NMR hyperpolarization techniques for biomedicine

Background-Free Proton NMR Spectroscopy with Radiofrequency Amplification by Stimulated Emission Radiation

We report on the utility of Radiofrequency Amplification by Stimulated Emission Radiation (RASER) for background-free proton detection of hyperpolarized biomolecules. We performed hyperpolarization of ≈0.3 M ethyl acetate via pairwise parahydrogen addition to vinyl acetate. A proton NMR signal with signal-to-noise ratio exceeding 100 000 was detected without radio-frequency excitation at the clinically relevant magnetic field of 1.4 T using a standard (non-cryogenic) inductive detector with quality factor of Q=68. No proton background signal was observed from protonated solvent (methanol) or other added co-solvents such as ethanol, water or bovine serum. Moreover, we demonstrate RASER detection without radio-frequency excitation of a bolus of hyperpolarized contrast agent in biological fluid. Completely background-free proton detection of hyperpolarized contrast agents in biological media paves the way to new applications in the areas of high-resolution NMR spectroscopy and in vivo spectroscopy and imaging.

Bimodal fluorescence/magnetic resonance molecular probes with extended spin lifetimes

Bimodal molecular probes combining nuclear magnetic resonance (NMR) and fluorescence have been widely studied in basic science, as well as clinical research. The investigation of spin phenomena holds promise to broaden the scope of available probes allowing deeper insights into physiological processes. Herein, a class of molecules with a bimodal character with respect to fluorescence and nuclear spin singlet states is introduced. Singlet states are NMR silent but can be probed indirectly. Symmetric, perdeuterated molecules, in which the singlet states can be populated by vanishingly small electron‐mediated couplings (below 1 Hz) are reported. The lifetimes of these states are an order of magnitude longer than the longitudinal relaxation times and up to four minutes at 7 T. Moreover, these molecules show either aggregation induced emission (AIE) or aggregation caused quenching (ACQ) with respect to their fluorescence. In the latter case, the existence of excited dimers, which are proposed to use in a switchable manner in combination with the quenching of nuclear spin singlet states, is observed

Steric and electronic effects on the 1 H hyperpolarisation of substituted pyridazines by signal amplification by reversible exchange

Utility of the pyridazine motif is growing in popularity as pharmaceutical and agrochemical agents. The detection and structural characterisation of such materials is therefore imperative for the successful development of new products. Signal amplification by reversible exchange (SABRE) offers a route to dramatically improve the sensitivity of magnetic resonance methods, and we apply it here to the rapid and cost-effective hyperpolarisation of substituted pyridazines. The 33 substrates investigated cover a range of steric and electronic properties and their capacity to perform highly effective SABRE is assessed. We find the method to be tolerant to a broad range of electron donating and withdrawing groups; however, good sensitivity is evident when steric bulk is added to the 3- and 6-positions of the pyridazine ring. We optimise the method by reference to a disubstituted ester that yields signal gains of >9000-fold at 9.4 T (>28% spin polarisation).

SABRE-enhanced real-time pure shift NMR spectroscopy

Pure shift nuclear magnetic resonance (NMR) methods suppress the effect of homonuclear scalar couplings to produce NMR spectra consisting solely of a single signal for each chemically distinct site. They are increasingly relied upon for analysis of complex molecules and mixtures as they overcome the extensive signal overlap that complicates proton NMR spectra of all but the simplest species. Current broadband pure shift methodologies for 1D proton spectra suffer from reduced sensitivity compared with their conventional counterparts and typically require a large amount of instrument time for low concentration samples. In this study, we demonstrate how the sensitivity limitation may be overcome by transiently increasing the bulk polarization using signal amplification by reversible exchange (SABRE) hyperpolarization. We utilize para-enriched dihydrogen to enhance the pure shift NMR resonances of pyridine by up to a factor of 60 in a single-scan experiment and extend this to propose a method to unambiguously determine mixture components based on the enhancement of their pure shift NMR signals.

Hyperpolarization of common antifungal agents with SABRE

Signal amplification by reversible exchange (SABRE) is a robust and inexpensive hyperpolarization (HP) technique to enhance nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) signals using parahydrogen (pH2 ). The substrate scope of SABRE is continually expanding. Here, we present the polarization of three antifungal drugs (voriconazole, clotrimazole, and fluconazole) and elicit the detailed HP mechanisms for 1 H and 15 N nuclei. In this exploratory work, 15 N polarization values of ~1% were achieved using 50% pH2 in solution of 3-mM catalyst and 60-mM substrate in perdeuterated methanol. All hyperpolarized 15 N sites exhibited long T1 in excess of 1 min at a clinically relevant field of 1 T. Hyperpolarizing common drugs is of interest due to their potential biomedical applications as MRI contrast agents or to enable studies on protein dynamics at physiological concentrations. We optimize the polarization with respect to temperature and the polarization transfer field (PTF) for 1 H nuclei in the millitesla regime and for 15 N nuclei in the microtesla regime, which provides detailed insights into exchange kinetics and spin evolution. This work broadens the SABRE substrate scope and provides mechanistic and kinetic insights into the HP process.

15 N hyperpolarisation of the antiprotozoal drug ornidazole by Signal Amplification By Reversible Exchange in aqueous medium

Signal amplification by reversible exchange (SABRE) offers a cost-effective route to boost nuclear magnetic resonance (NMR) signal by several orders of magnitude by employing readily available para-hydrogen as a source of hyperpolarisation. Although ¹ H spins have been the natural choice of SABRE hyperpolarisation since its inception due to its simplicity and accessibility, limited spin lifetimes of ¹ H makes it harder to employ them in a range of time-dependent NMR experiments. Heteronuclear spins, for example, ¹³ C and ¹⁵ N, in general have much longer T₁ lifetimes and thereby are found to be more suitable for hyperpolarised biological applications as demonstrated previously by para-hydrogen induced polarisation (PHIP) and dynamic nuclear polarisation (DNP). In this study we demonstrate a simple procedure to enhance ¹⁵ N signal of an antibiotic drug ornidazole by up to 71,000-folds with net ¹⁵ N polarisation reaching ~23%. Further, the effect of co-ligand strategy is studied in conjunction with the optimum field transfer protocols and consequently achieving ¹⁵ N hyperpolarised spin lifetime of >3 min at low field. Finally, we present a convenient route to harness the hyperpolarised solution in aqueous medium free from catalyst contamination leading to a strong ¹⁵ N signal detection for an extended duration of time.

Towards large-scale steady-state enhanced nuclear magnetization with in situ detection

Signal amplification by reversible exchange (SABRE) boosts NMR signals of various nuclei enabling new applications spanning from magnetic resonance imaging to analytical chemistry and fundamental physics. SABRE is especially well positioned for continuous generation of enhanced magnetization on a large scale; however, several challenges need to be addressed for accomplishing this goal. Specifically, SABRE requires (i) a specialized catalyst capable of reversible H₂ activation and (ii) physical transfer of the sample from the point of magnetization generation to the point of detection (e.g., a high-field or a benchtop nuclear magnetic resonance [NMR] spectrometer). Moreover, (iii) continuous parahydrogen bubbling accelerates solvent (e.g., methanol) evaporation, thereby limiting the experimental window to tens of minutes per sample. In this work, we demonstrate a strategy to rapidly generate the best-to-date precatalyst (a compound that is chemically modified in the course of the reaction to yield the catalyst) for SABRE, [Ir(IMes)(COD)Cl] (IMes = 1,3-bis-[2,4,6-trimethylphenyl]-imidazol-2-ylidene; COD = cyclooctadiene) via a highly accessible synthesis. Second, we measure hyperpolarized samples using a home-built zero-field NMR spectrometer and study the field dependence of hyperpolarization directly in the detection apparatus, eliminating the need to physically move the sample during the experiment. Finally, we prolong the measurement time and reduce evaporation by presaturating parahydrogen with the solvent vapor before bubbling into the sample. These advancements extend opportunities for exploring SABRE hyperpolarization by researchers from various fields and pave the way to producing large quantities of hyperpolarized material for long-lasting detection of SABRE-derived nuclear magnetization.

Determining the enantioselectivity of asymmetric hydrogenation through parahydrogen-induced hyperpolarization

Asymmetric hydrogenation plays an essential role for both academic research and industry to produce enantiomeric pure chiral molecules. Although nuclear magnetic resonance (NMR) is powerful in determining the yields of hydrogenation, it is still challenging to use NMR for chirality-related analysis. Herein, we applied parahydrogen-induced hyperpolarization (PHIP) NMR to determine the enantioselectivity of asymmetric hydrogenation and the absolute chirality of products. We hyperpolarized two types of unsaturated amino acid precursors, i.e., methyl-α-acetoamido cinnamate and (E)-ethyl 3-acetamidobut-2-enoate. Hydrogenation of prochiral substrates with parahydrogen gave temporary hyperpolarized diastereoisomers, which exhibit different PHIP patterns distinguishable in ¹H NMR. After assigning the NMR peaks by density functional theory calculations, we simulated the PHIP patterns of all the possible temporary hyperpolarized diastereoisomers and unambiguously assigned the chirality of the products and the enantioselectivity of asymmetric hydrogenation. Our work demonstrates the application and potential of PHIP in revealing the mechanism of asymmetric hydrogenation.

Density Functional Theory Study of Reaction Equilibria in Signal Amplification by Reversible Exchange

The front cover artwork is provided by the groups of Prof. Thomas Theis (North Carolina State University) Prof. Volker Blum (Duke University). The image shows the reaction network of Signal Amplification by Reversible Exchange (SABRE), elucidated by density functional theory (DFT). Read the full text of the Review at 10.1002/cphc.202100204.

Density Functional Theory Study of Reaction Equilibria in Signal Amplification by Reversible Exchange

An in-depth theoretical analysis of key chemical equilibria in Signal Amplification by Reversible Exchange (SABRE) is provided, employing density functional theory calculations to characterize the likely reaction network. For all reactions in the network, the potential energy surface is probed to identify minimum energy pathways. Energy barriers and transition states are calculated, and harmonic transition state theory is applied to calculate exchange rates that approximate experimental values. The reaction network energy surface can be modulated by chemical potentials that account for the dependence on concentration, temperature, and partial pressure of molecular constituents (hydrogen, methanol, pyridine) supplied to the experiment under equilibrium conditions. We show that, under typical experimental conditions, the Gibbs free energies of the two key states involved in pyridine-hydrogen exchange at the common Ir-IMes catalyst system in methanol are essentially the same, i. e., nearly optimal for SABRE. We also show that a methanol-containing intermediate is plausible as a transient species in the process.

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.

NMR spectroscopy of wastewater: A review, case study, and future potential

NMR spectroscopy is arguably the most powerful tool for the study of molecular structures and interactions, and is increasingly being applied to environmental research, such as the study of wastewater. With over 97% of the planet's water being saltwater, and two thirds of freshwater being frozen in the ice caps and glaciers, there is a significant need to maintain and reuse the remaining 1%, which is a precious resource, critical to the sustainability of most life on Earth. Sanitation and reutilization of wastewater is an important method of water conservation, especially in arid regions, making the understanding of wastewater itself, and of its treatment processes, a highly relevant area of environmental research. Here, the benefits, challenges and subtleties of using NMR spectroscopy for the analysis of wastewater are considered. First, the techniques available to overcome the specific challenges arising from the nature of wastewater (which is a complex and dilute matrix), including an examination of sample preparation and NMR techniques (such as solvent suppression), in both the solid and solution states, are discussed. Then, the arsenal of available NMR techniques for both structure elucidation (e.g., heteronuclear, multidimensional NMR, homonuclear scalar coupling-based experiments) and the study of intermolecular interactions (e.g., diffusion, nuclear Overhauser and saturation transfer-based techniques) in wastewater are examined. Examples of wastewater NMR studies from the literature are reviewed and potential areas for future research are identified. Organized by nucleus, this review includes the common heteronuclei (¹³C, ¹⁵N, ¹⁹F, ³¹P, ²⁹Si) as well as other environmentally relevant nuclei and metals such as ²⁷Al, ⁵¹V, ²⁰⁷Pb and ¹¹³Cd, among others. Further, the potential of additional NMR methods such as comprehensive multiphase NMR, NMR microscopy and hyphenated techniques (for example, LC-SPE-NMR-MS) for advancing the current understanding of wastewater are discussed. In addition, a case study that combines natural abundance (i.e. non-concentrated), targeted and non-targeted NMR to characterize wastewater, along with in vivo based NMR to understand its toxicity, is included. The study demonstrates that, when applied comprehensively, NMR can provide unique insights into not just the structure, but also potential impacts, of wastewater and wastewater treatment processes. Finally, low-field NMR, which holds considerable future potential for on-site wastewater monitoring, is briefly discussed. In summary, NMR spectroscopy is one of the most versatile tools in modern science, with abilities to study all phases (gases, liquids, gels and solids), chemical structures, interactions, interfaces, toxicity and much more. The authors hope this review will inspire more scientists to embrace NMR, given its huge potential for both wastewater analysis in particular and environmental research in general.

Ultrafast Laplace NMR to study metal-ligand interactions in reversible polarisation transfer from parahydrogen

Laplace Nuclear Magnetic Resonance (NMR) can determine relaxation parameters and diffusion constants, giving valuable information about molecular structure and dynamics. Information about relaxation times (T1 and T2) and the self-diffusion coefficient (D) can be extracted from exponentially decaying NMR signals by performing a Laplace transform, which is a different approach to traditional NMR involving Fourier transform of a free induction decay. Ultrafast Laplace NMR uses spatial encoding to collect the entire data set in just a single scan which provides orders of magnitude time savings. In this work we use ultrafast Laplace NMR D-T2 correlation sequences to measure key relaxation (T2) and diffusion (D) parameters of methanolic solutions containing pyridine. For the first time we combine this technique with the hyperpolarisation technique Signal Amplification By Reversible Exchange (SABRE), which employs an iridium catalyst to reversibly transfer polarisation from parahydrogen, to boost the 1H NMR signals of pyridine by up to 300-fold. We demonstrate use of ultrafast Laplace NMR to monitor changes in pyridine T2 and D associated with ligation to the iridium SABRE catalyst and kinetic isotope exchange reactions. The combined 1440-fold reduction in experiment time and 300-fold 1H NMR signal enhancement allow the determination of pyridine D coefficients and T2 values at 25 mM concentrations in just 3 seconds using SABRE hyperpolarised ultrafast Laplace NMR.

Synthetic Approaches for 15 N-Labeled Hyperpolarized Heterocyclic Molecular Imaging Agents for 15 N NMR Signal Amplification by Reversible Exchange in Microtesla Magnetic Fields

NMR hyperpolarization techniques enhance nuclear spin polarization by several orders of magnitude resulting in corresponding sensitivity gains. This enormous sensitivity gain enables new applications ranging from studies of small molecules by using high-resolution NMR spectroscopy to real-time metabolic imaging in vivo. Several hyperpolarization techniques exist for hyperpolarization of a large repertoire of nuclear spins, although the 13 C and 15 N sites of biocompatible agents are the key targets due to their widespread use in biochemical pathways. Moreover, their long T1 allows hyperpolarized states to be retained for up to tens of minutes. Signal amplification by reversible exchange (SABRE) is a low-cost and ultrafast hyperpolarization technique that has been shown to be versatile for the hyperpolarization of 15 N nuclei. Although large sensitivity gains are enabled by hyperpolarization, 15 N natural abundance is only ∼0.4 %, so isotopic labeling of the molecules to be hyperpolarized is required in order to take full advantage of the hyperpolarized state. Herein, we describe selected advances in the preparation of 15 N-labeled compounds with the primary emphasis on using these compounds for SABRE polarization in microtesla magnetic fields through spontaneous polarization transfer from parahydrogen. Also, these principles can certainly be applied for hyperpolarization of these emerging contrast agents using dynamic nuclear polarization and other techniques.

Heterogeneous 1 H and 13 C Parahydrogen-Induced Polarization of Acetate and Pyruvate Esters

Magnetic resonance imaging of [1-13 C]hyperpolarized carboxylates (most notably, [1-13 C]pyruvate) allows one to visualize abnormal metabolism in tumors and other pathologies. Herein, we investigate the efficiency of 1 H and 13 C hyperpolarization of acetate and pyruvate esters with ethyl, propyl and allyl alcoholic moieties using heterogeneous hydrogenation of corresponding vinyl, allyl and propargyl precursors in isotopically unlabeled and 1-13 C-enriched forms with parahydrogen over Rh/TiO2 catalysts in methanol-d4 and in D2 O. The maximum obtained 1 H polarization was 0.6±0.2 % (for propyl acetate in CD3 OD), while the highest 13 C polarization was 0.10±0.03 % (for ethyl acetate in CD3 OD). Hyperpolarization of acetate esters surpassed that of pyruvates, while esters with a triple carbon-carbon bond in unsaturated alcoholic moiety were less efficient as parahydrogen-induced polarization precursors than esters with a double bond. Among the compounds studied, the maximum 1 H and 13 C NMR signal intensities were observed for propyl acetate. Ethyl acetate yielded slightly less intense NMR signals which were dramatically greater than those of other esters under study.

Preparation of individual magnetic sub-levels of 4He(23S1) in a supersonic beam using laser optical pumping and magnetic hexapole focusing

We compare two different experimental techniques for the magnetic-sub-level preparation of metastable ⁴He in the 2³S₁ level in a supersonic beam, namely, magnetic hexapole focusing and optical pumping by laser radiation. At a beam velocity of v = 830 m/s, we deduce from a comparison with a particle trajectory simulation that up to 99% of the metastable atoms are in the M_J″ = +1 sub-level after magnetic hexapole focusing. Using laser optical pumping via the 2³P₂-2³S₁ transition, we achieve a maximum efficiency of 94% ± 3% for the population of the M_J″ = +1 sub-level. For the first time, we show that laser optical pumping via the 2³P₁-2³S₁ transition can be used to selectively populate each of the three M_J″ sub-levels (M_J″ = -1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific M_J″ sub-levels than magnetic hexapole focusing.

Developing Analytical Applications for Parahydrogen Hyperpolarization: Urinary Elimination Pharmacokinetics of Nicotine

Nuclear magnetic resonance spectroscopy (NMR) is a valuable analytical tool with applications in a vast array of research fields from chemistry and biology to medicine and beyond. NMR is renowned for its straightforward data interpretation and quantitative properties, making it attractive for pharmacokinetic applications, where drug metabolism pathways, concentrations, and kinetics need to be evaluated. However, pharmacologically active compounds and their metabolites in biofluids often appear in minute concentrations, well below the detection limit of NMR. Herein, we demonstrate how parahydrogen hyperpolarization overcomes this sensitivity barrier, allowing us to detect mid-nanomolar concentrations of a drug and a drug metabolite in a biofluid matrix. The performance of the method is demonstrated by monitoring nicotine and cotinine urinary elimination, reflected by their concentrations in urine during the onset and withdrawal from nicotine consumption. An NMR limit of detection of 0.1 μM and a limit of quantitation of 0.7 μM is achieved in a practical pharmacokinetics scenario where precise quantitative and qualitative analysis is desired.

Low-Cost High-Pressure Clinical-Scale 50% Parahydrogen Generator Using Liquid Nitrogen at 77 K

We report on a robust and low-cost parahydrogen generator design employing liquid nitrogen as a coolant. The core of the generator consists of catalyst-filled spiral copper tubing, which can be pressurized to 35 atm. Parahydrogen fraction >48% was obtained at 77 K with three nearly identical generators using paramagnetic hydrated iron oxide catalysts. Parahydrogen quantification was performed on the fly via benchtop NMR spectroscopy to monitor the signal from residual orthohydrogen-parahydrogen is NMR silent. This real-time quantification approach was also used to evaluate catalyst activation at up to 1.0 standard liter per minute flow rate. The reported inexpensive device can be employed for a wide range of studies employing parahydrogen as a source of nuclear spin hyperpolarization. To this end, we demonstrate the utility of this parahydrogen generator for hyperpolarization of concentrated sodium [1-13C]pyruvate, a metabolic contrast agent under investigation in numerous clinical trials. The reported pilot optimization of SABRE-SHEATH (signal amplification by reversible exchange-shield enables alignment transfer to heteronuclei) hyperpolarization yielded 13C signal enhancement of over 14,000-fold at a clinically relevant magnetic field of 1 T corresponding to approximately 1.2% 13C polarization-if near 100% parahydrogen would have been employed, the reported value would be tripled to 13C polarization of 3.5%.

Hyperpolarized Magnetic Resonance and Artificial Intelligence: Frontiers of Imaging in Pancreatic Cancer

BACKGROUND

There is an unmet need for noninvasive imaging markers that can help identify the aggressive subtype(s) of pancreatic ductal adenocarcinoma (PDAC) at diagnosis and at an earlier time point, and evaluate the efficacy of therapy prior to tumor reduction. In the past few years, there have been two major developments with potential for a significant impact in establishing imaging biomarkers for PDAC and pancreatic cancer premalignancy: (1) hyperpolarized metabolic (HP)-magnetic resonance (MR), which increases the sensitivity of conventional MR by over 10,000-fold, enabling real-time metabolic measurements; and (2) applications of artificial intelligence (AI).

OBJECTIVE

Our objective of this review was to discuss these two exciting but independent developments (HP-MR and AI) in the realm of PDAC imaging and detection from the available literature to date.

METHODS

A systematic review following the PRISMA extension for Scoping Reviews (PRISMA-ScR) guidelines was performed. Studies addressing the utilization of HP-MR and/or AI for early detection, assessment of aggressiveness, and interrogating the early efficacy of therapy in patients with PDAC cited in recent clinical guidelines were extracted from the PubMed and Google Scholar databases. The studies were reviewed following predefined exclusion and inclusion criteria, and grouped based on the utilization of HP-MR and/or AI in PDAC diagnosis.

RESULTS

Part of the goal of this review was to highlight the knowledge gap of early detection in pancreatic cancer by any imaging modality, and to emphasize how AI and HP-MR can address this critical gap. We reviewed every paper published on HP-MR applications in PDAC, including six preclinical studies and one clinical trial. We also reviewed several HP-MR-related articles describing new probes with many functional applications in PDAC. On the AI side, we reviewed all existing papers that met our inclusion criteria on AI applications for evaluating computed tomography (CT) and MR images in PDAC. With the emergence of AI and its unique capability to learn across multimodal data, along with sensitive metabolic imaging using HP-MR, this knowledge gap in PDAC can be adequately addressed. CT is an accessible and widespread imaging modality worldwide as it is affordable; because of this reason alone, most of the data discussed are based on CT imaging datasets. Although there were relatively few MR-related papers included in this review, we believe that with rapid adoption of MR imaging and HP-MR, more clinical data on pancreatic cancer imaging will be available in the near future.

CONCLUSIONS

Integration of AI, HP-MR, and multimodal imaging information in pancreatic cancer may lead to the development of real-time biomarkers of early detection, assessing aggressiveness, and interrogating early efficacy of therapy in PDAC.

Hyperpolarized MRI with silicon micro and nanoparticles: Principles and applications

Silicon-based micro and nanoparticles are ideally suited for use as biomedical imaging agents because of their biocompatibility, biodegradability, and simple surface chemistry that facilitates drug loading and targeting. A method to hyperpolarize silicon particles using dynamic nuclear polarization (DNP), which increases magnetic resonance (MR) imaging signals by several orders-of-magnitude through enhanced nuclear spin alignment, was developed to allow silicon particles to function as contrast agents for in vivo magnetic resonance imaging. In this review, we describe the application of the DNP technique to silicon particles and nanoparticles for background-free real-time molecular MR imaging. This review provides a summary of the state-of-the-science in silicon particle hyperpolarization with a detailed protocol for hyperpolarizing silicon particles. This information will foster awareness and spur interest in this emerging area of nanoimaging and provide a path to new developments and discoveries to further advance the field. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Singlet-Contrast Magnetic Resonance Imaging: Unlocking Hyperpolarization with Metabolism*

Hyperpolarization-enhanced magnetic resonance imaging can be used to study biomolecular processes in the body, but typically requires nuclei such as 13 C, 15 N, or 129 Xe due to their long spin-polarization lifetimes and the absence of a proton-background signal from water and fat in the images. Here we present a novel type of 1 H imaging, in which hyperpolarized spin order is locked in a nonmagnetic long-lived correlated (singlet) state, and is only liberated for imaging by a specific biochemical reaction. In this work we produce hyperpolarized fumarate via chemical reaction of a precursor molecule with para-enriched hydrogen gas, and the proton singlet order in fumarate is released as antiphase NMR signals by enzymatic conversion to malate in D2 O. Using this model system we show two pulse sequences to rephase the NMR signals for imaging and suppress the background signals from water. The hyperpolarization-enhanced 1 H-imaging modality presented here can allow for hyperpolarized imaging without the need for low-abundance, low-sensitivity heteronuclei.

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.

Clinical-Scale Production of Nearly Pure (>98.5%) Parahydrogen and Quantification by Benchtop NMR Spectroscopy

Because of the extensive chemical, physical, and biomedical applications of parahydrogen, the need exists for the development of highly enriched parahydrogen in a robust and efficient manner. Herein, we present a parahydrogen enrichment equipment which substantially improves upon the previous generators with its ability to enrich parahydrogen to >98.5% and a production rate of up to 4 standard liters per minute with the added advantage of real-time quantification. Our generator employs a pulsed injection system with a 3/16 in. outside diameter copper spiral tubing filled with iron-oxide catalyst. This tubing is mated to a custom-made copper attachment to provide efficient thermal coupling to the cold head. This device allows for robust operation at high pressures up to 34 atm. Real-time quantification by benchtop NMR spectroscopy is made possible by direct coupling of the p-H2 outlet from the generator to a 1.4 T NMR spectrometer using a regular 5 mm NMR tube that is continuously refilled with the exiting parahydrogen gas at ∼8 atm pressure. The use of high hydrogen gas pressure offers two critical NMR signal detection benefits: increased concentration and line narrowing. Our work presents a comprehensive description of the apparatus for a convenient and robust parahydrogen production, distribution, and quantification system, especially for parahydrogen-based hyperpolarization NMR research.

Low-Flammable Parahydrogen-Polarized MRI Contrast Agents

Many MRI contrast agents formed with the parahydrogen-induced polarization (PHIP) technique exhibit biocompatible profiles. In the context of respiratory imaging with inhalable molecular contrast agents, the development of nonflammable contrast agents would nonetheless be highly beneficial for the biomedical translation of this sensitive, high-throughput and affordable hyperpolarization technique. To this end, we assess the hydrogenation kinetics, the polarization levels and the lifetimes of PHIP hyperpolarized products (acids, ethers and esters) at various degrees of fluorine substitution. The results highlight important trends as a function of molecular structure that are instrumental for the design of new, safe contrast agents for in vivo imaging applications of the PHIP technique, with an emphasis on the highly volatile group of ethers used as inhalable anesthetics.

15 N NMR Hyperpolarization of Radiosensitizing Antibiotic Nimorazole by Reversible Parahydrogen Exchange in Microtesla Magnetic Fields

Nimorazole belongs to the imidazole-based family of antibiotics to fight against anaerobic bacteria. Moreover, nimorazole is now in Phase 3 clinical trial in Europe for potential use as a hypoxia radiosensitizer for treatment of head and neck cancers. We envision the use of [15 N3 ]nimorazole as a theragnostic hypoxia contrast agent that can be potentially deployed in the next-generation MRI-LINAC systems. Herein, we report the first steps to create long-lasting (for tens of minutes) hyperpolarized state on three 15 N sites of [15 N3 ]nimorazole with T1 of up to ca. 6 minutes. The nuclear spin polarization was boosted by ca. 67000-fold at 1.4 T (corresponding to P15N of 3.2 %) by 15 N-15 N spin-relayed SABRE-SHEATH hyperpolarization technique, relying on simultaneous exchange of [15 N3 ]nimorazole and parahydrogen on polarization transfer Ir-IMes catalyst. The presented results pave the way to efficient spin-relayed SABRE-SHEATH hyperpolarization of a wide range of imidazole-based antibiotics and chemotherapeutics.

Hydrogenative-PHIP polarized metabolites for biological studies

ParaHydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, but its application to biological investigations has been hampered, so far, due to chemical challenges. PHIP is obtained by means of the addition of hydrogen, enriched in the para-spin isomer, to an unsaturated substrate. Both hydrogen atoms must be transferred to the same substrate, in a pairwise manner, by a suitable hydrogenation catalyst; therefore, a de-hydrogenated precursor of the target molecule is necessary. This has strongly limited the number of parahydrogen polarized substrates. The non-hydrogenative approach brilliantly circumvents this central issue, but has not been translated to in-vivo yet. Recent advancements in hydrogenative PHIP (h-PHIP) considerably widened the possibility to hyperpolarize metabolites and, in this review, we will focus on substrates that have been obtained by means of this method and used in vivo. Attention will also be paid to the requirements that must be met and on the issues that have still to be tackled to obtain further improvements and to push PHIP substrates in biological applications.

Exploring the hyperpolarisation of EGTA-based ligands using SABRE

The design of molecules whose magnetic resonance (MR) signals report on their biological environment is receiving attention as a route to non-invasive functional MR. Hyperpolarisation techniques improve the sensitivity of MR and enable real time low concentration MR imaging, allowing for the development of novel functional imaging methodologies. In this work, we report on the synthesis of a series of EGTA-derived molecules (EGTA - ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid), whose core structures are known to bind biologically relevant metal ions in vivo, in addition to pyridyl rings that allow reversible ligation to an iridium dihydride complex. Consequently, they are amenable to hyperpolarisation through the parahydrogen-based signal amplification by reversible exchange (SABRE) process. We investigate how the proximity of EGTA and pyridine units, and the identity of the linker group, affect the SABRE hyperpolarisation attained for each agent. We also describe the effect of catalyst identity and co-ligand presence on these measurements and can achieve ¹H NMR signal enhancements of up to 160-fold. We rationalise these results to suggest the design elements needed for probes amenable to SABRE hyperpolarisation whose MR signals might in the future report on the presence of metal ions.

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.

Magnetic resonance microscopy of samples with translational symmetry with FOVs smaller than sample size

In MRI, usually the Field of View (FOV) has to cover the entire object. If this condition is not fulfilled, an infolding image artifact is observed, which suppresses visualization. In this study it is shown that for samples with translational symmetry, i.e., those consisting of identical objects in periodic unit cells, the FOV can be reduced to match the unit cell which enables imaging of an average object, of which the signal is originated from all unit cells of the sample, with no punishment by a loss in signal-to-noise ratio (SNR). This theoretical prediction was confirmed by experiments on a test sample with a 7 × 7 mm² unit cell arranged in a 3 × 3 matrix which was scanned by the spin-echo and by single point imaging methods. Effects of experimental imperfections in size and orientation mismatch between FOV and unit cell were studied as well. Finally, this method was demonstrated on a 3D periodic sample of tablets, which yielded well-resolved images of moisture distribution in an average tablet, while single tablet imaging provided no results. The method can be applied for SNR increase in imaging of any objects with inherently low signals provided they can be arranged in a periodic structure.

Reversible Hyperpolarization of Ketoisocaproate Using Sulfoxide-containing Polarization Transfer Catalysts

The substrate scope of sulfoxide-containing magnetisation transfer catalysts is extended to hyperpolarize α-ketoisocaproate and α-ketoisocaproate-1-[13 C]. This is achieved by forming [Ir(H)2 (κ2 -ketoisocaproate)(N-heterocyclic carbene)(sulfoxide)] which transfers latent magnetism from p-H2 via the signal amplification by reversible exchange (SABRE) process. The effect of polarization transfer field on the formation of enhanced 13 C magnetization is evaluated. Consequently, performing SABRE in a 0.5 μT field enabled most efficient magnetisation transfer. 13 C NMR signals for α-ketoisocaproate-1-[13 C] in methanol-d4 are up to 985-fold more intense than their traditional Boltzmann derived signal intensity (0.8 % 13 C polarisation). Single crystal X-ray diffraction reveals the formation of the novel catalyst decomposition products [Ir(μ-H)(H)2 (IMes)(SO(Ph)(Me)2 )]2 and [(Ir(H)2 (IMes)(SO(Me)2 ))2 (μ-S)] when the sulfoxides methylphenylsulfoxide and dimethylsulfoxide are used respectively.

Cardiac pH-Imaging With Hyperpolarized MRI

Regardless of the importance of acid-base disturbances in cardiac disease, there are currently no methods for clinical detection of pH in the heart. Several magnetic resonance imaging techniques hold translational promise and may enable in-vivo mapping of pH. We provide a brief overview of these emerging techniques. A particular focus is on the promising advance of magnetic resonance spectroscopy and imaging with hyperpolarized ¹³C-subtrates as biomarkers of myocardial pH. Hyperpolarization allows quantification of key metabolic substrates and their metabolites. Hereby, pH-sensitive reactions can be probed to provide a measure of acid-base alterations. To date, the most used substrates are [1-¹³C]pyruvate and ¹³C-labeled bicarbonate; however, others have been suggested. In cardiovascular medicine, hyperpolarized magnetic resonance spectroscopy has been used to probe acid-base disturbances following pharmacological stress, ischemia and heart failure in animals. In addition to pH-estimation, the technique can quantify fluxes such as the pivotal conversion of pyruvate to lactate via lactate dehydrogenase. This capability, a good safety profile and the fact that the technique is employable in clinical scanners have led to recent translation in early clinical trials. Thus, magnetic resonance spectroscopy and imaging may provide clinical pH-imaging in the near future.

Hyperpolarized NMR Metabolomics at Natural 13C Abundance

Metabolomics plays a pivotal role in systems biology, and NMR is a central tool with high precision and exceptional resolution of chemical information. Most NMR metabolomic studies are based on 1H 1D spectroscopy, severely limited by peak overlap. 13C NMR benefits from a larger signal dispersion but is barely used in metabolomics due to ca. 6000-fold lower sensitivity. We introduce a new approach, based on hyperpolarized 13C NMR at natural abundance, that circumvents this limitation. A new untargeted NMR-based metabolomic workflow based on dissolution dynamic nuclear polarization (d-DNP) for the first time enabled hyperpolarized natural abundance 13C metabolomics. Statistical analysis of resulting hyperpolarized 13C data distinguishes two groups of plant (tomato) extracts and highlights biomarkers, in full agreement with previous results on the same biological model. We also optimize parameters of the semiautomated d-DNP system suitable for high-throughput studies.

Quantitative NMR-Based Biomedical Metabolomics: Current Status and Applications

Nuclear Magnetic Resonance (NMR) spectroscopy is a quantitative analytical tool commonly utilized for metabolomics analysis. Quantitative NMR (qNMR) is a field of NMR spectroscopy dedicated to the measurement of analytes through signal intensity and its linear relationship with analyte concentration. Metabolomics-based NMR exploits this quantitative relationship to identify and measure biomarkers within complex biological samples such as serum, plasma, and urine. In this review of quantitative NMR-based metabolomics, the advancements and limitations of current techniques for metabolite quantification will be evaluated as well as the applications of qNMR in biomedical metabolomics. While qNMR is limited by sensitivity and dynamic range, the simple method development, minimal sample derivatization, and the simultaneous qualitative and quantitative information provide a unique landscape for biomedical metabolomics, which is not available to other techniques. Furthermore, the non-destructive nature of NMR-based metabolomics allows for multidimensional analysis of biomarkers that facilitates unambiguous assignment and quantification of metabolites in complex biofluids.

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.

Parahydrogen-Induced Hyperpolarization of Gases

Imaging of gases is a major challenge for any modality including MRI. NMR and MRI signals are directly proportional to the nuclear spin density and the degree of alignment of nuclear spins with applied static magnetic field, which is called nuclear spin polarization. The level of nuclear spin polarization is typically very low, i.e., one hundred thousandth of the potential maximum at 1.5 T and a physiologically relevant temperature. As a result, MRI typically focusses on imaging highly concentrated tissue water. Hyperpolarization methods transiently increase nuclear spin polarizations up to unity, yielding corresponding gains in MRI signal level of several orders of magnitude that enable the 3D imaging of dilute biomolecules including gases. Parahydrogen-induced polarization is a fast, highly scalable, and low-cost hyperpolarization technique. The focus of this Minireview is to highlight selected advances in the field of parahydrogen-induced polarization for the production of hyperpolarized compounds, which can be potentially employed as inhalable contrast agents.

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.

Rational ligand choice extends the SABRE substrate scope

Here we report on chelating ligands for Signal Amplification By Reversible Exchange (SABRE) catalysts that permit hyperpolarisation on otherwise sterically hindered substrates. We demonstrate 1H enhancements of ∼100-fold over 8.5 T thermal for 2-substituted pyridines, and smaller, yet significant enhancements for provitamin B6 and caffeine. We also show 15N-enhancements of ∼1000-fold and 19F-enhancements of 30-fold.

Nanodiamond-enabled biomedical imaging

Biomedical imaging allows in vivo studies of organisms, providing valuable information of biological processes at both cellular and tissue levels. Nanodiamonds have recently emerged as a new type of probe for fluorescence imaging and contrast agent for magnetic resonance and photoacoustic imaging. Composed of sp3-carbon atoms, diamond is chemically inert and inherently biocompatible. Uniquely, its matrix can host a variety of optically and magnetically active defects suited for bioimaging applications. Since the first production of fluorescent nanodiamonds in 2005, a large number of experiments have demonstrated that fluorescent nanodiamonds are useful as photostable markers and nanoscale sensors in living cells and organisms. In this review, we focus our discussion on the recent advancements of nanodiamond-enabled biomedical imaging for preclinical applications.

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.

High-sensitivity in vivo contrast for ultra-low field magnetic resonance imaging using superparamagnetic iron oxide nanoparticles

Magnetic resonance imaging (MRI) scanners operating at ultra-low magnetic fields (ULF; <10 mT) are uniquely positioned to reduce the cost and expand the clinical accessibility of MRI. A fundamental challenge for ULF MRI is obtaining high-contrast images without compromising acquisition sensitivity to the point that scan times become clinically unacceptable. Here, we demonstrate that the high magnetization of superparamagnetic iron oxide nanoparticles (SPIONs) at ULF makes possible relaxivity- and susceptibility-based effects unachievable with conventional contrast agents (CAs). We leverage these effects to acquire high-contrast images of SPIONs in a rat model with ULF MRI using short scan times. This work overcomes a key limitation of ULF MRI by enabling in vivo imaging of biocompatible CAs. These results open a new clinical translation pathway for ULF MRI and have broader implications for disease detection with low-field portable MRI scanners.

Resolving Metabolic Heterogeneity in Experimental Models of the Tumor Microenvironment from a Stable Isotope Resolved Metabolomics Perspective

The tumor microenvironment (TME) comprises complex interactions of multiple cell types that determines cell behavior and metabolism such as nutrient competition and immune suppression. We discuss the various types of heterogeneity that exist in solid tumors, and the complications this invokes for studies of TME. As human subjects and in vivo model systems are complex and difficult to manipulate, simpler 3D model systems that are compatible with flexible experimental control are necessary for studying metabolic regulation in TME. Stable Isotope Resolved Metabolomics (SIRM) is a valuable tool for tracing metabolic networks in complex systems, but at present does not directly address heterogeneous metabolism at the individual cell level. We compare the advantages and disadvantages of different model systems for SIRM experiments, with a focus on lung cancer cells, their interactions with macrophages and T cells, and their response to modulators in the immune microenvironment. We describe the experimental set up, illustrate results from 3D cultures and co-cultures of lung cancer cells with human macrophages, and outline strategies to address the heterogeneous TME.

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.