Electron paramagnetic resonance spectrometer for three-dimensional in vivo imaging at very low frequency

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Abstract
The paper describes an electron paramagnetic resonance apparatus for spectroscopy and imaging at very low frequency (283 MHz). The bridge operates in a reflection cavity homodyne configuration and can be used on a very broad frequency range. The sample cavity is a one loop‐two gap resonator and accepts samples up to 5 cm in diameter and 10 cm long. These sample dimensions make the apparatus suitable for observing living samples such as rats. For the main field and gradients the apparatus uses a newly designed multipolar magnet that can provide the main field and gradients for 2D image reconstruction. The third gradient is obtained by coils mounted in the bore of the magnet. Tests on peroxilamine disulfonate samples have shown micromolar sensitivity. The minimum spatial resolution is limited by the sample linewidth and by the signal/noise. In living samples the toxicity of nitroxide radicals restricts the maximum label concentrations to values of the order of 10−4 M, and the resolution seems to be limited by sensitivity to fractions of cm3.