Hyperpolarized and thermally polarized quadrupolar noble gas nuclei studied by nuclear magnetic resonance spectroscopy and magnetic resonance imaging
Stupic, Karl Francis
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This dissertation consists of several studies of two quadrupolar nuclei, 83Kr and 131Xe, with nuclear spin states of I = 9/2 and I = 3/2, respectively. These nuclei possess a nuclear electric quadrupole moment that strongly interacts with the surrounding electric field gradient (EFG). The quadrupolar interactions in these noble gas atoms dominate the longitudinal (T1) spin relaxation. To fully study these nuclei, high non-equilibrium nuclear spin polarization, referred to as hyperpolarization (hp), is generated using spin exchange optical pumping (SEOP). By employing this technique, enhanced nuclear magnetic resonance (NMR) signals many orders of magnitude above that of a thermally polarized (Boltzmann distribution of spin states) sample are possible and allow for experiments where signal averaging over long periods of time is prohibited (i.e. in vivo). The gas phase 83Kr T1 is shown to be sensitive to the surface composition/chemistry and the surface-to-volume ratio in an ideal system of closest packed glass beads. Understanding the behavior of 83Kr in these conditions allows for its development as a surface sensitive probe that could provide information in opaque porous media environments. Similar relaxation behavior can be observed in 131Xe; however, the quadrupolar interactions experienced by 131Xe also induce an observable splitting in the NMR spectrum. This quadrupolar splitting is extremely sensitive to surfaces during periods of adsorption as well as to the magnetic field strength when a 131Xe atom is present in the bulk gas phase. As the influence on the quadrupolar splitting can be more readily observed than the relaxation of either 83Kr or 131Xe, the observed splitting in 131Xe NMR can provide helpful insights into quadrupolar behavior experienced by both nuclei. To develop a better understanding of the quadrupolar behavior, both 131Xe quadrupolar splitting and 83Kr relaxation are explored as functions of magnetic field strength, gas phase composition and co-adsorbing species. In closing, improvements in polarization of 83Kr from line-narrowed diode array lasers as well as new delivery techniques have provided improvements that allow for the implementation of variable flip angle FLASH imaging sequence in an excised, intact rat lung. Additionally, initial evidence suggests the T1 of 83Kr can differentiate between the regions of the lung (the trachea, the bronchi and bronchioles, and the alveoli), which has potential as a diagnostic tool for the biomedical community. Improvements in signal intensity are needed to achieve in vivo studies, additional enhancements are possible through improved SEOP and by using isotopically enriched gases.