Abstract
For emerging biomedical applications of hyperpolarized xenon, the ability to obtain reliably high nuclear spin polarization levels is paramount. Yet, experimental nuclear spin polarization levels of xenon are highly variable and, more than often than not, well below what theory predicts. Despite rigorous and well-studied theoretical models for hyperpolarization and continuous-flow spin-exchange optical pumping (SEOP), there remains a substantial discrepancy between the theoretical and experimental polarization of 129Xe; inexplicably, seemingly similar experimental parameters can yield very different polarization values. In this paper, the validity of the assumptions typically made about the thermodynamic state of the Rb vapor inside the optical pumping cell and the gas dynamics are investigated through finite element analysis simulations of realistic optical pumping cell models, while in situ optical and nuclear magnetic resonance spectroscopy measurements are used to validate the results of the simulations. Our results show that shorter xenon gas residence times and lower Rb vapor densities than those predicted by empirical saturated vapor pressure curves, along with incorrect SEOP parameters, are the primary cause of the discrepancy between theoretical and experimental polarization values reported in the literature.
Highlights
The validity of the assumptions typically made about the thermodynamic state of the Rb vapor inside the optical pumping cell and the gas dynamics are investigated through finite element analysis simulations of realistic optical pumping cell models, while in situ optical and nuclear magnetic resonance spectroscopy measurements are used to validate the results of the simulations
While for a cell wall temperature of 110 C, the Killian formula predicts a density of 11:0 Â 1018 mÀ3 and an even higher density of 2:16 Â 1020 mÀ3 for a temperature of 165 C, our simulations predict a Rb density of 7:20 Â 1018 mÀ3 for a liquid Rb puddle of 15.7 mm2 located in the presaturation region
Our simulations and experimental measurements of Rb density inside the optical pumping cell show that, under experimental conditions typically used for continuous-flow spin-exchange optical pumping (SEOP) of Xe, the Rb density inside the main body of the cell is substantially lower than what is predicted by the Killian formula and other empirical formulas
Summary
Hyperpolarization of nuclear spins is used to increase the sensitivity of nuclear magnetic resonance (NMR) measurements by four to five orders of magnitude, enabling the detection of spin systems that otherwise would go undetected. Among all the nuclear spins that can be polarized, 129Xe presents unique features that make it an ideal probe for NMR investigations of materials and biological tissues, such as its inert nature and its high sensitivity to the chemical environment, which results in a range of chemical shifts 20-fold larger than other commonly used nuclei.2From its advent, the achievable polarization of 129Xe has increased tremendously, from less than 1% to a reported 70% for continuous-flow production and close to unity for stopped-flow production, enabling studies that were initially inconceivable. Yet, there remains a large discrepancy between the theoretically predicted and experimentally achieved 129Xe polarization for continuous-flow production. Hyperpolarization of nuclear spins is used to increase the sensitivity of nuclear magnetic resonance (NMR) measurements by four to five orders of magnitude, enabling the detection of spin systems that otherwise would go undetected.. Among all the nuclear spins that can be polarized, 129Xe presents unique features that make it an ideal probe for NMR investigations of materials and biological tissues, such as its inert nature and its high sensitivity to the chemical environment, which results in a range of chemical shifts 20-fold larger than other commonly used nuclei.. The achievable polarization of 129Xe has increased tremendously, from less than 1% to a reported 70% for continuous-flow production and close to unity for stopped-flow production, enabling studies that were initially inconceivable.. Hyperpolarized (HP) 129Xe is produced using a process called spin-exchange optical pumping (SEOP). First, the electronic spin of the valence electron of a mediating alkali metal, typically
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