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 10 − 20 mm and length 5 − 15 mm tuned to frequency of 22 − 23.4 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 30 − 40 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