Clinical-Scale Hyperpolarization of 129Xe and 131Xe via Stopped-Flow Spin Exchange Optical Pumping
Date of Award
Doctor of Philosophy
The fundamental physics of spin-exchange optical pumping (SEOP) has been explained in detail by many brilliant scientists since its discovery in the 50’s and 60’s. Although some interactions remain only tenuously understood, mathematical relationships have been mapped across many trajectories with meticulous care. Despite these foundational descriptions, many of the larger scale dynamics remain capricious in practice, especially as SEOP strives to take advantage of rapidly developing laser technologies. This presents a difficulty for implementing the large-scale production of hyperpolarized gases that is required for clinical and some specific experimental applications. This research, performed over the past four years, was designed to shed light on some of the practical effects that become critical for scaling up production while keeping polarizations high, particularly in a “stopped-flow” polarizer environment. This dissertation is divided into eight main chapters. The first chapter is written to provide a historical context for the SEOP field and summarize the evolutionary stages that have led to current methodologies. The second chapter provides a brief summary of SEOP theory and mathematically outlines the transfer of quantum order from photon polarization, to electron polarization via optical pumping, and finally to long-lived nuclear polarizations via spin-exchange. Chapter 3 discusses the practical implementation of SEOP, and the specific designs and techniques used throughout this project to create and monitor polarization. Chapter 4 presents data with some unexpected trends collected by Dr. Nicholas Whiting and Dr. Peter Nikolaou using high densities of xenon and high resonant laser powers. This data inspired a set of simulations designed to locate the cause of these trends, and map the expected trajectory for further studies. Chapter 5 features a clinical-scale polarizer with 170 W of highly resonant cw laser power, capable of producing >0.8 L of hyperpolarized gas per SEOP cycle with 129Xe polarization values of ~30-90% (depending on the xenon density). Multidimensional data maps were created over various temperatures, gas mixes, and laser powers; the results are used to guide optimal performance and describe the conditions that cause SEOP to fail. Chapter 6 reintroduces helium into stopped-flow gas mixes to help mitigate the central difficulties found in Chapter 5 with thermal regulation, and discusses the improvements and difficulties observed as a result. Chapter 7 contrasts the tactics for high 129Xe polarization with the strategies that lead to high 131Xe polarization. Specifically this study is designed to assess whether 131Xe is capable of becoming polarized via SEOP to sufficient levels—and in sufficient amounts—to be used for some specific fundamental physics studies and other biomedical applications. Finally Chapter 8 presents a short proof of concept for the use of an aluminum optical pumping cell instead of the glass optical pumping cells predominantly used for SEOP.
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