Owing to the detection sensitivity provided by their high, non-equilibrium nuclear spin polarizations, hyperpolarized (HP) noble gases (e.g. ¹²⁹Xe and ³He) are utilized in a growing number of MRS/MRI applications—ranging from biomedical imaging and spectroscopy to probing molecular and materials surfaces. Lung imaging with HP ¹²⁹Xe is of particular interest. Although ³He has a nearly 3-fold greater gyromagnetic ratio, ¹²⁹Xe has 26% natural abundance and higher solubility in blood and tissues. Moreover, ¹²⁹Xe’s proclivity for interacting with substances and its wide chemical shift range make it a more sensitive MR probe of biological environments.

HP ¹²⁹Xe is usually created by spin-exchange optical pumping (SEOP). It is traditionally expected that high ¹²⁹Xe polarizations (P_Xe) can only be obtained with low in-cell Xe densities because (i) higher Xe densities increase the destruction of the alkali metal polarization from non-spin-conserving collisions and (ii) higher total pressures tend to quench the more efficient van der Waals contribution to Rb-Xe spin exchange. Indeed, many polarizer designs tend to go to great lengths to produce large amounts of HP ¹²⁹Xe while still satisfying this condition within the cell. Such polarizers also tend to be complex, expensive, and strictly regulated by their controlling entities—factors that hinder large-scale HP ¹²⁹Xe generation for many unaffiliated laboratories.

Building upon our previous work exploring batch-mode or “stopped-flow” Rb/Xe SEOP under conditions of high resonant laser flux, we constructed a first-generation large-scale (>1 L/hr) “open-source” xenon polarizer for clinical, pre-clinical, and materials NMR/MRI applications comprised mostly of off-the-shelf components (including a 200 W VHG-narrowed LDA laser) [1–2]. This hyperpolarizer was cleared for preclinical work and approved by the FDA (IND #116,662) for operation at Brigham & Women’s Hospital. Unlike most clinical-scale ¹²⁹Xe polarizers, this first-generation device runs with Xe-rich gas mixtures in single-batch mode. In-cell P_Xe values during SEOP of up to ~90%, ~57%, ~50%, and ~28% have been measured for Xe partial pressures of ~300, ~500, ~760, and ~1570 Torr, respectively [1–2].

Experience gained in building this first-generation ¹²⁹Xe polarizer lead to construction of our second-generation ¹²⁹Xe polarizer (dubbed “XeUS”) [3–5]. The new design encompasses a variety of improvements and new technologies. For example, the water-cooled 200 W VHG-narrowed LDA laser possesses a novel integrated air-cooled optical train assembly. Implementation of a thermoelectric cooler (TEC) and incorporation of a quiet air compressor eliminates the need for external gas or liquid-N₂ supplies for heating/cooling of the OP cell oven or for pneumatic valve operation. The polycarbonate oven for the OP cell is manufactured via 3D printing and houses the TEC module. Further integration of the requisite instrumentation and optics into this 3D-printed oven in conjunction with the laser telescope greatly simplified system construction and laser alignment with the optical pumping cell. A high-pressure gas manifold using premixed gases also greatly simplifies the first-generation gas manifold design. Taken together, these various improvements have lead to the second-generation hyperpolarizer achieving higher performance, with P_Xe of ~74% at 1,000 Torr [3] and P_Xe of ~90% at 500 Torr [5] as well as the improvements in the production rate [3-5].