RF Front End for a 4.1 Tesla Clinical NMR Spectrometer

With increased signal-to-noise, spatial, temporal, and spectral resolution, blood oxygenation level contrast, and other benefits of high field NMR, 4T NMR systems enhance the potential for using multinuclear imaging and spectroscopy for medical science and clinical diagnostics. A new NMR spectrometer is needed however. The magnet aside, the key difference between present clinical or animal systems, and 4T+ clinical systems is the RF front end. Including the power amplifier, transmit/receive (T/R) switch, preamplifier, and RF coils, the front end is required to operate at power levels, bandwidths, and circuit lengths unique in the NMR field. New technology has been developed for these components to optimize the performance of the spectrometer. To cover broader spectral bandwidths, and to compensate for chemical shift dispersion error, two 15kW solid state amplifiers have been developed for implementation on a “home-built” 4.1T clinical system. A stripline transformed, nonmagnetic, tuned GaAsFET preamp has been built for achieving high gain and low noise at the RF coil. A nonmagnetic dual quadrature hybrid-PIN diode T/R switch was developed to isolate the RF power amplifier from the receiver. New high frequency coils have made use of tuned cavities and transmission lines.