Electron Paramagnetic Resonance Imaging

Abstract

Unpaired electrons can be measured by their absorption of electromagnetic radiation when the energy level separations established by a magnetic field match the energy of the incident electromagnetic radiation (usually microwave or radio frequency). The measurement method is called electron paramagnetic resonance (EPR). The EPR methodology can be continuous‐wave (CW), pulsed, or rapid‐scan EPR. Since the paramagnetic species are commonly not uniformly distributed in the material studied, the spatial distribution can be discerned by performing the EPR measurement in a magnetic‐field gradient, such that different parts of the sample resonate at different strengths of the applied magnetic field. Performing EPR in a gradient field and then constructing a spatial image from the acquired EPR spectra is called EPR imaging, sometimes abbreviated to EPRI. The method is analogous to MRI, but with several differences in implementation due to differences in spectral line widths, relaxation times, and unpaired spin concentrations. The goal of EPRI is to determine why the spins are where they are. Applications are as diverse as understanding physiology, measuring O 2 concentration in tumors, and determining the distribution of radiation‐induced defect centers in solid materials.