Interdependence of in-cell xenon density and temperature during Rb/129Xe spin-exchange optical pumping using VHG-narrowed laser diode arrays

Continuous Delivery of Hyperpolarized Xenon-129 Gas Using a "Stopped-Flow" Clinical-Scale Cryogen-Free Hyperpolarizer

In 2022, the FDA approved hyperpolarized (HP) 129Xe gas as an inhalable contrast agent for functional lung imaging. For clinical imaging, HP 129Xe is usually given as a bolus inhalation. However, for preclinical applications (e.g., pulmonary imaging in small rodents), the continuous delivery of HP 129Xe is greatly desired to enable MRI scanning under conditions of physiological continuous animal breathing patterns. Moreover, HP 129Xe gas can be utilized for other applications including materials science and bioanalytical chemistry, where a continuous flow of hyperpolarized gas through an NMR sample over several minutes is also desired for sensing of 129Xe inside an NMR spectrometer. 129Xe is often hyperpolarized using continuous-flow spin-exchange optical pumping, which employs a lean (1-2%) mixture of Xe and a carrier gas (e.g., He and N2). The low Xe concentration in the produced output reduces the NMR detection sensitivity, and thus, Xe cryo-collection is typically employed to achieve near-100% pure gas-phase Xe before administration to the sample or subject. However, the need for cryo-collection undermines a key advantage of continuous-flow production, i.e., the continuous flowing in a hyperpolarizer HP 129Xe gas is trapped inside the hyperpolarizer, and the produced HP 129Xe gas is released at once when the production cycle (30-60 min) is completed. An alternative HP 129Xe production technology employs a "stopped-flow" approach, where a batch of HP gas is hyperpolarized over time and quickly released from a hyperpolarizer. Here, a clinical-scale "stopped-flow" 129Xe hyperpolarizer was employed to hyperpolarize a 1.3 L-atm batch of 50:50 Xe:N2 gas mixture inside a glass cell with an ultralong lifetime of the HP 129Xe state (T1 > 2 h). The produced HP 129Xe gas was slowly delivered into a 5 mm NMR tube via PEEK tubing under a wide range of gas flow rates: 3-180 standard cubic centimeters per minute (sccm). The polarization of the gas ejected from the hyperpolarizer was quantified using in situ low-field NMR polarimetry and additionally verified using a 0.35 T clinical MRI scanner. Continuous-flow delivery of HP 129Xe was demonstrated for up to 15 min with a gas flow rate of 45-150 sccm over a 2.5-m length of PEEK tubing, suffering only small losses in 129Xe polarization. These observations are additionally supported by 129Xe relaxation measurements inside the PEEK tubing employed for gas delivery and the 5 mm NMR tube employed for polarimetry. 129Xe polarization of 16-19% was obtained in the delivered gas, starting with an "in-polarizer" 129Xe polarization of 19%. We envision that this method can be employed for on-demand cryogen-free delivery of hyperpolarized gas using "stopped-flow" 129Xe hyperpolarizers for a broad range of applications, from preclinical imaging to biosensors, and to spectroscopy of materials surfaces.

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.

Hyperpolarised gas filling station for medical imaging using polarised 129Xe and 3He

We report the design, construction, and initial tests of a hyperpolariser to produce polarised 129Xe and 3He gas for medical imaging of the lung. The hyperpolariser uses the Spin-Exchange Optical Pumping method to polarise the nuclear spins of the isotopic gas. Batch mode operation was chosen for the design to produce polarised 129Xe and polarised 3He. Two-side pumping, electrical heating and a piston to transfer the polarised gas were some of the implemented techniques that are not commonly used in hyperpolariser designs. We have carried out magnetic resonance imaging experiments demonstrating that the 3He and 129Xe polarisation reached were sufficient for imaging, in particular for in vivo lung imaging using 129Xe. Further improvements to the hyperpolariser have also been discussed.

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.

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.

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

Spin-exchange optical pumping (SEOP) can enhance the NMR sensitivity of noble gases by up to five orders of magnitude at Tesla-strength magnetic fields. SEOP-generated hyperpolarised (HP) ¹²⁹Xe is a promising contrast agent for lung imaging but an ongoing barrier to widespread clinical usage has been economical production of sufficient quantities with high ¹²⁹Xe polarisation. Here, the 'standard model' of SEOP, which was previously used in the optimisation of continuous-flow ¹²⁹Xe polarisers, is modified for validation against two Xe-rich stopped-flow SEOP datasets. We use this model to examine ways to increase HP Xe production efficiency in stopped-flow ¹²⁹Xe polarisers and provide further insight into the underlying physics of Xe-rich stopped-flow SEOP at high laser fluxes.

NMR Hyperpolarization Techniques of Gases

Nuclear spin polarization can be significantly increased through the process of hyperpolarization, leading to an increase in the sensitivity of nuclear magnetic resonance (NMR) experiments by 4-8 orders of magnitude. Hyperpolarized gases, unlike liquids and solids, can often be readily separated and purified from the compounds used to mediate the hyperpolarization processes. These pure hyperpolarized gases enabled many novel MRI applications including the visualization of void spaces, imaging of lung function, and remote detection. Additionally, hyperpolarized gases can be dissolved in liquids and can be used as sensitive molecular probes and reporters. This Minireview covers the fundamentals of the preparation of hyperpolarized gases and focuses on selected applications of interest to biomedicine and materials science.

Observing and preventing rubidium runaway in a direct-infusion xenon-spin hyperpolarizer optimized for high-resolution hyper-CEST (chemical exchange saturation transfer using hyperpolarized nuclei) NMR

Xenon is well known to undergo host-guest interactions with proteins and synthetic molecules. As xenon can also be hyperpolarized by spin exchange optical pumping, allowing the investigation of highly dilute systems, it makes an ideal nuclear magnetic resonance probe for such host molecules. The utility of xenon as a probe can be further improved using Chemical Exchange Saturation Transfer using hyperpolarized nuclei (Hyper-CEST), but for highly accurate experiments requires a polarizer and xenon infusion system optimized for such measurements. We present the design of a hyperpolarizer and xenon infusion system specifically designed to meet the requirements of Hyper-CEST measurements. One key element of this design is preventing rubidium runaway, a chain reaction induced by laser heating that prevents efficient utilization of high photon densities. Using thermocouples positioned along the pumping cell we identify the sources of heating and conditions for rubidium runaway to occur. We then demonstrate the effectiveness of actively cooling the optical cell to prevent rubidium runaway in a compact setup. This results in a 2-3-fold higher polarization than without cooling, allowing us to achieve a polarization of 25% at continuous flow rates of 9 ml/min of (129)Xe. The simplicity of this design also allows it to be retrofitted to many existing polarizers. Combined with a direction infusion system that reduces shot-to-shot noise down to 0.56% we have captured Hyper-CEST spectra in unprecedented detail, allowing us to completely resolve peaks separated by just 1.62 ppm. Due to its high polarization and excellent stability, our design allows the comparison of underlying theories of host-guest systems with experiment at low concentrations, something extremely difficult with previous polarizers.

Temperature-ramped (129)Xe spin-exchange optical pumping

We describe temperature-ramped spin-exchange optical pumping (TR-SEOP) in an automated high-throughput batch-mode (129)Xe hyperpolarizer utilizing three key temperature regimes: (i) "hot"-where the (129)Xe hyperpolarization rate is maximal, (ii) "warm"-where the (129)Xe hyperpolarization approaches unity, and (iii) "cool"-where hyperpolarized (129)Xe gas is transferred into a Tedlar bag with low Rb content (<5 ng per ∼1 L dose) suitable for human imaging applications. Unlike with the conventional approach of batch-mode SEOP, here all three temperature regimes may be operated under continuous high-power (170 W) laser irradiation, and hyperpolarized (129)Xe gas is delivered without the need for a cryocollection step. The variable-temperature approach increased the SEOP rate by more than 2-fold compared to the constant-temperature polarization rate (e.g., giving effective values for the exponential buildup constant γSEOP of 62.5 ± 3.7 × 10(-3) min(-1) vs 29.9 ± 1.2 × 10(-3) min(-1)) while achieving nearly the same maximum %PXe value (88.0 ± 0.8% vs 90.1% ± 0.8%, for a 500 Torr (67 kPa) Xe cell loading-corresponding to nuclear magnetic resonance/magnetic resonance imaging (NMR/MRI) enhancements of ∼3.1 × 10(5) and ∼2.32 × 10(8) at the relevant fields for clinical imaging and HP (129)Xe production of 3 T and 4 mT, respectively); moreover, the intercycle "dead" time was also significantly decreased. The higher-throughput TR-SEOP approach can be implemented without sacrificing the level of (129)Xe hyperpolarization or the experimental stability for automation-making this approach beneficial for improving the overall (129)Xe production rate in clinical settings.

Characterizing and modeling the efficiency limits in large-scale production of hyperpolarized 129Xe

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.

Multidimensional mapping of spin-exchange optical pumping in clinical-scale batch-mode 129Xe hyperpolarizers

We present a systematic, multiparameter study of Rb/¹²⁹Xe spin-exchange optical pumping (SEOP) in the regimes of high xenon pressure and photon flux using a 3D-printed, clinical-scale stopped-flow hyperpolarizer. In situ NMR detection was used to study the dynamics of ¹²⁹Xe polarization as a function of SEOP-cell operating temperature, photon flux, and xenon partial pressure to maximize ¹²⁹Xe polarization (P_Xe). P_Xe values of 95 ± 9%, 73 ± 4%, 60 ± 2%, 41 ± 1%, and 31 ± 1% at 275, 515, 1000, 1500, and 2000 Torr Xe partial pressure were achieved. These P_Xe polarization values were separately validated by ejecting the hyperpolarized ¹²⁹Xe gas and performing low-field MRI at 47.5 mT. It is shown that P_Xe in this high-pressure regime can be increased beyond already record levels with higher photon flux and better SEOP thermal management, as well as optimization of the polarization dynamics, pointing the way to further improvements in hyperpolarized ¹²⁹Xe production efficiency.

Sensitivity enhancement in solution NMR: emerging ideas and new frontiers

Modern NMR spectroscopy has reached an unprecedented level of sophistication in the determination of biomolecular structure and dynamics at atomic resolution in liquids. However, the sensitivity of this technique is still too low to solve a variety of cutting-edge biological problems in solution, especially those that involve viscous samples, very large biomolecules or aggregation-prone systems that need to be kept at low concentration. Despite the challenges, a variety of efforts have been carried out over the years to increase sensitivity of NMR spectroscopy in liquids. This review discusses basic concepts, recent developments and future opportunities in this exciting area of research.

XeNA: an automated 'open-source' (129)Xe hyperpolarizer for clinical use

Here we provide a full report on the construction, components, and capabilities of our consortium's "open-source" large-scale (~1L/h) (129)Xe hyperpolarizer for clinical, pre-clinical, and materials NMR/MRI (Nikolaou et al., Proc. Natl. Acad. Sci. USA, 110, 14150 (2013)). The 'hyperpolarizer' is automated and built mostly of off-the-shelf components; moreover, it is designed to be cost-effective and installed in both research laboratories and clinical settings with materials costing less than $125,000. The device runs in the xenon-rich regime (up to 1800Torr Xe in 0.5L) in either stopped-flow or single-batch mode-making cryo-collection of the hyperpolarized gas unnecessary for many applications. In-cell (129)Xe nuclear spin polarization values of ~30%-90% have been measured for Xe loadings of ~300-1600Torr. Typical (129)Xe polarization build-up and T1 relaxation time constants were ~8.5min and ~1.9h respectively under our spin-exchange optical pumping conditions; such ratios, combined with near-unity Rb electron spin polarizations enabled by the high resonant laser power (up to ~200W), permit such high PXe values to be achieved despite the high in-cell Xe densities. Importantly, most of the polarization is maintained during efficient HP gas transfer to other containers, and ultra-long (129)Xe relaxation times (up to nearly 6h) were observed in Tedlar bags following transport to a clinical 3T scanner for MR spectroscopy and imaging as a prelude to in vivo experiments. The device has received FDA IND approval for a clinical study of chronic obstructive pulmonary disease subjects. The primary focus of this paper is on the technical/engineering development of the polarizer, with the explicit goals of facilitating the adaptation of design features and operative modes into other laboratories, and of spurring the further advancement of HP-gas MR applications in biomedicine.

A 3D-printed high power nuclear spin polarizer

Three-dimensional printing with high-temperature plastic is used to enable spin exchange optical pumping (SEOP) and hyperpolarization of xenon-129 gas. The use of 3D printed structures increases the simplicity of integration of the following key components with a variable temperature SEOP probe: (i) in situ NMR circuit operating at 84 kHz (Larmor frequencies of (129)Xe and (1)H nuclear spins), (ii) <0.3 nm narrowed 200 W laser source, (iii) in situ high-resolution near-IR spectroscopy, (iv) thermoelectric temperature control, (v) retroreflection optics, and (vi) optomechanical alignment system. The rapid prototyping endowed by 3D printing dramatically reduces production time and expenses while allowing reproducibility and integration of "off-the-shelf" components and enables the concept of printing on demand. The utility of this SEOP setup is demonstrated here to obtain near-unity (129)Xe polarization values in a 0.5 L optical pumping cell, including ∼74 ± 7% at 1000 Torr xenon partial pressure, a record value at such high Xe density. Values for the (129)Xe polarization exponential build-up rate [(3.63 ± 0.15) × 10(-2) min(-1)] and in-cell (129)Xe spin-lattice relaxation time (T1 = 2.19 ± 0.06 h) for 1000 Torr Xe were in excellent agreement with the ratio of the gas-phase polarizations for (129)Xe and Rb (PRb ∼ 96%). Hyperpolarization-enhanced (129)Xe gas imaging was demonstrated with a spherical phantom following automated gas transfer from the polarizer. Taken together, these results support the development of a wide range of chemical, biochemical, material science, and biomedical applications.

Cryogenics free production of hyperpolarized 129Xe and 83Kr for biomedical MRI applications

As an alternative to cryogenic gas handling, hyperpolarized (hp) gas mixtures were extracted directly from the spin exchange optical pumping (SEOP) process through expansion followed by compression to ambient pressure for biomedical MRI applications. The omission of cryogenic gas separation generally requires the usage of high xenon or krypton concentrations at low SEOP gas pressures to generate hp (129)Xe or hp (83)Kr with sufficient MR signal intensity for imaging applications. Two different extraction schemes for the hp gasses were explored with focus on the preservation of the nuclear spin polarization. It was found that an extraction scheme based on an inflatable, pressure controlled balloon is sufficient for hp (129)Xe handling, while (83)Kr can efficiently be extracted through a single cycle piston pump. The extraction methods were tested for ex vivo MRI applications with excised rat lungs. Precise mixing of the hp gases with oxygen, which may be of interest for potential in vivo applications, was accomplished during the extraction process using a piston pump. The (83)Kr bulk gas phase T1 relaxation in the mixtures containing more than approximately 1% O2 was found to be slower than that of (129)Xe in corresponding mixtures. The experimental setup also facilitated (129)Xe T1 relaxation measurements as a function of O2 concentration within excised lungs.

Near-unity nuclear polarization with an open-source 129Xe hyperpolarizer for NMR and MRI

The exquisite NMR spectral sensitivity and negligible reactivity of hyperpolarized xenon-129 (HP(129)Xe) make it attractive for a number of magnetic resonance applications; moreover, HP(129)Xe embodies an alternative to rare and nonrenewable (3)He. However, the ability to reliably and inexpensively produce large quantities of HP(129)Xe with sufficiently high (129)Xe nuclear spin polarization (P(Xe)) remains a significant challenge--particularly at high Xe densities. We present results from our "open-source" large-scale (∼1 L/h) (129)Xe polarizer for clinical, preclinical, and materials NMR and MRI research. Automated and composed mostly of off-the-shelf components, this "hyperpolarizer" is designed to be readily implementable in other laboratories. The device runs with high resonant photon flux (up to 200 W at the Rb D1 line) in the xenon-rich regime (up to 1,800 torr Xe in 500 cc) in either single-batch or stopped-flow mode, negating in part the usual requirement of Xe cryocollection. Excellent agreement is observed among four independent methods used to measure spin polarization. In-cell P(Xe) values of ∼90%, ∼57%, ∼50%, and ∼30% have been measured for Xe loadings of ∼300, ∼500, ∼760, and ∼1,570 torr, respectively. P(Xe) values of ∼41% and ∼28% (with ∼760 and ∼1,545 torr Xe loadings) have been measured after transfer to Tedlar bags and transport to a clinical 3 T scanner for MR imaging, including demonstration of lung MRI with a healthy human subject. Long "in-bag" (129)Xe polarization decay times have been measured (T1 ∼38 min and ∼5.9 h at ∼1.5 mT and 3 T, respectively)--more than sufficient for a variety of applications.

NMR of hyperpolarised probes

Increasing the sensitivity of NMR experiments is an ongoing field of research to help realise the exquisite molecular specificity of this technique. Hyperpolarisation of various nuclei is a powerful approach that enables the use of NMR for molecular and cellular imaging. Substantial progress has been achieved over recent years in terms of both tracer preparation and detection schemes. This review summarises recent developments in probe design and optimised signal encoding, and promising results in sensitive disease detection and efficient therapeutic monitoring. The different methods have great potential to provide molecular specificity not available by other diagnostic modalities.

Pathway to cryogen free production of hyperpolarized Krypton-83 and Xenon-129

Hyperpolarized (hp) ¹²⁹Xe and hp ⁸³Kr for magnetic resonance imaging (MRI) are typically obtained through spin-exchange optical pumping (SEOP) in gas mixtures with dilute concentrations of the respective noble gas. The usage of dilute noble gases mixtures requires cryogenic gas separation after SEOP, a step that makes clinical and preclinical applications of hp ¹²⁹Xe MRI cumbersome. For hp ⁸³Kr MRI, cryogenic concentration is not practical due to depolarization that is caused by quadrupolar relaxation in the condensed phase. In this work, the concept of stopped flow SEOP with concentrated noble gas mixtures at low pressures was explored using a laser with 23.3 W of output power and 0.25 nm linewidth. For ¹²⁹Xe SEOP without cryogenic separation, the highest obtained MR signal intensity from the hp xenon-nitrogen gas mixture was equivalent to that arising from 15.5±1.9% spin polarized ¹²⁹Xe in pure xenon gas. The production rate of the hp gas mixture, measured at 298 K, was 1.8 cm³/min. For hp ⁸³Kr, the equivalent of 4.4±0.5% spin polarization in pure krypton at a production rate of 2 cm³/min was produced. The general dependency of spin polarization upon gas pressure obtained in stopped flow SEOP is reported for various noble gas concentrations. Aspects of SEOP specific to the two noble gas isotopes are discussed and compared with current theoretical opinions. A non-linear pressure broadening of the Rb D₁ transition was observed and taken into account for the qualitative description of the SEOP process.