Characterizing and modeling the efficiency limits in large-scale production of hyperpolarizedXe129

Abstract

The ability to produce liter volumes of highly spin-polarized 129Xe enables a wide range of investigations, most notably in the fields of materials science and biomedical MRI. However, for nearly all polarizers built to date, both peak 129Xe polarization and the rate at which it is produced fall far below those predicted by the standard model of Rb metal vapor, spin-exchange optical pumping (SEOP). In this work, we comprehensively characterized a high-volume, flow-through 129Xe polarizer using three different SEOP cells with internal volumes of 100, 200 and 300 cc and two types of optical sources: a broad-spectrum 111-W laser (FWHM = 1.92 nm) and a line-narrowed 71-W laser (FWHM = 0.39 nm). By measuring 129Xe polarization as a function of gas flow rate, we extracted peak polarization and polarization production rate across a wide range of laser absorption levels. Peak polarization for all cells consistently remained a factor of 2-3 times lower than predicted at all absorption levels. Moreover, although production rates increased with laser absorption, they did so much more slowly than predicted by the standard theoretical model and basic spin exchange efficiency arguments. Underperformance was most notable in the smallest optical cells. We propose that all these systematic deviations from theory can be explained by invoking the presence of paramagnetic Rb clusters within the vapor. Cluster formation within saturated alkali vapors is well established and their interaction with resonant laser light was recently shown to create plasma-like conditions. Such cluster systems cause both Rb and 129Xe depolarization, as well as excess photon scattering. These effects were incorporated into the SEOP model by assuming that clusters are activated in proportion to excited-state Rb number density and by further estimating physically reasonable values for the nanocluster-induced, velocity-averaged spin-destruction cross-section for Rb (<σcluster-Rbv> ≈4×10-7 cm3s-1), 129Xe relaxation cross-section (<σcluster-Xev> ≈ 4×10-13 cm3s-1), and a non-wavelength-specific, photon-scattering cross-section (σcluster ≈ 1×10-12 cm2). The resulting modified SEOP model now closely matches experimental observations.

Dose and pulse sequence considerations for hyperpolarized 129Xe ventilation MRI

“…Alternatively, one could employ 500 ml of xenon at natural abundance (26% 129 Xe) polarized to 23%. Both are well within reach of emerging polarizer technology (40). Of course, as pulse sequences and transmit/receive coil technology improves, dose efficiency will increase, and smaller volumes of xenon can be used.…”

A Genetically Encoded β‐Lactamase Reporter for Ultrasensitive 129Xe NMR in Mammalian Cells

“…Use of isotopically enriched 129 Xe and new 129 Xe hyperpolarization technology should increase detection sensitivity more than 10-fold. [24] We also aim to improve bla as a Hyper-CEST reporter for biomolecular imaging: The introduction of site-specific mutations, for example, can increase bla affinity for Xe and/or shift its Hyper-CEST response peak to achieve multiplexing or discriminate against 129 Xe-mammalian cell background signals.…”

Molecular hydrogen and catalytic combustion in the production of hyperpolarized 83 Kr and 129 Xe MRI contrast agents

“…Technological improvements (6)(7)(8)(9)(10)(11)(12)(13)(14)(15) have enabled pulmonary hp 129 Xe MRI at high spatial resolution, thereby reducing the need for use of the scarcely available 3 He isotope. Furthermore, tissue solubility, large chemical shift range, and interaction with specific sensor molecules allow for a variety of hp 129 Xe applications in biomedical sciences and beyond (1-6, 16, 17).…”