High Xe density, high photon flux, stopped-flow spin-exchange optical pumping: Simulations versus experiments

A simple setup for in situ alkali metal electronic spin polarimetry

Faraday rotation is considered a gold standard measurement of the electronic spin polarization of an alkali metal vapor produced under optical pumping. However, during the production of large volumes of hyperpolarized xenon gas, transmission monitoring measurements, otherwise known as field cycling measurements, are generally employed to measure the spin polarization of alkali metal atoms in situ as this method is easier to implement than Faraday rotation on standard polarizer setups. Here, we present a simple, low-cost experimental setup to perform Faraday rotation measurements of the electronic spin polarization of alkali metal atoms that can be easily implemented on standard polarizer setups. We then compare Rb polarization measurements obtained with the Faraday rotation method to those obtained with the transmission monitoring method. To our knowledge, a direct comparison of these methods has never been made. Overall, we found good agreement between the two methods, but at low Rb density and high laser power, we found evidence of nonlinear magneto-optical effects that may prevent Faraday rotation from being used under these conditions.

Portable Hyperpolarized Xe-129 Apparatus with Long-Time Stable Polarization Mediated by Adaptable Rb Vapor Density

The extraordinary sensitivity of ¹²⁹Xe, hyperpolarized by spin-exchange optical pumping, is essential for magnetic resonance imaging and spectroscopy in life and materials sciences. However, fluctuations of the polarization over time still limit the reproducibility and quantification with which the interconnectivity of pore spaces can be analyzed. Here, we present a polarizer that not only produces a continuous stream of hyperpolarized ¹²⁹Xe but also maintains stable polarization levels on the order of hours, independent of gas flow rates. The polarizer features excellent magnetization production rates of about 70 mL/h and ¹²⁹Xe polarization values on the order of 40% at moderate system pressures. Key design features include a vertically oriented, large-capacity two-bodied pumping cell and a separate Rb presaturation chamber having its own temperature control, independent of the main pumping cell oven. The separate presaturation chamber allows for precise control of the Rb vapor density by restricting the Rb load and varying the temperature. The polarizer is both compact and transportable─making it easily storable─and adaptable for use in various sample environments. Time-evolved two-dimensional (2D) exchange spectra of ¹²⁹Xe absorbed in the microporous metal-organic framework CAU-1-AmMe are presented to highlight the quantitative nature of the device.

Pilot Quality-Assurance Study of a Third-Generation Batch-Mode Clinical-Scale Automated Xenon-129 Hyperpolarizer

We present a pilot quality assurance (QA) study of a clinical-scale, automated, third-generation (GEN-3) ¹²⁹Xe hyperpolarizer employing batch-mode spin-exchange optical pumping (SEOP) with high-Xe densities (50% natural abundance Xe and 50% N₂ in ~2.6 atm total pressure sourced from Nova Gas Technologies) and rapid temperature ramping enabled by an aluminum heating jacket surrounding the 0.5 L SEOP cell. ¹²⁹Xe hyperpolarization was performed over the course of 700 gas loading cycles of the SEOP cell, simulating long-term hyperpolarized contrast agent production in a clinical lung imaging setting. High levels of ¹²⁹Xe polarization (avg. %P_Xe = 51.0% with standard deviation σ_PXe = 3.0%) were recorded with fast ¹²⁹Xe polarization build-up time constants (avg. T_b = 25.1 min with standard deviation σ_Tb = 3.1 min) across the first 500 SEOP cell refills, using moderate temperatures of 75 °C. These results demonstrate a more than 2-fold increase in build-up rate relative to previously demonstrated results in a comparable QA study on a second-generation (GEN-2) ¹²⁹Xe hyperpolarizer device, with only a minor reduction in maximum achievable %P_Xe and with greater consistency over a larger number of SEOP cell refill processes at a similar polarization lifetime duration (avg. T₁ = 82.4 min, standard deviation σ_T1 = 10.8 min). Additionally, the effects of varying SEOP jacket temperatures, distribution of Rb metal, and preparation and operation of the fluid path are quantified in the context of device installation, performance optimization and maintenance to consistently produce high ¹²⁹Xe polarization values, build-up rates (T_b as low as 6 min) and lifetimes over the course of a typical high-throughput ¹²⁹Xe polarization SEOP cell life cycle. The results presented further demonstrate the significant potential for hyperpolarized ¹²⁹Xe contrast agent in imaging and bio-sensing applications on a clinical scale.

Enabling Clinical Technologies for Hyperpolarized 129 Xenon Magnetic Resonance Imaging and Spectroscopy

Hyperpolarization is a technique that can increase nuclear spin polarization with the corresponding gains in nuclear magnetic resonance (NMR) signals by 4-8 orders of magnitude. When this process is applied to biologically relevant samples, the hyperpolarized molecules can be used as exogenous magnetic resonance imaging (MRI) contrast agents. A technique called spin-exchange optical pumping (SEOP) can be applied to hyperpolarize noble gases such as 129 Xe. Techniques based on hyperpolarized 129 Xe are poised to revolutionize clinical lung imaging, offering a non-ionizing, high-contrast alternative to computed tomography (CT) imaging and conventional proton MRI. Moreover, CT and conventional proton MRI report on lung tissue structure but provide little functional information. On the other hand, when a subject breathes hyperpolarized 129 Xe gas, functional lung images reporting on lung ventilation, perfusion and diffusion with 3D readout can be obtained in seconds. In this Review, the physics of SEOP is discussed and the different production modalities are explained in the context of their clinical application. We also briefly compare SEOP to other hyperpolarization methods and conclude this paper with the outlook for biomedical applications of hyperpolarized 129 Xe to lung imaging and beyond.

A semi-empirical model to optimize continuous-flow hyperpolarized 129Xe production under practical cryogenic-accumulation conditions

Continuous-flow spin exchange optical pumping (SEOP) with cryogenic accumulation is a powerful technique to generate multiple, large volumes of hyperpolarized (HP) 129Xe in rapid succession. It enables a range of studies, from dark matter tracking to preclinical and clinical MRI. Multiple analytical models based on first principles atomic physics and device-specific design features have been proposed for individual processes within HP 129Xe production. However, the modeling efforts have not yet integrated all the steps involved in practical, large volume HP 129Xe production process (e.g., alkali vapor generation, continuous-flow SEOP, and cryogenic accumulation). Here, we use a simplified analytical model that couples both SEOP and cryogenic accumulation, incorporating only two system-specific empirical parameters: the longitudinal relaxation time of the polycrystalline 129Xe "snow', T1snow, generated during cryogenic accumulation, and 2) the average Rb density during active, continuous-flow polarization. By fitting the model to polarization data collected from >140 L of 129Xe polarized across a range of flow and volume conditions, the estimates for Rb density and T1snow were 1.6 ± 0.1 × 1013 cm-3 and 84 ± 5 min, respectively - each notably less than expected based on previous literature. Together, these findings indicate that 1) earlier polarization predictions were hindered by miscalculated Rb densities, and 2) polarization is not optimized by maximizing SEOP efficiency with a low concentration 129Xe, but rather by using richer 129Xe-buffer gas blends that enable faster accumulation. Accordingly, modeling and experimentation revealed the optimal fraction of 129Xe, f, in the 129Xe-buffer gas blend was ~2%. Further, if coupled with modest increases in laser power, the model predicts liter volumes of HP 129Xe with polarizations exceeding 60% could be generated routinely in only tens of minutes.

XeUS: A second-generation automated open-source batch-mode clinical-scale hyperpolarizer

We present a second-generation open-source automated batch-mode ¹²⁹Xe hyperpolarizer (XeUS GEN-2), designed for clinical-scale hyperpolarized (HP) ¹²⁹Xe production via spin-exchange optical pumping (SEOP) in the regimes of high Xe density (0.66-2.5 atm partial pressure) and resonant photon flux (~170 W, Δλ = 0.154 nm FWHM), without the need for cryo-collection typically employed by continuous-flow hyperpolarizers. An Arduino micro-controller was used for hyperpolarizer operation. Processing open-source software was employed to program a custom graphical user interface (GUI), capable of remote automation. The Arduino Integrated Development Environment (IDE) was used to design a variety of customized automation sequences such as temperature ramping, NMR signal acquisition, and SEOP cell refilling for increased reliability. A polycarbonate 3D-printed oven equipped with a thermo-electric cooler/heater provides thermal stability for SEOP for both binary (Xe/N₂) and ternary (⁴He-containing) SEOP cell gas mixtures. Quantitative studies of the ¹²⁹Xe hyperpolarization process demonstrate that near-unity polarization can be achieved in a 0.5 L SEOP cell. For example, %P_Xe of 93.2 ± 2.9% is achieved at 0.66 atm Xe pressure with polarization build-up rate constant γ_SEOP = 0.040 ± 0.005 min^-1, giving a max dose equivalent ≈ 0.11 L/h 100% hyperpolarized, 100% enriched ¹²⁹Xe; %P_Xe of 72.6 ± 1.4% is achieved at 1.75 atm Xe pressure with γ_SEOP of 0.041 ± 0.001 min^-1, yielding a corresponding max dose equivalent of 0.27 L/h. Quality assurance studies on this device have demonstrated the potential to refill SEOP cells hundreds of times without significant losses in performance, with average %P_Xe = 71.7%, (standard deviation σ_P = 1.52%) and mean polarization lifetime T₁ = 90.5 min, (standard deviation σ_T = 10.3 min) over the first ~200 gas mixture refills, with sufficient performance maintained across a further ~700 refills. These findings highlight numerous technological developments and have significant translational relevance for efficient production of gaseous HP ¹²⁹Xe contrast agents for use in clinical imaging and bio-sensing techniques.

Hyperpolarized 129Xe Time-of-Flight MR Imaging of Perfusion and Brain Function

Perfusion measurements can provide vital information about the homeostasis of an organ and can therefore be used as biomarkers to diagnose a variety of cardiovascular, renal, and neurological diseases. Currently, the most common techniques to measure perfusion are ¹⁵O positron emission tomography (PET), xenon-enhanced computed tomography (CT), single photon emission computed tomography (SPECT), dynamic contrast enhanced (DCE) MRI, and arterial spin labeling (ASL) MRI. Here, we show how regional perfusion can be quantitively measured with magnetic resonance imaging (MRI) using time-resolved depolarization of hyperpolarized (HP) xenon-129 (¹²⁹Xe), and the application of this approach to detect changes in cerebral blood flow (CBF) due to a hemodynamic response in response to brain stimuli. The investigated HP ¹²⁹Xe Time-of-Flight (TOF) technique produced perfusion images with an average signal-to-noise ratio (SNR) of 10.35. Furthermore, to our knowledge, the first hemodynamic response (HDR) map was acquired in healthy volunteers using the HP ¹²⁹Xe TOF imaging. Responses to visual and motor stimuli were observed. The acquired HP TOF HDR maps correlated well with traditional proton blood oxygenation level-dependent functional MRI. Overall, this study expands the field of HP MRI with a novel dynamic imaging technique suitable for rapid and quantitative perfusion imaging.

Pilot multi-site quality assurance study of batch-mode clinical-scale automated xenon-129 hyperpolarizers

We present a pilot quality assurance (QA) study of spin-exchange optical pumping (SEOP) performed on two nearly identical second-generation (GEN-2) automated batch-mode clinical-scale ¹²⁹Xe hyperpolarizers, each utilizing a convective forced air oven, high-power (~170 W) continuous pump laser irradiation, and xenon-rich gas mixtures (~1.30 atm partial pressure). In one study, the repeatability of SEOP in a 1000 Torr Xe/900 Torr N₂/100 Torr ⁴He (2000 Torr total pressure) gas mixture is evaluated over the course of ~700 gas loading cycles, with negligible decrease in performance during the first ~200 cycles, and with high ¹²⁹Xe polarization levels (avg. %P_Xe = 71.7% with standard deviation σ_PXe = 1.5%), build-up rates (avg. γ_SEOP = 0.019 min^-1 with standard deviation σ_γ = 0.003 min^-1) and polarization lifetimes (avg. T₁ = 90.5 min with standard deviation σ_T = 10.3 min) reported at moderate oven temperature of ~70 °C. Although the SEOP cell in this study exhibited a detectable performance decrease after 400 cycles, the cell continued to produce potentially useable HP ¹²⁹Xe with %P_Xe = 42.3 ± 0.6% even after nearly 700 refill cycles. The possibility of "regenerating" "dormant" (i.e., not used for an extended period of time) SEOP cells using repeated temperature cycling methods to recover %P_Xe is also demonstrated. The quality and consistency of results show significant promise for translation to clinical-scale production of hyperpolarized ¹²⁹Xe contrast agents for imaging and bio-sensing applications.

Helium-rich mixtures for improved batch-mode clinical-scale spin-exchange optical pumping of Xenon-129

We present studies of spin-exchange optical pumping (SEOP) using ternary xenon-nitrogen-helium gas mixtures at high xenon partial pressures (up to 1330 Torr partial pressure at loading, out of 2660 Torr total pressure) in a 500-mL volume SEOP cell, using two automated batch-mode clinical-scale ¹²⁹Xe hyperpolarizers operating under continuous high-power (~170 W) pump laser irradiation. In this pilot study, we explore SEOP in gas mixtures with up to 45% ⁴He content under a wide range of experimental conditions. When an aluminum jacket cooling/heating design was employed (GEN-3 hyperpolarizer), ¹²⁹Xe polarization (%P_Xe) of 55.9 ± 0.9% was observed with mono-exponential build-up rate γ_SEOP of 0.049 ± 0.001 min^-1 for the ⁴He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N₂), compared to %P_Xe of 49.3 ± 3.3% at γ_SEOP of 0.035 ± 0.004 min^-1 for the N₂-rich gas mixture (1000 Torr Xe/100 Torr He, 900 Torr N₂). When forced-air cooling/heating was used (GEN-2 hyperpolarizer), %P_Xe of 83.9 ± 2.7% was observed at γ_SEOP of 0.045 ± 0.005 min^-1 for the ⁴He-rich mixture (1000 Torr Xe/900 Torr He, 100 Torr N₂), compared to %P_Xe of 73.5 ± 1.3% at γ_SEOP of 0.028 ± 0.001 min^-1 for the N₂-rich gas mixture (1000 Torr Xe and 1000 Torr N₂). Additionally, %P_Xe of 72.6 ± 1.4% was observed at a build-up rate γ_SEOP of 0.041 ± 0.003 min^-1 for a super-high-density ⁴He-rich mixture (1330 Torr Xe/1200 Torr ⁴He/130 Torr N₂), compared to %P_Xe = 56.6 ± 1.3% at a build-up rate of γ_SEOP of 0.034 ± 0.002 min^-1 for an N₂-rich mixture (1330 Torr Xe/1330 Torr N₂) using forced air cooling/heating. The observed SEOP hyperpolarization performance under these conditions corresponds to %P_Xe improvement by a factor of 1.14 ± 0.04 at 1000 Torr Xe density and by up to a factor of 1.28 ± 0.04 at 1330 Torr Xe density at improved SEOP build-up rates by factors of 1.61 ± 0.18 and 1.21 ± 0.11 respectively. Record %P_Xe levels have been obtained here: 83.9 ± 2.7% at 1000 Torr Xe partial pressure and 72.6 ± 1.4% at 1330 Torr Xe partial pressure. In addition to improved thermal stability for SEOP, the use of ⁴He-rich gas mixtures also reduces the overall density of produced inhalable HP contrast agents; this property may be desirable for HP ¹²⁹Xe inhalation by human subjects in clinical settings-especially in populations with heavily impaired lung function. The described approach should enjoy ready application in the production of inhalable ¹²⁹Xe contrast agent with near-unity ¹²⁹Xe nuclear spin polarization.

Batch-Mode Clinical-Scale Optical Hyperpolarization of Xenon-129 Using an Aluminum Jacket with Rapid Temperature Ramping

We present spin-exchange optical pumping (SEOP) using a third-generation (GEN-3) automated batch-mode clinical-scale ¹²⁹Xe hyperpolarizer utilizing continuous high-power (∼170 W) pump laser irradiation and a novel aluminum jacket design for rapid temperature ramping of xenon-rich gas mixtures (up to 2 atm partial pressure). The aluminum jacket design is capable of heating SEOP cells from ambient temperature (typically 25 °C) to 70 °C (temperature of the SEOP process) in 4 min, and perform cooling of the cell to the temperature at which the hyperpolarized gas mixture can be released from the hyperpolarizer (with negligible amounts of Rb metal leaving the cell) in approximately 4 min, substantially faster (by a factor of 6) than previous hyperpolarizer designs relying on air heat exchange. These reductions in temperature cycling time will likely be highly advantageous for the overall increase of production rates of batch-mode (i.e., stopped-flow) ¹²⁹Xe hyperpolarizers, which is particularly beneficial for clinical applications. The additional advantage of the presented design is significantly improved thermal management of the SEOP cell. Accompanying the heating jacket design and performance, we also evaluate the repeatability of SEOP experiments conducted using this new architecture, and present typically achievable hyperpolarization levels exceeding 40% at exponential build-up rates on the order of 0.1 min^-1.