Optimization of electromagnetic coupling to ceramic resonators for magnetic resonance imaging applications

Research into the properties of dielectric resonators can provide insight to potential applications in magnetic resonance imaging (MRI) technology as replacements for the radiofrequency (RF) coils used in current designs. Increasing the strength of the external magnetic field offers several advantages, including improved signal-to-noise ratio (SNR) and spectral resolution. However, this increase in field strength may require an alternative coil design as traditional RF coils have numerous difficulties at higher frequencies such as low quality factor. A potential solution may be to replace these coils with dielectric resonators. The objective of this research project is aimed at gathering information pertinent to dielectric resonators with various boundary conditions. Specifically, probe design and alternative coupling methods were investigated using network analyzers to provide insight into methods that could increase power supplied to the resonator, allowing it to generate strong RF magnetic fields within MRI equipment. By implementing a full loop probe design around the ceramic resonator, the effective power transmission was increased by 70.6% to -9.60 dB in the experimental design and by 75.7% to -15.6 dB when it was used in the MRI probe. However, this strong increase in signal transmission, made possible through the replacement of the original 12 mm diameter coil, had unintentional consequences in that the resonant frequency could be tuned to a minimum of 605 MHz instead of the ideal 600 MHz. In order to correct for this, a thin plate of CaTiO₃ was added to the side of the existing resonator disk to achieve a lower resonant frequency.