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Modern medicine relies upon a variety of body scanners with which to visualise the internal structures of our bodies. Similar instruments are used by research scientists to study materials of significance across the whole range of the biological and medical sciences. For many purposes, however, anatomical or other structural information is not enough: we seek functional imaging methods that report on the processes that are taking place. Our project is concerned with the development of such a functional imaging method of exceptionally broad applicability: electron spin resonance imaging. It is the electrons furthest away from the atomic nucleus, the so-call 'valence' electrons, which confer most of the chemical and physical properties on atoms, ions, molecules and materials, since it is these electrons that interact most strongly with the environment. When an odd number of valence electrons are present (or in some special cases an even number) the electrons impart a net magnetic moment onto the atom/molecule/material. Electron spin resonance is a sensitive and precise method of measuring this magnetism. It is possible to relate these measurements to the environment of the valence electrons and hence use it to report on chemical structure and dynamics, and ultimately to provide functional information on chemical and other dynamical processes. Electron spin resonance determines the magnetic properties of electrons by measuring the applied magnetic field that is required to allow resonant absorption of microwave radiation. Spatial information is obtained by applying different magnetic fields to different parts of the object under investigation. If the microwave absorption is measured in a large number of magnetic field gradients a three-dimensional image of the magnetic properties of the object can be calculated. This overall approach is similar to that employed in nuclear magnetic resonance imaging (MRI). However, electron spin resonance imaging is technically much more challenging because valence electrons interact much more strongly with their environment than do nuclei. This important difference means that new, extremely sensitive and efficient, methods of measuring microwave absorption are required. Our project is focused on the necessary technologies. The sensitivity of an electron spin resonance imaging instrument should properly be expressed as the number of electrons that can be detected in a spatially resolved volume element in a given time. Thus an instrument with substantially improved sensitivity will also be able to make higher resolution images and/or faster measurements. Insufficient resolution and speed have been limiting factors in earlier approaches. The motivation behind our project is the extraordinary range of important applications to which ESR imaging can potentially be put. This work will enable substantial advances in our understanding of major human diseases such as cardio-vascular disease, cancer, diabetes, and septic shock, and also allow the efficacy of new therapies for these conditions to be assessed.