Design and Optimise a GARField MRI Resonator

Abstract

The design of a Nuclear Magnetic Resonance (NMR) sensor coil for a GARField NMR system was examined. The target design has a diameter about  mm and length  mm tuned to frequency of  MHz at 50 Ω total impedance. Nine different sets of coils were built with different numbers of turns (3, 5, and 7) and different thickness of wire to vary the wire resistance. The report was to examine based on the design parameters the best resonant circuit for a GARField MRI system. The acquired tuning characteristics from these resonant circuits were interpreted using MATLAB scripts and Excel spreadsheet to compare each coil with already existing theory of resonators. This was achieved by matching each resonant circuit using a match and tuning capacitor to the required frequency (22-23.4 MHz) and to 50 Ω total impedance at resonance. It was found that there is no easy method to estimate the inductance of the coil of wire. The result for the experimental inductance was found to be 0.5 µF and resistance of 0.4 Ω for a medium coil of wire with 5 numbers of turns, diameter of 0.45 and length of 0.7 mm. The initial attempt to fit the experimental data to that of the theory failed due to the absence of stray capacitance in the theory. However, when stray capacitor with value ranging between  pF was incorporated in parallel with the tank circuit, it was found that both the experiment and theory fit as expected. Three coils were tested in the NMR laboratory using a GARField spectrometer to examine the best coil that will be suitable for NMR experiment. Coils were compared on the basis of signal to noise ratio (SNR) and P90 pulse length. It was found that medium coil of wire with 3 number of turns has the biggest SNR of 177 which is good for NMR procedures. On the other hand, coil with 5 numbers of turns has the shortest P90 pulse length of 2.0 µs which is good for spatial resolution. At all rate, this research have shown how theories are verified through experiment.