Basics of Optically Pumped MR
Rosa Tamara Branca1
1Physics and Astronomy, University of North Carolina at Chapel Hill, United States

Synopsis

Spin-exchange optical pumping (SEOP) of mixtures of alkali-metal vapors and noble gases can be used to efficiently polarize the nuclei of noble-gas atoms. Liters of noble gases at standard temperature and pressure can now be produced with nuclear spin polarization levels of several tens of percents. In this talk we will review the physics of the SEOP process, and then discuss our understanding on how experimental conditions affect final polarization levels.

Abstract

Spin-exchange optical pumping (SEOP) is a two-step process whereby angular momentum is transferred from resonant, circularly polarized laser light to the electronic spins of an alkali metal vapor (optical pumping). Spin polarization is subsequently transferred from the valence electrons of the optically pumped alkali-metal atoms to the nuclei of the noble-gas atoms (129Xe or 3He) via collisions (spin exchange)(1).

In the past 20-years, the production of large volumes of highly polarized noble gases has enabled a wide range of applications, from high energy physics to medical imaging. Despite progressive improvements in polarization, polarization levels by SEOP have remained well below the theoretical maximum. While some systems have demonstrated polarization levels that can approach values predicted by theory(2, 3), it is still not clear how system parameters drive the polarization levels.

In this talk I will give a broad overview of:
· The physics of the SEOP process (from the interaction of alkali atoms with circularly polarized laser light, to the spin exchange process for both 3He and 129Xe atoms).
· The experimental setups used for SEOP, in particular stopped flow vs. continuous flow pumping.
· The relaxation processes that occur during SEOP (alkali metal spin destruction processes, magnetic field inhomogeneities, wall collisions, self-collision relaxation processes, and the X-factor used to reconcile experimental polarization values with predicted theoretical values).
· The inter-play of different experimental conditions and how they affect final polarization level.

Acknowledgements

We gratefully acknowledge support from the National Institute of Health through grant R01-DK108231

References

1. Walker TG, Happer W (1997) Spin-exchange optical pumping of noble-gas nuclei. Rev Mod Phys 69(2):629–642.

2. Ruset IC, Ketel S, Hersman FW (2006) Optical pumping system design for large production of hyperpolarized Xe129. Phys Rev Lett 96(5). doi:10.1103/PhysRevLett.96.053002.

3. Nikolaou P, et al. (2013) Near-unity nuclear polarization with an open-source 129Xe hyperpolarizer for NMR and MRI. Proc Natl Acad Sci U S A 110(35):14150–5.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)