RF and Electronic Design Perspective on Ultra-High Field MRI systems.

摘要

Magnetic Resonance Imaging (MRI) is one of the most sophisticated technologies to produce detailed images of the human body's organs and structures. There were many items of pioneering work in MRI over the last few decades. However, many challenges still remain.;This dissertation includes two studies concerning the challenges: the fully automatic matched / tuned RF head coil design and RF coil design with improved B1 field uniformity. Both have been being considered as critical problems at ultra-high field (7T and beyond) MRI systems.;The first study aims to build a real-time, high-speed, electrically controlled, and fully stand-alone system of the impedance matched / frequency tuned RF coil that is applicable and compatible to an existing MRI system. To achieve this project, subjects including the basic background of Nuclear Magnetic Resonance (NMR) physics, MRI systems, RF/analog circuit theory. have been studied and presented in this dissertation. The 8-channel RF head coil was successfully built and tested. The fully automatic tuning and matching RF coil offers fast operation (less than 550ms per each channel), accurate impedance matching / frequency tuning (less than -20dB in the reflected coefficient, S11, at the Larmor frequency) resulting in the high power efficiency (4%∼21% improved at each channel), and higher Signal-to-Noise Ratio (SNR) in MR images (about 3% in the whole object region).;In the second study, the double trapezoidal-like shape along the length of the microstrip resonator is proposed to obtain the gradual impedance variation and flatten the near field distributions. The conventional and proposed 8-channel volume coils have been built and investigated with a phantom in 7T MRI scanner. The results show successful flat field distributions with about 35% increased local transmission magnetic field strength at the ends of the RF coil as well as about 13% improvement at its center. The quality factor ratio between unloaded and loaded is also increased about 45% (from 1.46 to 2.13) compared to the conventional structure.;The proposed and demonstrated approaches are a meaningful step in order to overcome difficulties at ultra-high field MRI systems, and one critical contribution to the area.