Active coil isolation in NMR imaging and spectroscopy using PIN diodes and tuned transmission line: a practical approach

To improve signal-to-noise (S/N) ratios in biological NMR experiments we have regularly employed close-fitting receiver coils. The poor RF (radio-frequency) homogeneity often exhibited by these coils can be partly overcome by using them with large transmitter coils, provided that good between-coil isolation during the RF transmission and receive periods is achieved. With this in mind, we have used combined PIN diodes and tuned line to isolate transmitter and receiver and to remove transmitter noise. A series of experiments reported here demonstrate (a) distortion-free receiver detuning during free-induction decay, (b) the reduced effect of the receiver coil on the transmitter pulse, (c) an increase in S/N from 71:1 to 158:1, and (d) the effectiveness of transmitter noise isolation. Improvements in S/N, isolation, and image homogeneity illustrate the value of utilizing these devices. Hardware to allow PIN diode switching under computer control is described, utilizing mostly nonmagnetic materials and batteries.