Novel radio frequency resonators for in vivo magnetic resonance imaging and spectroscopy at very high magnetic fields.

摘要

Since the discovery of nuclear magnetic resonance (NMR) in 1946, the magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) technique has evolved to one of the most powerful tools for diagnosing and studying human diseases non-invasively. Due to the superior intrinsic signal-to-noise ratio (SNR) of high magnetic fields, the field strength of human MRI system has been pushed from less than 1 Tesla to 4 Tesla, even to today's newly developed 7 Tesla (University of Minnesota at Twin Cities) and 8 Tesla (Ohio State University at Columbus). To realize this high field advantage in human MR studies, design of efficient high-frequency RF coil, a key device in the MR system, is demanded. The performance of RF coils, especially large volume coils for human head or body MR imaging is essential to the success of the high field human MR imaging and spectroscopy. A number of problems related to RF coil designs such as decreased RF penetration, pronounced dielectric resonance effect, ohmic losses, electromagnetic radiation losses and B, field inhomogeneity arises at high fields. The traditional RF coils operating at low frequencies for low field MR applications apparently appear limitations at high fields. All these problems call for new RF coil designs for high field MR studies.; This dissertation investigates these high-field RF coil problems and solutions. A new RF coil design concept using microstrip transmission line (MTL) is developed for the use of in vivo MR applications at high fields. Various RF coil designs using this concept for human and animal MRI/MRS at high magnetic field strengths of 4T, 7T, 9.4T and 28T are successfully developed and validated. The outcomes from this thesis work provide an efficient solution for designing high-frequency RF coils for high-field MR applications. In addition, an implanted micro coil with a region-defined (REDE) B1 field for determining input function of rat carotid artery with high temporal resolution is also introduced. This REDE implanted micro coil provides a new and efficient means in the cerebral metabolic rate of oxygen utilization (CMRO₂) studies.