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Theoretical models for continuous-flow and stopped-flow spin-exchange optical pumping of 129 Xe have long predicted much higher 129 Xe polarization values than are measured experimentally, leading to a search for additional depolarization mechanisms. In this work, we show that a misapplication of the general theory of spin-exchange optical pumping along with the incorrect use of previously measured spin-exchange constants has been perpetuated in the past 20 years and is the main cause of the long-held discrepancy between theoretical and experimental 129 Xe polarization values. Following the standard theory of spin-exchange optical pumping developed almost 40 years ago by Happer et al., we outline the common mistake made in the application of this theory in modern theoretical models and derive a simplified expression of the spin-exchange cross section that can be used to correctly predict 129 Xe polarization values under any set of experimental conditions. We show that the complete expression of the spin-exchange cross section derived using the work of Happer et al. predicts spin-exchange rates tenfold higher than those previously assumed in theoretical models of continuous-flow and stopped-flow spin-exchange optical pumping and can fully rectify the long-standing discrepancy between theoretical and experimental polarization values. [ABSTRACT FROM AUTHOR]

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 129 Xe; 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. [ABSTRACT FROM AUTHOR]

Purpose: To compare the effect of superparamagnetic iron oxide nanoparticles (SPIONs) on the T1 of 129Xe and 1H and to measure the relaxation of 129Xe in blood at low and high magnetic field strengths. Methods: 129Xe and 1H T1 relaxometry was performed at low‐ and high‐field strengths in samples containing different SPION concentrations, while imaging was used to compare the contrast obtainable in these two field regimes. In vivo experiments at variable field strengths were performed to determine the depolarization of 129Xe in blood and the feasibility of in vivo dissolved‐phase spectroscopy and imaging at low field. Results: The SPION relaxivity was substantially greater at low field for 1H, increasing from 0.92 ± 0.06 mM s−1 at 11.7T to 31.5 ± 1.8 mM s−1 at 0.6 mT, and for 129Xe, which increased from 0.13 ± 0.03 mM s−1 at 11.7T to 7.32 ± 0.71 mM s−1 at 2.1 mT. The additional MR signal loss increased from 0.7% at 9.4T to 20.6 ± 4.2% at 0.6 mT for 1H and from −0.7 ± 3.4% at 9.4T to 12.7 ± 3.5% at 2.1 mT for 129Xe. Blood was found to depolarize 129Xe below 3T in a manner inversely proportional to the field strength. In vitro studies at 2.1 mT suggest 129Xe relaxation times below 5 s in blood dilutions as low as 0.4% volume. Conclusion: SPIONs longitudinal relaxivity increases at low field both for 1H and 129Xe. The depolarization of xenon in blood, which is found to increase below 3T, effectively prevents in vivo dissolved‐phase spectroscopy and imaging at low‐field strengths. [ABSTRACT FROM AUTHOR]

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Purpose: To measure dissolved‐phase 129Xe T1 values at high and low magnetic fields and the field dependence of 129Xe depolarization by hollow fiber membranes used to infuse hyperpolarized xenon in solution. Methods: Dissolved‐phase T1 measurements were made at 11.7T and 2.1 mT by bubbling xenon in solution and by using a variable delay to allow spins to partially relax back to thermal equilibrium before probing their magnetization. At high field, relaxation values were compared to those obtained by using the small flip angle method. For depolarization studies, we probed the magnetization of the polarized gas diffusing through an exchange membrane module placed at different field strengths. Results: Total loss of polarization was observed for xenon diffusing through hollow fiber membranes at low field, while significant polarization loss (>20%) was observed at magnetic fields up to 2T. Dissolved‐phase 129Xe T1 values were found consistently shorter at 2.1 mT compared to 11.7T. In addition, both O2 and Xe gas concentrations in solution were found to significantly affect dissolved‐phase 129Xe T1 values. Conclusion: Dissolved‐phase 129Xe measurements are feasible at low field, but to assess the feasibility of in vivo dissolved‐phase imaging and spectroscopy the T1 of xenon in blood will need to be measured. Both O2 and Xe concentrations in solution are found to greatly affect dissolved‐phase 129Xe T1 values and may explain, along with RF miscalibration, the large discrepancy in previously reported results. [ABSTRACT FROM AUTHOR]