Technical note: a moderately sized resonator/surface coil for radiofrequency electron paramagnetic resonance spectroscopy and imaging in vivo.

A resonator for the radiofrequency (250 MHz) electron paramagnetic resonance spectroscopy (EPRS) and imaging (EPRI) of biological samples is described. It has been designed for use with a 0.5 m bore EPR spectrometer. It is of the double split-ring type, 65 mm in diameter and 100 mm in length, and is equipped with a tuning-lock mechanism for fixed frequency operation. It can be used in the whole-body mode for small animals and as a surface coil for larger samples. The extent of sensitivity in the latter mode is ca. 20 mm. Larger resonators are being developed for whole-body human studies. A phase-locked crystal oscillator frequency source with high spectral purity is used to minimize noise demodulation and automatic tuning; coupling and phase controls have been included to compensate for motional artefacts. For purely aqueous nitroxides, the minimum detectable concentration is ca. 4 x 10(-7) M using an internal sample 100 ml in volume. Sensitivity in the surface coil mode is discussed and spectra from phantoms using both physiological saline and a human volunteer to induce realistic conduction losses are shown.