Abstract
Magnetic resonance imaging (MRI) is a widely used medical diagnostic technique that utilizes strong magnetic fields and radiofrequency (RF) waves to obtain anatomical details of the human body. Signal-to-noise ratio (SNR) is an essential parameter in MRI that substantially affects the image quality. Varied magnetic field strengths impose different requirements and engineering challenges on the RF system of the MRI scanner. The magnetic flux density (B1 field) produced by the RF coils plays a key role in the SNR of an MRI system and the imaging quality. In this PhD thesis, several approaches have been proposed to improve the RF coil sensitivity (B1 sensitivity). In particular, by optimizing the design of the RF coil itself and by introducing passive resonant structures in the space between the RF coil and a subject under a scan. In the first approach, a novel RF coil design was proposed that is based on a folded dipole geometry. It is able to produce a magnetic flux density over a large area of interest, similar to a loop coil, without the need for distributed lumped elements. In the second approach, a passive resonator based on a space-filling curve geometry was added between an RF coil and a subject under a scan to redistribute the B1 field. The proposed passive resonator offers a compact solution to improve the magnetic flux density of a traditional RF coil. Moreover, it leads to a relatively lower specific absorption rate (SAR) and a longer signal penetration depth. In the third approach, tuning capability was introduced to a spiral-based passive resonator with the help of a high permittivity liquid. The proposed passive resonator not only improves the magnetic flux density of a traditional RF coil but also offers a wide-range frequency tuning mechanism. In this thesis, it has been demonstrated through well designed numerical simulations and successfully validated by bench experiments, that the SNR can be improved with novel RF coil designs, as well as by altering the magnetic flux density of traditional RF coils with the help of passive resonators. This thesis has advanced the RF technology of MRI scanners in terms of the SNR enhancement and safety improvement. Frontier physical concepts have been reshaped towards the application for MRI. The techniques and approaches developed in this thesis contribute towards a sound performance improvement of existing high field systems, and towards the safety of ultra-high field systems.