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

Hyperpolarized 129Xe Atoms Sense the Presence of Drug Molecules in Nanohosts Revealed by Magnetic Resonance Imaging

Assessing the effectiveness of nanomedicines involves evaluating the drug content at the target site. Currently, most research focuses on monitoring the signal responses from loaded drugs, neglecting the changes caused by the nanohosts. Here, we propose a strategy to quantitatively evaluate the content of loaded drugs by detecting the signal variations resulting from the alterations in the microenvironment of the nanohosts. Specifically, hyperpolarized (HP) 129Xe atoms are employed as probes to sense the nanohosts' environment and generate a specific magnetic resonance (MR) signal that indicates their accessibility. The introduction of drugs reduces the available space in the nanohosts, leading to a crowded microenvironment that hinders the access of the 129Xe atoms. By employing 129Xe atoms as a signal source to detect the alterations in the microenvironment, we constructed a three-dimensional (3D) map that indicated the concentration of the nanohosts and established a linear relationship to quantitatively measure the drug content within the nanohosts based on the corresponding MR signals. Using the developed strategy, we successfully quantified the uptake of the nanohosts and drugs in living cells through HP 129Xe MR imaging. Overall, the proposed HP 129Xe atom-sensing approach can be used to monitor alterations in the microenvironment of nanohosts induced by loaded drugs and provides a new perspective for the quantitative evaluation of drug presence in various nanomedicines.

129Xe Dynamic Nuclear Polarization Demystified: The Influence of the Glassing Matrix on the Radical Properties

129Xe dissolution dynamic nuclear polarization (DNP) is a controversial topic. The gold standard technique for hyperpolarized xenon magnetic resonance imaging (MRI) is spin exchange optical pumping, which received FDA approval in 2022. Nevertheless, the versatility of DNP for enhancing the signal of any NMR active nucleus might provide new perspectives for hyperpolarized 129Xe NMR/MRI. Initial publications about 129Xe DNP underlined the increased complexity in the sample preparation and lower polarization levels when compared to more conventional 13C-labeled molecules, at same experimental conditions, despite very close gyromagnetic ratios. Herein, we introduce, using a Custom Fluid Path system, a user-friendly and very robust sample preparation method. Moreover, investigating the radical properties at real DNP conditions by means of LOngitudinal Detected Electron Spin Resonance, we discovered a dramatic shortening of the electron spin longitudinal relaxation time (T1e) of nitroxyl radicals in xenon DNP samples' matrices, with respect to more commonly used water:glycerol ones. Mitigating those challenges through microwave frequency modulation, we achieved over 20% 129Xe polarization without employing any deuterated solvent.

Ultra-Low-Cost Disposable Hand-Held Clinical-Scale Propane Gas Hyperpolarizer for Pulmonary Magnetic Resonance Imaging Sensing

Hyperpolarized magnetic resonance imaging (MRI) contrast agents are revolutionizing the field of biomedical imaging. Hyperpolarized Xe-129 was recently FDA approved as an inhalable MRI contrast agent for functional lung imaging sensing. Despite success in research settings, modern Xe-129 hyperpolarizers are expensive (up to $1M), large, and complex to site and operate. Moreover, Xe-129 sensing requires specialized MRI hardware that is not commonly available on clinical MRI scanners. Here, we demonstrate that proton-hyperpolarized propane gas can be produced on demand using a disposable, hand-held, clinical-scale hyperpolarizer via parahydrogen-induced polarization, which relies on parahydrogen as a source of hyperpolarization. The device consists of a heterogeneous catalytic reactor connected to a gas mixture storage can containing pressurized hyperpolarization precursors: propylene and parahydrogen (10 bar total pressure). Once the built-in flow valve of the storage can is actuated, the precursors are ejected from the can into a reactor, and a stream of hyperpolarized propane gas is ejected from the reactor. Robust operation of the device is demonstrated for producing proton sensing polarization of 1.2% in a wide range of operational pressures and gas flow rates. We demonstrate that the propylene/parahydrogen gas mixture can retain potency for days in the storage can with a monoexponential decay time constant of 6.0 ± 0.5 days, which is limited by the lifetime of the parahydrogen singlet spin state in the storage container. The utility of the produced sensing agent is demonstrated for phantom imaging on a 3 T clinical MRI scanner located 100 miles from the agent/device preparation site and also for ventilation imaging of excised pig lungs using a 0.35 T clinical MRI scanner. The cost of the device components is less than $35, which we envision can be reduced to less than $5 for mass-scale production. The hyperpolarizer device can be reused, recycled, or disposed.

Absolute thermometry of human brown adipose tissue by magnetic resonance with laser polarized 129Xe

BACKGROUND

Absolute temperature measurements of tissues inside the human body are difficult to perform non-invasively. Yet, for brown adipose tissue (BAT), these measurements would enable direct monitoring of its thermogenic activity and its association with metabolic health.

METHODS

Here, we report direct measurement of absolute BAT temperature in humans during cold exposure by magnetic resonance (MR) with laser polarized xenon gas. This methodology, which leverages on the sensitivity of the chemical shift of the 129Xe isotope to temperature-induced changes in fat density, is first calibrated in vitro and then tested in vivo in rodents. Finally, it is used in humans along with positron emission tomography (PET) scans with fluorine-18-fluorodeoxyglucose to detect BAT thermogenic activity during cold exposure.

RESULTS

Absolute temperature measurements, obtained in rodents with an experimental error of 0.5 °C, show only a median deviation of 0.12 °C against temperature measurements made using a pre-calibrated optical temperature probe. In humans, enhanced uptake of 129Xe in BAT during cold exposure leads to background-free detection of this tissue by MR. Global measurements of supraclavicular BAT temperature, made over the course of four seconds and with an experimental error ranging from a minimum of 0.4 °C to more than 2 °C, in case of poor shimming, reveal an average BAT temperature of 38.8° ± 0.8 °C, significantly higher (p < 0.02 two-sided t test) than 37.7 °C. Hot BAT is also detected in participants with a PET scan negative for BAT.

CONCLUSIONS

Non-invasive, radiation-free measurements of BAT temperature by MRI with hyperpolarized 129Xe may enable longitudinal monitoring of human BAT activity under various stimulatory conditions.

Dramatic improvement in the "Bulk" hyperpolarization of 131Xe via spin exchange optical pumping probed using in situ low-field NMR

We report on hyperpolarization of quadrupolar (I=3/2) 131Xe via spin-exchange optical pumping. Observations of the 131Xe polarization dynamics via in situ low-field NMR show that the estimated alkali-metal/131Xe spin-exchange rates can be large enough to compete with 131Xe spin relaxation. 131Xe polarization up to 7.6±1.5% was achieved in ∼8.5×1020 spins-a ∼100-fold improvement in the total spin angular momentum-potentially enabling various applications, including: measurement of spin-dependent neutron-131Xe s-wave scattering; sensitive searches for time-reversal violation in neutron-131Xe interactions beyond the Standard Model; and surface-sensitive pulmonary MRI.

Reversible Parahydrogen Induced Hyperpolarization of 15 N in Unmodified Amino Acids Unraveled at High Magnetic Field

Amino acids (AAs) and ammonia are metabolic markers essential for nitrogen metabolism and cell regulation in both plants and humans. NMR provides interesting opportunities to investigate these metabolic pathways, yet lacks sensitivity, especially in case of 15 N. In this study, spin order embedded in p-H2 is used to produce on-demand reversible hyperpolarization in 15 N of pristine alanine and ammonia under ambient protic conditions directly in the NMR spectrometer. This is made possible by designing a mixed-ligand Ir-catalyst, selectively ligating the amino group of AA by exploiting ammonia as a strongly competitive co-ligand and preventing deactivation of Ir by bidentate ligation of AA. The stereoisomerism of the catalyst complexes is determined by hydride fingerprinting using 1 H/D scrambling of the associated N-functional groups on the catalyst (i.e., isotopological fingerprinting), and unravelled by 2D-ZQ-NMR. Monitoring the transfer of spin order from p-H2 to 15 N nuclei of ligated and free alanine and ammonia targets using SABRE-INEPT with variable exchange delays pinpoints the monodentate elucidated catalyst complexes to be most SABRE active. Also RF-spin locking (SABRE-SLIC) enables transfer of hyperpolarization to 15 N. The presented high-field approach can be a valuable alternative to SABRE-SHEATH techniques since the obtained catalytic insights (stereochemistry and kinetics) will remain valid at ultra-low magnetic fields.

Functional lung imaging using novel and emerging MRI techniques

Respiratory diseases are leading causes of death and disability in the world. While early diagnosis is key, this has proven difficult due to the lack of sensitive and non-invasive tools. Computed tomography is regarded as the gold standard for structural lung imaging but lacks functional information and involves significant radiation exposure. Lung magnetic resonance imaging (MRI) has historically been challenging due to its short T2 and low proton density. Hyperpolarised gas MRI is an emerging technique that is able to overcome these difficulties, permitting the functional and microstructural evaluation of the lung. Other novel imaging techniques such as fluorinated gas MRI, oxygen-enhanced MRI, Fourier decomposition MRI and phase-resolved functional lung imaging can also be used to interrogate lung function though they are currently at varying stages of development. This article provides a clinically focused review of these contrast and non-contrast MR imaging techniques and their current applications in lung disease.

Hyperpolarized 129Xe MRI at low field: Current status and future directions

Magnetic Resonance Imaging (MRI) is dictated by the magnetization of the sample, and is thus a low-sensitivity imaging method. Inhalation of hyperpolarized (HP) noble gases, such as helium-3 and xenon-129, is a non-invasive, radiation-risk free imaging technique permitting high resolution imaging of the lungs and pulmonary functions, such as the lung microstructure, diffusion, perfusion, gas exchange, and dynamic ventilation. Instead of increasing the magnetic field strength, the higher spin polarization achievable from this method results in significantly higher net MR signal independent of tissue/water concentration. Moreover, the significantly longer apparent transverse relaxation time T₂* of these HP gases at low magnetic field strengths results in fewer necessary radiofrequency (RF) pulses, permitting larger flip angles; this allows for high-sensitivity imaging of in vivo animal and human lungs at conventionally low (<0.5 T) field strengths and suggests that the low field regime is optimal for pulmonary MRI using hyperpolarized gases. In this review, theory on the common spin-exchange optical-pumping method of hyperpolarization and the field dependence of the MR signal of HP gases are presented, in the context of human lung imaging. The current state-of-the-art is explored, with emphasis on both MRI hardware (low field scanners, RF coils, and polarizers) and image acquisition techniques (pulse sequences) advancements. Common challenges surrounding imaging of HP gases and possible solutions are discussed, and the future of low field hyperpolarized gas MRI is posed as being a clinically-accessible and versatile imaging method, circumventing the siting restrictions of conventional high field scanners and bringing point-of-care pulmonary imaging to global facilities.

Nanomaterials for hyperpolarized nuclear magnetic resonance and magnetic resonance imaging

Nanomaterials play an important role in the development and application of hyperpolarized materials for magnetic resonance imaging (MRI). In this context they can not only act as hyperpolarized materials which are directly imaged but also play a role as carriers for hyperpolarized gases and catalysts for para-hydrogen induced polarization (PHIP) to generate hyperpolarized substrates for metabolic imaging. Those three application possibilities are discussed, focusing on carbon-based materials for the directly imaged particles. An overview over recent developments in all three fields is given, including the early developments in each field as well as important steps towards applications in MRI, such as making the initially developed methods more biocompatible and first imaging experiments with spatial resolution in either phantoms or in vivo studies. Focusing on the important features nanomaterials need to display to be applicable in the MRI context, a wide range of different approaches to that extent is covered, giving the reader a general idea of different possibilities as well as recent developments in those different fields of hyperpolarized magnetic resonance. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Investigating Rubidium Density and Temperature Distributions in a High-Throughput 129Xe-Rb Spin-Exchange Optical Pumping Polarizer

Accurate knowledge of the rubidium (Rb) vapor density, [Rb], is necessary to correctly model the spin dynamics of 129Xe-Rb spin-exchange optical pumping (SEOP). Here we present a systematic evaluation of [Rb] within a high-throughput 129Xe-Rb hyperpolarizer during continuous-flow SEOP. Near-infrared (52S1/2→52P1/2 (D1)/52P3/2 (D2)) and violet (52S1/2→62P1/2/62P3/2) atomic absorption spectroscopy was used to measure [Rb] within 3.5 L cylindrical SEOP cells containing different spatial distributions and amounts of Rb metal. We were able to quantify deviation from the Beer-Lambert law at high optical depth for D2 and 62P3/2 absorption by comparison with measurements of the D1 and 62P1/2 absorption lines, respectively. D2 absorption deviates from the Beer-Lambert law at [Rb]D2>4×1017 m−3 whilst 52S1/2→62P3/2 absorption deviates from the Beer-Lambert law at [Rb]6P3/2>(4.16±0.01)×1019 m−3. The measured [Rb] was used to estimate a 129Xe-Rb spin exchange cross section of γ′=(1.2±0.1)×10−21 m3 s−1, consistent with spin-exchange cross sections from the literature. Significant [Rb] heterogeneity was observed in a SEOP cell containing 1 g of Rb localized at the back of the cell. While [Rb] homogeneity was improved for a greater surface area of the Rb source distribution in the cell, or by using a Rb presaturator, the measured [Rb] was consistently lower than that predicted by saturation Rb vapor density curves. Efforts to optimize [Rb] and thermal management within spin polarizer systems are necessary to maximize potential future enhancements of this technology.

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.

Nichtinvasive funktionelle Lungenbildgebung mit hyperpolarisiertem Xenon

Hintergrund

Die Magnetresonanztomographie (MRT) ist ein nichtinvasives Verfahren mit hervorragendem Weichteilkontrast. Aufgrund der geringen Protonendichte und vielen Luft-Gewebe-Übergängen ist die Anwendung in der Lunge jedoch eingeschränkt, so dass hier häufig röntgenbasierte Methoden eingesetzt werden (mit den bekannten Nachteilen ionisierender Strahlung).

Fragestellung

In dieser Übersichtsarbeit wird die Lungen-MRT mit hyperpolarisiertem Xenon-129 (Xe-MRT) dargestellt. Die Xe-MRT erlaubt einzigartige wertvolle Einblicke in die Mikrostruktur und Funktion der Lunge, einschließlich des Gasaustauschs mit roten Blutkörperchen – Parameter, die mit klinischen Standardmethoden nicht zugänglich sind.

Material und Methoden

Durch die magnetische Markierung, die Hyperpolarisierung, wird das Signal von Xenon-129 um bis zu 100.000-fach verstärkt. Hierbei werden die Elektronen von Rubidium mittels Laserlicht zunächst auf 100 % polarisiert und dann durch Stöße auf Xenon übertragen. Danach wird das hyperpolarisierte Gas in einem Beutel zum Patienten gebracht und eingeatmet, kurz bevor die MRT-Aufnahmen beginnen.

Ergebnisse

Durch spezielle Programmierungen (Sequenzen) in der MRT kann die Ventilation, Mikrostruktur oder der Gasaustausch der Lunge in 3‑D dargestellt werden. Dies ermöglicht z. B. die quantitative Darstellung von Belüftungsdefekten, der Größe der Alveolen, der Gasaufnahme im Gewebe und des Gastransfers ins Blut.

Schlussfolgerung

Die Xe-MRT liefert einzigartige Informationen über den Zustand der Lunge – nichtinvasiv, in vivo und in weniger als einer Minute.

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.

Standalone portable xenon-129 hyperpolariser for multicentre clinical magnetic resonance imaging of the lungs

OBJECTIVES

Design and build a portable xenon-129 (¹²⁹Xe) hyperpolariser for clinically accessible ¹²⁹Xe lung MRI.

METHODS

The polariser system consists of six main functional components: (i) a laser diode array and optics; (ii) a B₀ coil assembly; (iii) an oven containing an optical cell; (iv) NMR and optical spectrometers; (v) a gas-handling manifold; and (vi) a cryostat within a permanent magnet. All components run without external utilities such as compressed air or three-phase electricity, and require just three mains sockets for operation. The system can be manually transported in a lightweight van and rapidly installed on a small estates footprint in a hospital setting.

RESULTS

The polariser routinely provides polarised ¹²⁹Xe for routine clinical lung MRI. To test the concept of portability and rapid deployment, it was transported 200 km, installed at a hospital with no previous experience with the technology and ¹²⁹Xe MR images of a diagnostic quality were acquired the day after system transport and installation.

CONCLUSION

This portable ¹²⁹Xe hyperpolariser system could form the basis of a cost-effective platform for wider clinical dissemination and multicentre evaluation of ¹²⁹Xe lung MR imaging.

ADVANCES IN KNOWLEDGE

Our work successfully demonstrates the feasibility of multicentre clinical ¹²⁹Xe MRI with a portable hyperpolariser system.

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.

Hyperpolarized Solution-State NMR Spectroscopy with Optically Polarized Crystals

Nuclear spin hyperpolarization provides a promising route to overcome the challenges imposed by the limited sensitivity of nuclear magnetic resonance. Here we demonstrate that dissolution of spin-polarized pentacene-doped naphthalene crystals enables transfer of polarization to target molecules via intermolecular cross-relaxation at room temperature and moderate magnetic fields (1.45 T). This makes it possible to exploit the high spin polarization of optically polarized crystals, while mitigating the challenges of its transfer to external nuclei. With this method, we inject the highly polarized mixture into a benchtop NMR spectrometer and observe the polarization dynamics for target ¹H nuclei. Although the spectra are radiation damped due to the high naphthalene magnetization, we describe a procedure to process the data to obtain more conventional NMR spectra and extract the target nuclei polarization. With the entire process occurring on a time scale of 1 min, we observe NMR signals enhanced by factors between -200 and -1730 at 1.45 T for a range of small molecules.

Synergies between Hyperpolarized NMR and Microfluidics: A Review

Hyperpolarized nuclear magnetic resonance and lab-on-a-chip microfluidics are two dynamic, but until recently quite distinct, fields of research. Recent developments in both areas increased their synergistic overlap. By microfluidic integration, many complex experimental steps can be brought together onto a single platform. Microfluidic devices are therefore increasingly finding applications in medical diagnostics, forensic analysis, and biomedical research. In particular, they provide novel and powerful ways to culture cells, cell aggregates, and even functional models of entire organs. Nuclear magnetic resonance is a non-invasive, high-resolution spectroscopic technique which allows real-time process monitoring with chemical specificity. It is ideally suited for observing metabolic and other biological and chemical processes in microfluidic systems. However, its intrinsically low sensitivity has limited its application. Recent advances in nuclear hyperpolarization techniques may change this: under special circumstances, it is possible to enhance NMR signals by up to 5 orders of magnitude, which dramatically extends the utility of NMR in the context of microfluidic systems. Hyperpolarization requires complex chemical and/or physical manipulations, which in turn may benefit from microfluidic implementation. In fact, many hyperpolarization methodologies rely on processes that are more efficient at the micro-scale, such as molecular diffusion, penetration of electromagnetic radiation into a sample, or restricted molecular mobility on a surface. In this review we examine the confluence between the fields of hyperpolarization-enhanced NMR and microfluidics, and assess how these areas of research have mutually benefited one another, and will continue to do so.

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.

Protocols for multi-site trials using hyperpolarized 129 Xe MRI for imaging of ventilation, alveolar-airspace size, and gas exchange: A position paper from the 129 Xe MRI clinical trials consortium

Hyperpolarized (HP) ¹²⁹ Xe MRI uniquely images pulmonary ventilation, gas exchange, and terminal airway morphology rapidly and safely, providing novel information not possible using conventional imaging modalities or pulmonary function tests. As such, there is mounting interest in expanding the use of biomarkers derived from HP ¹²⁹ Xe MRI as outcome measures in multi-site clinical trials across a range of pulmonary disorders. Until recently, HP ¹²⁹ Xe MRI techniques have been developed largely independently at a limited number of academic centers, without harmonizing acquisition strategies. To promote uniformity and adoption of HP ¹²⁹ Xe MRI more widely in translational research, multi-site trials, and ultimately clinical practice, this position paper from the ¹²⁹ Xe MRI Clinical Trials Consortium (https://cpir.cchmc.org/XeMRICTC) recommends standard protocols to harmonize methods for image acquisition in HP ¹²⁹ Xe MRI. Recommendations are described for the most common HP gas MRI techniques-calibration, ventilation, alveolar-airspace size, and gas exchange-across MRI scanner manufacturers most used for this application. Moreover, recommendations are described for ¹²⁹ Xe dose volumes and breath-hold standardization to further foster consistency of imaging studies. The intention is that sites with HP ¹²⁹ Xe MRI capabilities can readily implement these methods to obtain consistent high-quality images that provide regional insight into lung structure and function. While this document represents consensus at a snapshot in time, a roadmap for technical developments is provided that will further increase image quality and efficiency. These standardized dosing and imaging protocols will facilitate the wider adoption of HP ¹²⁹ Xe MRI for multi-site pulmonary research.

Automated Low-Cost In Situ IR and NMR Spectroscopy Characterization of Clinical-Scale 129Xe Spin-Exchange Optical Pumping

We present on the utility of in situ nuclear magnetic resonance (NMR) and near-infrared (NIR) spectroscopic techniques for automated advanced analysis of the ¹²⁹Xe hyperpolarization process during spin-exchange optical pumping (SEOP). The developed software protocol, written in the MATLAB programming language, facilitates detailed characterization of hyperpolarized contrast agent production efficiency based on determination of key performance indicators, including the maximum achievable ¹²⁹Xe polarization, steady-state Rb-¹²⁹Xe spin-exchange and ¹²⁹Xe polarization build-up rates, ¹²⁹Xe spin-relaxation rates, and estimates of steady-state Rb electron polarization. Mapping the dynamics of ¹²⁹Xe polarization and relaxation as a function of SEOP temperature enables systematic optimization of the batch-mode SEOP process. The automated analysis of a typical experimental data set, encompassing ∼300 raw NMR and NIR spectra combined across six different SEOP temperatures, can be performed in under 5 min on a laptop computer. The protocol is designed to be robust in operation on any batch-mode SEOP hyperpolarizer device. In particular, we demonstrate the implementation of a combination of low-cost NIR and low-frequency NMR spectrometers (∼$1,100 and ∼$300 respectively, ca. 2020) for use in the described protocols. The demonstrated methodology will aid in the characterization of NMR hyperpolarization hardware in the context of SEOP and other hyperpolarization techniques for more robust and less expensive clinical production of HP ¹²⁹Xe and other contrast agents.

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.

In vivo methods and applications of xenon-129 magnetic resonance

Hyperpolarised gas lung MRI using xenon-129 can provide detailed 3D images of the ventilated lung airspaces, and can be applied to quantify lung microstructure and detailed aspects of lung function such as gas exchange. It is sensitive to functional and structural changes in early lung disease and can be used in longitudinal studies of disease progression and therapy response. The ability of ¹²⁹Xe to dissolve into the blood stream and its chemical shift sensitivity to its local environment allow monitoring of gas exchange in the lungs, perfusion of the brain and kidneys, and blood oxygenation. This article reviews the methods and applications of in vivo¹²⁹Xe MR in humans, with a focus on the physics of polarisation by optical pumping, radiofrequency coil and pulse sequence design, and the in vivo applications of ¹²⁹Xe MRI and MRS to examine lung ventilation, microstructure and gas exchange, blood oxygenation, and perfusion of the brain and kidneys.

Heterogeneous Parahydrogen-Induced Polarization of Diethyl Ether for Magnetic Resonance Imaging Applications

Magnetic resonance imaging (MRI) with the use of hyperpolarized gases as contrast agents provides valuable information on lungs structure and function. While the technology of 129 Xe hyperpolarization for clinical MRI research is well developed, it requires the expensive equipment for production and detection of hyperpolarized 129 Xe. Herein we present the 1 H hyperpolarization of diethyl ether vapor that can be imaged on any clinical MRI scanner. 1 H nuclear spin polarization of up to 1.3 % was achieved using heterogeneous hydrogenation of ethyl vinyl ether with parahydrogen over Rh/TiO2 catalyst. Liquefaction of diethyl ether vapor proceeds with partial preservation of hyperpolarization and prolongs its lifetime by ≈10 times. The proof-of-principle 2D 1 H MRI of hyperpolarized diethyl ether was demonstrated with 0.1×1.1 mm2 spatial and 120 ms temporal resolution. The long history of use of diethyl ether for anesthesia is expected to facilitate the clinical translation of the presented approach.

Cyclodextrin-Based Contrast Agents for Medical Imaging

Cyclodextrins (CDs) are naturally occurring cyclic oligosaccharides consisting of multiple glucose subunits. CDs are widely used in host–guest chemistry and biochemistry due to their structural advantages, biocompatibility, and ability to form inclusion complexes. Recently, CDs have become of high interest in the field of medical imaging as a potential scaffold for the development of a large variety of the contrast agents suitable for magnetic resonance imaging, ultrasound imaging, photoacoustic imaging, positron emission tomography, single photon emission computed tomography, and computed tomography. The aim of this review is to summarize and highlight the achievements in the field of cyclodextrin-based contrast agents for medical imaging.

Parahydrogen-Induced Polarization of Diethyl Ether Anesthetic

The growing interest in magnetic resonance imaging (MRI) for assessing regional lung function relies on the use of nuclear spin hyperpolarized gas as a contrast agent. The long gas-phase lifetimes of hyperpolarized 129 Xe make this inhalable contrast agent acceptable for clinical research today despite limitations such as high cost, low throughput of production and challenges of 129 Xe imaging on clinical MRI scanners, which are normally equipped with proton detection only. We report on low-cost and high-throughput preparation of proton-hyperpolarized diethyl ether, which can be potentially employed for pulmonary imaging with a nontoxic, simple, and sensitive overall strategy using proton detection commonly available on all clinical MRI scanners. Diethyl ether is hyperpolarized by pairwise parahydrogen addition to vinyl ethyl ether and characterized by 1 H NMR spectroscopy. Proton polarization levels exceeding 8 % are achieved at near complete chemical conversion within seconds, causing the activation of radio amplification by stimulated emission radiation (RASER) throughout detection. Although gas-phase T1 relaxation of hyperpolarized diethyl ether (at partial pressure of 0.5 bar) is very efficient, with T1 of ca. 1.2 second, we demonstrate that, at low magnetic fields, the use of long-lived singlet states created via pairwise parahydrogen addition extends the relaxation decay by approximately threefold, paving the way to bioimaging applications and beyond.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

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.

Detecting protein-protein interactions by Xe-129 NMR

Detection of protein-protein interactions (PPIs) is limited by current bioanalytical methods. A protein complementation assay (PCA), split TEM-1 β-lactamase, interacts with xenon at the interface of the TEM-1 fragments. Reconstitution of TEM-1-promoted here by cFos/cJun leucine zipper interaction-gives rise to sensitive 129Xe NMR signal in bacterial cells.

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.

Parahydrogen-Induced Radio Amplification by Stimulated Emission of Radiation

Radio amplification by stimulated emission of radiation (RASER) was recently discovered in a low-field NMR spectrometer incorporating a highly specialized radio-frequency resonator, where a high degree of proton-spin polarization was achieved by reversible parahydrogen exchange. RASER activity, which results from the coherent coupling between the nuclear spins and the inductive detector, can overcome the limits of frequency resolution in NMR. Here we show that this phenomenon is not limited to low magnetic fields or the use of resonators with high-quality factors. We use a commercial bench-top 1.4 T NMR spectrometer in conjunction with pairwise parahydrogen addition producing proton-hyperpolarized molecules in the Earth's magnetic field (ALTADENA condition) or in a high magnetic field (PASADENA condition) to induce RASER without any radio-frequency excitation pulses. The results demonstrate that RASER activity can be observed on virtually any NMR spectrometer and measures most of the important NMR parameters with high precision.

Functionalized Lipopeptide Micelles as Highly Efficient NMR Depolarization Seed Points for Targeted Cell Labelling in Xenon MRI

Improving diagnostic imaging and therapy by targeted compound delivery to pathological areas and across biological barriers is of urgent need. A lipopeptide, P-CrA-A2, composed of a highly cationic peptide sequence (A2), an N-terminally attached palmitoyl chain (P) and cryptophane molecule (CrA) for preferred uptake into blood-brain barrier (BBB) capillary endothelial cells, was generated. CrA allows reversible binding of Xe for NMR detection with hyperpolarized nuclei. The lipopeptide forms size-optimized micelles with a diameter of about 11 nm at low micromolar concentration. Their high local CrA payload has a strong and switchable impact on the bulk magnetization through Hyper-CEST detection. Covalent fixation of CrA does not impede micelle formation and does not hamper its host functionality but simplifies Xe access to hosts for inducing saturation transfer. Xe Hyper-CEST magnetic resonance imaging (MRI) allows for distinguishing BBB endothelial cells from control aortic endothelial cells, and the small micelle volume with a sevenfold improved CrA-loading density compared to liposomal carriers allows preferred cell labelling with a minimally invasive volume (≈16 000-fold more efficient than ¹⁹ F cell labelling). Thus, these nanoscopic particles combine selectivity for human brain capillary endothelial cells with great sensitivity of Xe Hyper-CEST MRI and might be a potential MRI tool in brain diagnostics.

Multiplexed 129Xe HyperCEST MRI Detection of Genetically Reconstituted Bacterial Protein Nanoparticles in Human Cancer Cells

Gas vesicle nanoparticles (GVs) are gas-containing protein assemblies expressed in bacteria and archaea. Recently, GVs have gained considerable attention for biotechnological applications as genetically encodable contrast agents for MRI and ultrasonography. However, at present, the practical use of GVs is hampered by a lack of robust methodology for their induction into mammalian cells. Here, we demonstrate the genetic reconstitution of protein nanoparticles with characteristic bicone structures similar to natural GVs in a human breast cancer cell line KPL-4 and genetic control of their size and shape through expression of reduced sets of humanized gas vesicle genes cloned into Tol2 transposon vectors, referencing the natural gas vesicle gene clusters of the cyanobacteria planktothrix rubescens/agardhii. We then report the utility of these nanoparticles as multiplexed, sensitive, and genetically encoded contrast agents for hyperpolarized xenon chemical exchange saturation transfer (HyperCEST) MRI.

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.

Probing Reversible Guest Binding with Hyperpolarized 129Xe-NMR: Characteristics and Applications for Cucurbit[n]urils

Cucurbit[n]urils (CB[n]s) are a family of macrocyclic host molecules that find various applications in drug delivery, molecular switching, and dye displacement assays. The CB[n]s with n = 5-7 have also been studied with 129Xe-NMR. They bind the noble gas with a large range of exchange rates. Starting with insights from conventional direct detection of bound Xe, this review summarizes recent achievements with chemical exchange saturation transfer (CEST) detection of efficiently exchanging Xe in various CB[n]-based supramolecular systems. Unprecedented sensitivity has been reached by combining the CEST method with hyperpolarized Xe, the production of which is also briefly described. Applications such as displacement assays for enzyme activity detection and rotaxanes as emerging types of Xe biosensors are likewise discussed in the context of biomedical applications and pinpoint future directions for translating this field to preclinical studies.

Nanoparticle-Based Contrast Agents for 129Xe HyperCEST NMR and MRI Applications

Spin hyperpolarization techniques have enabled important advancements in preclinical and clinical MRI applications to overcome the intrinsic low sensitivity of nuclear magnetic resonance. Functionalized xenon biosensors represent one of these approaches. They combine two amplification strategies, namely, spin exchange optical pumping (SEOP) and chemical exchange saturation transfer (CEST). The latter one requires host structures that reversibly bind the hyperpolarized noble gas. Different nanoparticle approaches have been implemented and have enabled molecular MRI with 129Xe at unprecedented sensitivity. This review gives an overview of the Xe biosensor concept, particularly how different nanoparticles address various critical aspects of gas binding and exchange, spectral dispersion for multiplexing, and targeted reporter delivery. As this concept is emerging into preclinical applications, comprehensive sensor design will be indispensable in translating the outstanding sensitivity potential into biomedical molecular imaging applications.

Low-Concentration Measurements of Nuclear Spin-Induced Optical Rotation Using SABRE Hyperpolarization

Nuclear spin-induced optical rotation (NSOR) is a promising phenomenon for molecular structure elucidation due to its sensitivity to electronic structure near atomic nuclei. It is the only experimentally verified nuclear magneto-optic effect (NMOE), so far observed usually in neat liquids or in concentrated binary mixtures, with the proportion of the minor component at least 10%. We report a method to extend the lower-concentration range of NSOR measurements by 2 orders of magnitude by employing continuous-flow SABRE (signal amplification by reversible exchange) hyperpolarization. This approach significantly increases the sensitivity of NSOR and enables its detection in dilute samples, as demonstrated with measurements of NSOR of 90 mmol/L solutions of pyridine and pyrazine. The results are compared with first-principles calculations, and good agreement is found. The possibility to measure low-concentration solutions significantly extends the pool of samples available for further studies of NMOEs.

Relayed hyperpolarization from para-hydrogen improves the NMR detectability of alcohols

The detection of alcohols by magnetic resonance techniques is important for their characterization and the monitoring of chemical change. Hyperpolarization processes can make previously inpractical measurements, such as the determination of low concentration intermediates, possible. Here, we investigate the SABRE-Relay method in order to define its key characteristics and improve the resulting 1H NMR signal gains which subsequently approach 103 per proton. We identify optimal amine proton transfer agents for SABRE-Relay and show how catalyst structure influences the outcome. The breadth of the method is revealed by expansion to more complex alcohols and the polarization of heteronuclei.

Stability Enhancement of 129Xe Hyperpolarizing System Using Alkali Metal Vapor in Spin-Exchange Optical Pumping Cell to Achieve High NMR Sensitivity

Hyperpolarized (HP) ¹²⁹Xe NMR and MRI have enabled ¹²⁹Xe studies with extraordinarily enhanced sensitivity, stimulating new developments in magnetic resonance in chemistry, physics, biology and medicine. However, the standard method of HP ¹²⁹Xe production inevitably demands Rb vapor for the excitation, which has made the method very sensitive to impurities such as water or oxygen. This is the case especially in the recirculating system. In the present study, stability of the hyperpolarizing system is discussed by proposing the "cell decay constant", which symbolizes the decay rate of the NMR signal obtained from the system. The cell decay constant is effectively decreased to 1/3 by introducing separated chambers and mechanical stirring of the alkali metals used in the system, making it effective for accumulating FIDs over 30 to 100 h. The newly developed hyperpolarizing system has been successfully applied for newly detecting a broad signal at 190 ppm with an industrial material Nanofiber.

Rotaxane Probes for the Detection of Hydrogen Peroxide by 129 Xe HyperCEST NMR Spectroscopy

The development of sensitive and chemically selective MRI contrast agents is imperative for the early detection and diagnosis of many diseases. Conventional responsive contrast agents used in 1 H MRI are impaired by the high abundance of protons in the body. 129 Xe hyperCEST NMR/MRI comprises a highly sensitive complement to traditional 1 H MRI because of its ability to report specific chemical environments. To date, the scope of responsive 129 Xe NMR contrast agents lacks breadth in the specific detection of small molecules, which are often important markers of disease. Herein, we report the synthesis and characterization of a rotaxane-based 129 Xe hyperCEST NMR contrast agent that can be turned on in response to H2 O2 , which is upregulated in several disease states. Added H2 O2 was detected by 129 Xe hyperCEST NMR spectroscopy in the low micromolar range, as well as H2 O2 produced by HEK 293T cells activated with tumor necrosis factor.

Laser- and cryogenic probe-assisted NMR enables hypersensitive analysis of biomolecules at submicromolar concentration

Solution-state NMR typically requires 100 μM to 1 mM samples. This limitation prevents applications to mass-limited and aggregation-prone target molecules. Photochemically induced dynamic nuclear polarization was adapted to data collection on low-concentration samples by radiofrequency gating, enabling rapid 1D NMR spectral acquisition on aromatic amino acids and proteins bearing aromatic residues at nanomolar concentration, i.e., a full order of magnitude below other hyperpolarization techniques in liquids. Both backbone H¹-C¹³ and side-chain resonances were enhanced, enabling secondary and tertiary structure analysis of proteins with remarkable spectral editing, via the ¹³C PREPRINT pulse sequence. Laser-enhanced 2D NMR spectra of 5 μM proteins at 600 MHz display 30-fold better S/N than conventional 2D data collected at 900 MHz. Sensitivity enhancements achieved with this technology, denoted as low-concentration photo-CIDNP (LC-photo-CIDNP), depend only weakly on laser intensity, highlighting the opportunity of safer and more cost-effective hypersensitive NMR applications employing low-power laser sources.

Single breath-hold measurement of pulmonary gas exchange and diffusion in humans with hyperpolarized 129 Xe MR

Pulmonary diseases usually result in changes of the blood-gas exchange function in the early stages. Gas exchange across the respiratory membrane and gas diffusion in the alveoli can be quantified using hyperpolarized ¹²⁹ Xe MR via chemical shift saturation recovery (CSSR) and diffusion-weighted imaging (DWI), respectively. Generally, CSSR and DWI data have been collected in separate breaths in humans. Unfortunately, the lung inflation level cannot be the exactly same in different breaths, which causes fluctuations in blood-gas exchange and pulmonary microstructure. Here we combine CSSR and DWI obtained with compressed sensing, to evaluate the gas diffusion and exchange function within a single breath-hold in humans. A new parameter, namely the perfusion factor of the respiratory membrane (SVR_d/g ), is proposed to evaluate the gas exchange function. Hyperpolarized ¹²⁹ Xe MR data are compared with pulmonary function tests and computed tomography examinations in healthy young, age-matched control, and chronic obstructive pulmonary disease human cohorts. SVR_d/g decreases as the ventilation impairment and emphysema index increase. Our results indicate that the proposed method has the potential to detect the extent of lung parenchyma destruction caused by age and pulmonary diseases, and it would be useful in the early diagnosis of pulmonary diseases in clinical practice.