An open volume, high isolation, radio frequency surface coil system for pulsed magnetic resonance

We present an open volume, high isolation, RF system suitable for pulsed NMR and EPR spectrometers with reduced dead time. It comprises a set of three RF surface coils disposed with mutually parallel RF fields and a double-channel receiver (RX). Theoretical and experimental results obtained with a prototype operating at about 100 MHz are reported. Each surface RF coil (diameter 5.5 cm) was tuned to f0=100.00+/-0.01 MHz when isolated. Because of the mutual coupling and the geometry of the RF coils, only two resonances at f1=97.94 MHz and f2=101.85 MHz were observed. We show they are associated with two different RF field spatial distributions. In continuous mode (CW) operation the isolation between the TX coil and one of the RX coils (single-channel) was about -10 dB. By setting the double-channel RF assembly in subtraction mode the isolation values at f1 or f2 could be optimised to about -75 dB. Following a TX RF pulse (5 micros duration) an exponential decay with time constant of about 600 ns was observed. The isolation with single-channel RX coil was about -11 dB and it increased to about -47 dB with the double-channel RX in subtraction mode. Similar results were obtained with the RF pulse frequency selected to f2 and also with shorter (500 ns) RF pulses. The above geometrical parameters and operating frequency of the RF assembly were selected as a model for potential applications in solid state NMR and in free radical EPR spectroscopy and imaging.