Comparison of hyperpolarization techniques for ultralow-field magnetic resonance

Abstract

Magnetic resonance (MR) studies are well established in numerous industrial, medical and scientific applications. Examples include MR spectroscopy (MRS), which is utilized for non-destructive chemical analysis, and MR imaging (MRI), which is a common noninvasive, medical imaging technique with great contrast in soft tissue. Conventional systems are bulky and expensive, because large magnet coils are utilized to generate high magnetic fields and signal amplitudes. The project presented in this thesis seeks to address these issues by combining the use of ultralow magnetic fields (ULF), with two signal enhancing hyperpolarization techniques. First, the experimental ULF-MR setup was established. It employs an open magnet coil assembly in combination with a superconducting quantum interference device (SQUID) as sensor, allowing for the quantitative measurement of the MR signal. Spectroscopic signal amplification by reversible exchange (SABRE) experiments and Overhauser dynamic nuclear polarization (ODNP) enhanced MRI showcased the successful implementation of these hyperpolarization techniques, and the imaging capabilities of the system. The results outline future applications and emphasize how the ultralowfield approach benefits from enhanced signal amplitude by hyperpolarization methods, while in turn facilitating the investigation and refinement of these techniques. Next, the simultaneous SABRE enhanced measurement of fluor and proton spins was performed. After investigating the influence of some measurement parameters on signal enhancement, correlation spectroscopy was utilized for a more detailed examination of the polarization transfer mechanisms. The studies demonstrated the capability of the system to perform multinuclear correlation spectroscopy experiments and the results are proof for the hyperpolarization of multiple-spin states by SABRE. The last part of this thesis focused on ODNP. With this technique, free radicals can facilitate an increase in nuclear spin polarization, enhancing the MR signal. Here, the polarization transfer efficacy of a broad range of nitroxide radicals was characterized. The comprehensive study allowed for a correlation of chemico-physical features with hyperpolarization-related properties. The results provide a catalog of polarizing agents and give direction for predicting and optimizing free radical performance in the future, especially for the development of functionalized polarizing agents. Reviewing the results allowed for a discussion of future utilization and direction of research for both hyperpolarization techniques. While possible applications differ greatly, they both share the prospect of profoundly enhancing MR signal and contrast in not only ultralow-fields but also in higher field regimes.