Characterizing Hyperpolarized Xenon-129 Depolarization Mechanisms During Continuous-flow Spin Exchange Optical Pumping and as a Source of Image Contrast
Public Deposited- Last Modified
- March 21, 2019
- Creator
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Burant, Alex
- Affiliation: College of Arts and Sciences, Department of Physics and Astronomy
- Abstract
- Xenon-129 has become the isotope of choice for applications of hyperpolarized (HP) noble gases in magnetic resonance imaging (MRI) and spectroscopy due to its lower cost and higher availability compared to helium-3, relatively high tissue solubility, and wide range of chemical shifts. As the signal achieved in HP gas MRI is directly related to the nuclear spin polarization, the production of large volumes of highly polarized xenon-129 is paramount. While near unity levels of xenon polarization have been achieved in optical cells using stopped-flow spin exchange optical pumping (SEOP), continuous-flow SEOP is the most widely used method for clinical applications as it enables the production of large volumes of hyperpolarized gas, which are necessary for imaging applications in humans. However, polarization levels achieved via continuous-flow SEOP are well below the theoretically predicted values. In this dissertation, xenon-129 relaxation mechanisms that are often ignored during continuous-flow SEOP are investigated, through both simulations and experiments, to quantify their effects. First, computational fluid dynamics simulations are used to better characterize the SEOP process inside the optical cells. This work reveals turbulence inside the optical cell occurs at much lower flow rates than previously predicted. Turbulence leads to a wide distribution of xenon residency times in the cell, previously assumed to be constant for a given flow rate. This could be a cause for the discrepancy between the theoretical model for the final xenon polarization and the levels achieved experimentally. Then, the effect of diffusion-mediated depolarization of xenon-129 gas in magnetic field inhomogeneities during continuous-flow SEOP is determined. The results indicate that xenon diffusion in regions in which the magnetic field abruptly changes strength and direction can be a major source of depolarization during continuous-flow SEOP. As such, care should be taken in the design of the SEOP setup to avoid these gradients in the flow path of the HP gas. In the absence of such large magnetic field gradients, wall collisions remain the major contributing factor to gas-phase spin relaxation. Depolarization in magnetic field gradients can also be a source of image contrast for magnetic resonance imaging. To this end, the effects of longitudinal and transverse spin relaxation are separated and characterized for hyperpolarized xenon-129 diffusing near SPIONs using finite element analysis and Monte Carlo simulations. Simulations demonstrate that signal loss near SPIONs is dominated by transverse relaxation, with little contribution from longitudinal relaxation. In addition, experimental and computational work clearly show that the high diffusion coefficient of xenon does not provide appreciable sensitivity enhancement to SPIONs at the length scales typically probed by MRI. This work provides a better understanding of often-ignored relaxation mechanisms during continuous-flow hyperpolarization and will aid in the effort to bridge the gap between theoretical and experimental xenon polarization levels.
- Date of publication
- August 2018
- Keyword
- DOI
- Resource type
- Rights statement
- In Copyright
- Advisor
- Henning, Reyco
- Ng, Jack
- Clegg, Thomas
- Branca, Rosa
- McNeil, Laurie
- Degree
- Doctor of Philosophy
- Degree granting institution
- University of North Carolina at Chapel Hill Graduate School
- Graduation year
- 2018
- Language
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