A RADIOFREQUENCY ESR SPECTROMETER FOR INVIVO IMAGING

A radiofrequency ESR spectrometer suitable for in vivo studies is described. The spectrometer utilizes a large loop-gap resonator, 10 cm in diameter and 15 cm in length, with an active volume of about 300 ml. This can be tuned to resonate at exactly 300 MHz for a wide range of samples and enables a fixed frequency source with high spectral purity to be used, minimizing noise demodulation. Automatic tuning, coupling, and phase control systems have been incorporated to compensate for motional artifacts in subsequent experiments using animals. Sensitivity is compared to that obtained using an aqueous cell at X band. For aqueous nitroxides, the minimum detectable concentration is about 2 × 10-7 M (sample volume - 200 ml, time constant = 0.01 s). The effects of physiological salt solution on sensitivity and RE depth penetration are reported and discussed. Coils for producing magnetic field gradients in three dimensions are also described and preliminary imaging results are shown. © 1991.