Propane-d6 Heterogeneously Hyperpolarized by Parahydrogen

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.

Recent advances in the application of parahydrogen in catalysis and biochemistry

Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) are analytical and diagnostic tools that are essential for a very broad field of applications, ranging from chemical analytics, to non-destructive testing of materials and the investigation of molecular dynamics, to in vivo medical diagnostics and drug research. One of the major challenges in their application to many problems is the inherent low sensitivity of magnetic resonance, which results from the small energy-differences of the nuclear spin-states. At thermal equilibrium at room temperature the normalized population difference of the spin-states, called the Boltzmann polarization, is only on the order of 10-5. Parahydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, which has widespread applications in Chemistry, Physics, Biochemistry, Biophysics, and Medical Imaging. PHIP creates its signal-enhancements by means of a reversible (SABRE) or irreversible (classic PHIP) chemical reaction between the parahydrogen, a catalyst, and a substrate. Here, we first give a short overview about parahydrogen-based hyperpolarization techniques and then review the current literature on method developments and applications of various flavors of the PHIP experiment.

Mechanisms of Methylenecyclobutane Hydrogenation over Supported Metal Catalysts Studied by Parahydrogen-Induced Polarization Technique

In this work the mechanism of methylenecyclobutane hydrogenation over titania-supported Rh, Pt and Pd catalysts was investigated using parahydrogen-induced polarization (PHIP) technique. It was found that methylenecyclobutane hydrogenation leads to formation of a mixture of reaction products including cyclic (1-methylcyclobutene, methylcyclobutane), linear (1-pentene, cis-2-pentene, trans-2-pentene, pentane) and branched (isoprene, 2-methyl-1-butene, 2-methyl-2-butene, isopentane) compounds. Generally, at lower temperatures (150-350 °C) the major reaction product was methylcyclobutane while higher temperature of 450 °C favors formation of branched products isoprene, 2-methyl-1-butene and 2-methyl-2-butene. PHIP effects were detected for all reaction products except methylenecyclobutane isomers 1-methylcyclobutene and isoprene implying that the corresponding compounds can incorporate two atoms from the same parahydrogen molecule in a pairwise manner in the course of the reaction in particular positions. The mechanisms were proposed for the formation of these reaction products based on PHIP results.

Heterogeneous Catalysis and Parahydrogen-Induced Polarization

Parahydrogen-induced polarization with heterogeneous catalysts (HET-PHIP) has been a subject of extensive research in the last decade since its first observation in 2007. While NMR signal enhancements obtained with such catalysts are currently below those achieved with transition metal complexes in homogeneous hydrogenations in solution, this relatively new field demonstrates major prospects for a broad range of advanced fundamental and practical applications, from providing catalyst-free hyperpolarized fluids for biomedical magnetic resonance imaging (MRI) to exploring mechanisms of industrially important heterogeneous catalytic processes. This review covers the evolution of the heterogeneous catalysts used for PHIP observation, from metal complexes immobilized on solid supports to bulk metals and single-atom catalysts and discusses the general visions for maximizing the obtained NMR signal enhancements using HET-PHIP. Various practical applications of HET-PHIP, both for catalytic studies and for potential production of hyperpolarized contrast agents for MRI, are described.

Bridging the Gap: From Homogeneous to Heterogeneous Parahydrogen-induced Hyperpolarization and Beyond

Demonstration of parahydrogen-induced polarization effects in hydrogenations catalyzed by heterogeneous catalysts instead of metal complexes in a homogeneous solution has opened an entirely new dimension for parahydrogen-based research, demonstrating its applicability not only for the production of catalyst-free hyperpolarized liquids and gases and long-lived non-equilibrium spin states for potential biomedical applications, but also for addressing challenges of modern fundamental and industrial catalysis including advanced mechanistic studies of catalytic reactions and operando NMR and MRI of reactors. This essay summarizes the progress achieved in this field by highlighting the research contributed to it by our colleague and friend Kirill V. Kovtunov whose scientific career ended unexpectedly and tragically at the age of 37. His role in this research was certainly crucial, further enhanced by a vast network of his contacts and collaborations at the national and international level.

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.

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.

Diffusion measurements with continuous hydrogenation in PHIP

DOSY is a powerful spectroscopic NMR technique that resolves components in mixtures through the evaluation of different diffusion coefficients. The application of DOSY to dilute mixtures is hampered by the low signal to noise ratios (SNR), leading to long acquisition times. The use of PHIP may resolve this issue as long as reproducible signals are obtained in order to perform 2D experiments. Here we show that the use of hollow membranes and adequate gas flow produce constant polarization for a time-span that enables the acquisition of 2D experiments. A pressure gradient is evidenced by the presence of convection, which is accounted for by using a DPGSE sequence. The influence of J-coupling evolution during the sequence is studied both numerically and experimentally, to determine the optimum echo-time. The applicability of the method for samples with poor SNR is explored by setting the reaction rate to achieve a low intensity of polarized signals.

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.

Catalytic hydrogenation with parahydrogen: a bridge from homogeneous to heterogeneous catalysis

One of the essential themes in modern catalysis is that of bridging the gap between its homogeneous and heterogeneous counterparts to combine their individual advantages and overcome shortcomings. One more incentive can now be added to the list, namely the ability of transition metal complexes to provide strong nuclear magnetic resonance (NMR) signal enhancement upon their use in homogeneous hydrogenations of unsaturated compounds with parahydrogen in solution. The addition of both H atoms of a parahydrogen molecule to the same substrate, a prerequisite for such effects, is implemented naturally with metal complexes that operate via the formation of a dihydride intermediate, but not with most heterogeneous catalysts. Despite that, it has been demonstrated in recent years that various types of heterogeneous catalysts are able to perform the required pairwise H2 addition at least to some extent. This has opened a major gateway for developing highly sensitive and informative tools for mechanistic studies of heterogeneous hydrogenations and other processes involving H2. Besides, production of catalyst-free fluids with NMR signals enhanced by 3-4 orders of magnitude is essential for modern applications of magnetic resonance imaging (MRI), including biomedical research and practice. The ongoing efforts to design heterogeneous catalysts which can implement the homogeneous (pairwise) hydrogenation mechanism are reported.

Biomedical Applications of the Dynamic Nuclear Polarization and Parahydrogen Induced Polarization Techniques for Hyperpolarized 13C MR Imaging

Since the first pioneering report of hyperpolarized [1-¹³C]pyruvate magnetic resonance imaging (MRI) of the Warburg effect in prostate cancer patients, clinical dissemination of the technique has been rapid; close to 10 sites worldwide now possess a polarizer fit for the clinic, and more than 30 clinical trials, predominantly for oncological applications, are already registered on the US and European clinical trials databases. Hyperpolarized ¹³C probes to study pathophysiological processes beyond the Warburg effect, including tricarboxylic acid cycle metabolism, intra-cellular pH and cellular necrosis have also been demonstrated in the preclinical arena and are pending clinical translation, and the simultaneous injection of multiple co-polarized agents is opening the door to high-sensitivity, multi-functional molecular MRI with a single dose. Here, we review the biomedical applications to date of the two polarization methods that have been used for in vivo hyperpolarized ¹³C molecular MRI; namely, dissolution dynamic nuclear polarization and parahydrogen-induced polarization. The basic concept of hyperpolarization and the fundamental theory underpinning these two key ¹³C hyperpolarization methods, along with recent technological advances that have facilitated biomedical realization, are also covered.

Multiple Quantum Coherences Hyperpolarized at Ultra-Low Fields

The development of hyperpolarization technologies enabled several yet exotic NMR applications at low and ultra-low fields (ULF), where without hyperpolarization even the detection of a signal from analytes is a challenge. Herein, we present a method for the simultaneous excitation and observation of homo- and heteronuclear multiple quantum coherences (from zero up to the third-order), which give an additional degree of freedom for ULF NMR experiments, where the chemical shift variation is negligible. The approach is based on heteronuclear correlated spectroscopy (COSY); its combination with a phase-cycling scheme allows the selective observation of multiple quantum coherences of different orders. The nonequilibrium spin state and multiple spin orders are generated by signal amplification by reversible exchange (SABRE) and detected at ULF with a superconducting quantum interference device (SQUID)-based NMR system.

A versatile synthetic route to the preparation of 15 N heterocycles

A robust medium-scale (approximately 3 g) synthetic method for 15 N labeling of pyridine (15 N-Py) is reported based on the Zincke reaction. 15 N enrichment in excess of 81% was achieved with approximately 33% yield. 15 N-Py serves as a standard substrate in a wide range of studies employing a hyperpolarization technique for efficient polarization transfer from parahydrogen to heteronuclei; this technique, called SABRE (signal amplification by reversible exchange), employs a simultaneous chemical exchange of parahydrogen and a to-be-hyperpolarized substrate (e.g., pyridine) on metal centers. In studies aimed at the development of hyperpolarized contrast agents for in vivo molecular imaging, pyridine is often employed either as a model substrate (for hyperpolarization technique development, quality assurance, and phantom imaging studies) or as a co-substrate to facilitate more efficient hyperpolarization of a wide range of emerging contrast agents (e.g., nicotinamide). Here, the produced 15 N-Py was used for the feasibility study of spontaneous 15 N hyperpolarization at high magnetic (HF) fields (7 T and 9.4 T) of an NMR spectrometer and an MRI scanner. SABRE hyperpolarization enabled acquisition of 2D MRI imaging of catalyst-bound 15 N-pyridine with 75 × 75 mm2 field of view (FOV), 32 × 32 matrix size, demonstrating the feasibility of 15 N HF-SABRE molecular imaging with 2.4 × 2.4 mm2 spatial resolution.

NMR Spectroscopy for Metabolomics Research

Over the past two decades, nuclear magnetic resonance (NMR) has emerged as one of the three principal analytical techniques used in metabolomics (the other two being gas chromatography coupled to mass spectrometry (GC-MS) and liquid chromatography coupled with single-stage mass spectrometry (LC-MS)). The relative ease of sample preparation, the ability to quantify metabolite levels, the high level of experimental reproducibility, and the inherently nondestructive nature of NMR spectroscopy have made it the preferred platform for long-term or large-scale clinical metabolomic studies. These advantages, however, are often outweighed by the fact that most other analytical techniques, including both LC-MS and GC-MS, are inherently more sensitive than NMR, with lower limits of detection typically being 10 to 100 times better. This review is intended to introduce readers to the field of NMR-based metabolomics and to highlight both the advantages and disadvantages of NMR spectroscopy for metabolomic studies. It will also explore some of the unique strengths of NMR-based metabolomics, particularly with regard to isotope selection/detection, mixture deconvolution via 2D spectroscopy, automation, and the ability to noninvasively analyze native tissue specimens. Finally, this review will highlight a number of emerging NMR techniques and technologies that are being used to strengthen its utility and overcome its inherent limitations in metabolomic applications.

Relaxation Dynamics of Nuclear Long-Lived Spin States in Propane and Propane-d6 Hyperpolarized by Parahydrogen

We report a systematic study of relaxation dynamics of hyperpolarized (HP) propane and HP propane-d6 prepared by heterogeneous pairwise parahydrogen addition to propylene and propylene-d6 respectively. Long-lived spin states (LLS) created for these molecules at the low magnetic field of 0.0475 T were employed for this study. The parahydrogen-induced overpopulation of a HP propane LLS decays exponentially with time constant (TLLS) approximately 3-fold greater than the corresponding T1 values. Both TLLS and T1 increase linearly with propane pressure in the range from 1 atm (the most biomedically relevant conditions for pulmonary MRI) to 5 atm. The TLLS value of HP propane gas at 1 atm is ~3 s. Deuteration of the substrate (propylene-d6) yields hyperpolarized propane-d6 gas with TLLS values approximately 20% shorter than those of hyperpolarized fully protonated propane gas, indicating that deuteration does not benefit the lifetime of the LLS HP state. The use of pH2 or Xe/N2 buffering gas during heterogeneous hydrogenation reaction (leading to production of 100% HP propane (no buffering gas) versus 43% HP propane gas (with 57% buffering gas) composition mixtures) results in (i) no significant changes in T1, (ii) decrease of TLLS values (by 35±7% and 8±7% respectively); and (iii) an increase of the polarization levels of HP propane gas with a propane concentration decrease (by 1.6±0.1-fold and 1.4±0.1-fold respectively despite the decrease in TLLS, which leads to disproportionately greater polarization losses during HP gas transport). Moreover, we demonstrate the feasibility of HP propane cryo-collection (which can be potentially useful for preparing larger amounts of concentrated HP propane, when buffering gas is employed), and TLLS of liquefied HP propane reaches 14.7 seconds, which is greater than the TLLS value of HP propane gas at any pressure studied. Finally, we have explored the utility of using a partial Spin-Lock Induced Crossing (SLIC) radio frequency (RF) pulse sequence for converting the overpopulated LLS into observable 1H nuclear magnetization at low magnetic field. We find that (i) the bulk of the overpopulated LLS is retained even when the optimal or near-optimal values of SLIC pulse duration are employed, and (ii) the overpopulated LLS of propane is also relatively immune to strong RF pulses-thereby, indicating that LLS is highly suitable as a spin-polarization reservoir in the context of NMR/MRI detection applications. The presented findings may be useful for improving the levels of polarization of HP propane produced by HET-PHIP via the use of an inert buffer gas; increasing the lifetime of the HP state during preparation and storage; and developing efficient approaches for ultrafast MR imaging of HP propane in the context of biomedical applications of HP propane gas, including its potential use as an inhalable contrast agent.

Simulating Non-linear Chemical and Physical (CAP) Dynamics of Signal Amplification By Reversible Exchange (SABRE)

The hyperpolarization of nuclear spins by using parahydrogen (pH₂ ) is a fascinating technique that allows spin polarization and thus the magnetic resonance signal to be increased by several orders of magnitude. Entirely new applications have become available. Signal amplification by reversible exchange (SABRE) is a relatively new method that is based on the reversible exchange of a substrate, catalyst and parahydrogen. SABRE is particularly interesting for in vivo medical and industrial applications, such as fast and low-cost trace analysis or continuous signal enhancement. Ever since its discovery, many attempts have been made to model and understand SABRE, with various degrees of simplifications. In this work, we reduced the simplifications further, taking into account non-linear chemical and physical (CAP) dynamics of several multi-spin systems. A master equation was derived and realized using the MOIN open-source software. The effects of different parameters (exchange rates, concentrations, spin-spin couplings) on relaxation and the polarization level have been evaluated and the results provide interesting insights into the mechanism of SABRE.

Clinical-Scale Batch-Mode Production of Hyperpolarized Propane Gas for MRI

NMR spectroscopy and imaging (MRI) are two of the most important methods to study structure, function, and dynamics from atom to organism scale. NMR approaches often suffer from an insufficient sensitivity, which, however, can be transiently boosted using hyperpolarization techniques. One of these techniques is parahydrogen-induced polarization, which has been used to produce catalyst-free hyperpolarized propane gas with proton polarization that is 3 orders of magnitude greater than equilibrium thermal polarization at a 1.5 T field of a clinical MRI scanner. Here we show that more than 0.3 L of hyperpolarized propane gas can be produced in 2 s. This production rate is more than an order of magnitude greater than that demonstrated previously, and the reported production rate is comparable to that employed for in-human MRI using HP noble gas (e.g., ¹²⁹Xe) produced via a spin exchange optical pumping (SEOP) hyperpolarization technique. We show that high polarization values can be retained despite the significant increase in the production rate of hyperpolarized propane. The enhanced signals of produced hyperpolarized propane gas were revealed by stopped-flow MRI visualization at 4.7 T. Achieving this high production rate enables the future use of this compound (already approved for unlimited use in foods by the corresponding regulating agencies, e.g., FDA in the USA, and more broadly as an E944 food additive) as a new inhalable contrast agent for diagnostic detection via MRI.

Magnetic resonance imaging of catalytically relevant processes

The main aim of this article is to provide a state-of-the-art review of the magnetic resonance imaging (MRI) utilization in heterogeneous catalysis. MRI is capable to provide very useful information about both living and nonliving objects in a noninvasive way. The studies of an internal heterogeneous reactor structure by MRI help to understand the mass transport and chemical processes inside the working catalytic reactor that can significantly improve its efficiency. However, one of the serious disadvantages of MRI is low sensitivity, and this obstacle dramatically limits possible MRI application. Fortunately, there are hyperpolarization methods that eliminate this problem. Parahydrogen-induced polarization approach, for instance, can increase the nuclear magnetic resonance signal intensity by four to five orders of magnitude; moreover, the obtained polarization can be stored in long-lived spin states and then transferred into an observable signal in MRI. An in-depth account of the studies on both thermal and hyperpolarized MRI for the investigation of heterogeneous catalytic processes is provided in this review as part of the special issue emphasizing the research performed to date in Russia/USSR.

Production of highly concentrated and hyperpolarized metabolites within seconds in high and low magnetic fields

We introduce two experiments that allow for the rapid production of hyperpolarized metabolites. More than 50% ¹³C polarization in 50 mM concentrations is achieved. This can be translated to portable low field NMR devices.

OnlyParahydrogen SpectrosopY (OPSY) pulse sequences - One does not fit all

The hyperpolarization of nuclear spins using parahydrogen is an interesting effect that allows to increase the magnetic resonance signal by several orders of magnitude. Known as ParaHydrogen And Synthesis Allow Dramatically Enhanced Nuclear Alignment (PASADENA) and ParaHydrogen Induced Polarization (PHIP), the method was successfully used for in vitro analysis and in vivo imaging. In this contribution, we investigated four known and four new variants of Only Parahydrogen SpectroscopY (OPSY) sequences (Aguilar et al., 2007) with respect to the selective preparation of hyperpolarized NMR signal and background suppression. Depending on the method chosen, either anti-phase, in-phase or a mixture of both signals are obtained: anti-phase signals are beneficial to identify hyperpolarized signals and the structure or J-coupling constants; in-phase signals are useful for imaging applications or when the lines are broad. This comprehensive overview of sequences new and old facilitates selecting the right sequence for the task at hand.

Only Para-Hydrogen Spectroscopy (OPSY) Revisited: In-Phase Spectra for Chemical Analysis and Imaging

We revisited only para-hydrogen spectroscopy (OPSY) for the analysis of para-hydrogen-enhanced NMR spectra at high magnetic fields. We found that the sign of the gradients and interpulse delays are pivotal for the performance of the sequence: the variant of double-quantum filter OPSY, where the second time interval is twice as long as the first one (OPSYd-12) converts the antiphase spectrum to in-phase and efficiently suppresses the background signal in a single scan better than the other variants. OPSYd-12 strongly facilitates the analysis of para-hydrogen-derived NMR spectra in homogeneous and inhomogeneous magnetic fields. Furthermore, the net magnetization produced is essential for subsequent applications such as imaging, e.g., in a reaction chamber or in vivo.

Chemical Exchange Reaction Effect on Polarization Transfer Efficiency in SLIC-SABRE

Signal Amplification By Reversible Exchange (SABRE) is a new and rapidly developing hyperpolarization technique. The recent discovery of Spin-Lock Induced Crossing SABRE (SLIC-SABRE) showed that high field hyperpolarization transfer techniques developed so far were optimized for singlet spin order that does not coincide with the experimentally produced spin state. Here, we investigated the SLIC-SABRE approach and the most advanced quantitative theoretical SABRE model to date. Our goal is to achieve the highest possible polarization with SLIC-SABRE at high field using the standard SABRE system, IrIMes catalyst with pyridine. We demonstrated the accuracy of the SABRE model describing the effects of various physical parameters such as the amplitude and frequency of the radio frequency field, and the effects of chemical parameters such as the exchange rate constants. By fitting the model to the experimental data, the effective life time of the SABRE complex was estimated, as well as the entropy and enthalpy of the complex-dissociation reaction. We show, for the first time, that this SLIC-SABRE model can be useful for the evaluation of the chemical exchange parameters that are very important for the production of highly polarized contrast agents via SABRE.

Heterogeneous Parahydrogen Pairwise Addition to Cyclopropane

Hyperpolarized gases revolutionize functional pulmonary imaging. Hyperpolarized propane is a promising emerging contrast agent for pulmonary MRI. Unlike hyperpolarized noble gases, proton-hyperpolarized propane gas can be imaged using conventional MRI scanners with proton imaging capability. Moreover, it is non-toxic odorless anesthetic. Furthermore, propane hyperpolarization can be accomplished by pairwise addition of parahydrogen to propylene. Here, we demonstrate the feasibility of propane hyperpolarization via hydrogenation of cyclopropane with parahydrogen. 1 H propane polarization up to 2.4 % is demonstrated here using 82 % parahydrogen enrichment and heterogeneous Rh/TiO2 hydrogenation catalyst. This level of polarization is several times greater than that obtained with propylene as a precursor under the same conditions despite the fact that direct pairwise addition of parahydrogen to cyclopropane may also lead to formation of propane with NMR-invisible hyperpolarization due to magnetic equivalence of nascent parahydrogen protons in two CH3 groups. NMR-visible hyperpolarized propane demonstrated here can be formed only via a reaction pathway involving cleavage of at least one C-H bond in the reactant molecule. The resulting NMR signal enhancement of hyperpolarized propane was sufficient for 2D gradient echo MRI of ∼5.5 mL phantom with 1×1 mm2 spatial resolution and 64×64 imaging matrix despite relatively low chemical conversion of cyclopropane substrate.

Quasi-Resonance Signal Amplification by Reversible Exchange

Here we present the feasibility of NMR signal amplification by reversible exchange (SABRE) using radio frequency irradiation at low magnetic field (0.05 T) in the regime where the chemical shifts of free and catalyst-bound species are similar. In SABRE, the 15N-containing substrate and parahydrogen perform simultaneous chemical exchange on an iridium hexacoordinate complex. A shaped spin-lock induced crossing (SLIC) radio frequency pulse sequence followed by a delay is applied at quasi-resonance (QUASR) conditions of 15N spins of a 15N-enriched substrate. As a result of this pulse sequence application, 15N z-magnetization is created from the spin order of parahydrogen-derived hyperpolarized hydrides. The repetition of the pulse sequence block consisting of a shaped radio frequency pulse and the delay leads to the buildup of 15N magnetization. The modulation of this effect by the irradiation frequency, pulse duration and amplitude, delay duration, and number of pumping cycles was demonstrated. Pyridine-15N, acetonitrile-15N, and metronidazole-15N2-13C2 substrates were studied representing three classes of compounds (five- and six-membered heterocycles and nitrile), showing the wide applicability of the technique. Metronidazole-15N2-13C2 is an FDA-approved antibiotic that can be injected in large quantities, promising noninvasive and accurate hypoxia sensing. The 15N hyperpolarization levels attained with QUASR-SABRE on metronidazole-15N2-13C2 were more than 2-fold greater than those with SABRE-SHEATH (SABRE in shield enables alignment transfer to heteronuclei), demonstrating that QUASR-SABRE can deliver significantly more efficient means of SABRE hyperpolarization.

Parahydrogen-Based Hyperpolarization for Biomedicine

Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2 ), which is inherently stable and long-lived. When brought into contact with another molecule, this "spin order on demand" allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2 -based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.

Hyperpolarized NMR Spectroscopy: d-DNP, PHIP, and SABRE Techniques

The intensity of NMR signals can be enhanced by several orders of magnitude by using various techniques for the hyperpolarization of different molecules. Such approaches can overcome the main sensitivity challenges facing modern NMR/magnetic resonance imaging (MRI) techniques, whilst hyperpolarized fluids can also be used in a variety of applications in material science and biomedicine. This Focus Review considers the fundamentals of the preparation of hyperpolarized liquids and gases by using dissolution dynamic nuclear polarization (d-DNP) and parahydrogen-based techniques, such as signal amplification by reversible exchange (SABRE) and parahydrogen-induced polarization (PHIP), in both heterogeneous and homogeneous processes. The various new aspects in the formation and utilization of hyperpolarized fluids, along with the possibility of observing NMR signal enhancement, are described.

A pulse programmable parahydrogen polarizer using a tunable electromagnet and dual channel NMR spectrometer

Applications of parahydrogen induced polarization (PHIP) often warrant conversion of the chemically-synthesized singlet-state spin order into net heteronuclear magnetization. In order to obtain optimal yields from the overall hyperpolarization process, catalytic hydrogenation must be tightly synchronized to subsequent radiofrequency (RF) transformations of spin order. Commercial NMR consoles are designed to synchronize applied waves on multiple channels and consequently are well-suited as controllers for these types of hyperpolarization experiments that require tight coordination of RF and non-RF events. Described here is a PHIP instrument interfaced to a portable NMR console operating with a static field electromagnet in the milliTesla regime. In addition to providing comprehensive control over chemistry and RF events, this setup condenses the PHIP protocol into a pulse-program that in turn can be readily shared in the manner of traditional pulse sequences. In this device, a TTL multiplexer was constructed to convert spectrometer TTL outputs into 24 VDC signals. These signals then activated solenoid valves to control chemical shuttling and reactivity in PHIP experiments. Consolidating these steps in a pulse-programming environment speeded calibration and improved quality assurance by enabling the B₀/B₁ fields to be tuned based on the direct acquisition of thermally polarized and hyperpolarized NMR signals. Performance was tested on the parahydrogen addition product of 2-hydroxyethyl propionate-1-¹³C-d₃, where the ¹³C polarization was estimated to be P_13C=20±2.5% corresponding to ¹³C signal enhancement approximately 25 million-fold at 9.1 mT or approximately 77,000-fold ¹³C enhancement at 3 T with respect to thermally induced polarization at room temperature.

High-resolution hyperpolarized in vivo metabolic 13C spectroscopy at low magnetic field (48.7mT) following murine tail-vein injection

High-resolution ¹³C NMR spectroscopy of hyperpolarized succinate-1-¹³C-2,3-d₂ is reported in vitro and in vivo using a clinical-scale, biplanar (80cm-gap) 48.7mT permanent magnet with a high homogeneity magnetic field. Non-localized ¹³C NMR spectra were recorded at 0.52MHz resonance frequency over the torso of a tumor-bearing mouse every 2s. Hyperpolarized ¹³C NMR signals with linewidths of ∼3Hz (corresponding to ∼6ppm) were recorded in vitro (2mL in a syringe) and in vivo (over a mouse torso). Comparison of the full width at half maximum (FWHM) for ¹³C NMR spectra acquired at 48.7mT and at 4.7T in a small-animal MRI scanner demonstrates a factor of ∼12 improvement for the ¹³C resonance linewidth attainable at 48.7mT compared to that at 4.7T in vitro. ¹³C hyperpolarized succinate-1-¹³C resonance linewidths in vivo are at least one order of magnitude narrower at 48.7mT compared to those observed in high-field (≥3T) studies employing HP contrast agents. The demonstrated high-resolution ¹³C in vivo spectroscopy could be useful for high-sensitivity spectroscopic studies involving monitoring HP agent uptake or detecting metabolism using HP contrast agents with sufficiently large ¹³C chemical shift differences.

Extending the Lifetime of Hyperpolarized Propane Gas through Reversible Dissolution

Hyperpolarized (HP) propane produced by the parahydrogen-induced polarization (PHIP) technique has been recently introduced as a promising contrast agent for functional lung magnetic resonance (MR) imaging. However, its short lifetime due to a spin-lattice relaxation time T₁ of less than 1 s in the gas phase is a significant translational challenge for its potential biomedical applications. The previously demonstrated approach for extending the lifetime of the HP propane state through long-lived spin states allows the HP propane lifetime to be increased by a factor of ∼3. Here, we demonstrate that a remarkable increase in the propane hyperpolarization decay time at high magnetic field (7.1 T) can be achieved by its dissolution in deuterated organic solvents (acetone-d₆ or methanol-d₄). The approximate values of the HP decay time for propane dissolved in acetone-d₆ are 35.1 and 28.6 s for the CH₂ group and the CH₃ group, respectively (similar values were obtained for propane dissolved in methanol-d₄), which are ∼50 times larger than the gaseous propane T₁ value. Furthermore, we show that it is possible to retrieve HP propane from solution to the gas phase with the preservation of hyperpolarization.

NMR Spin-Lock Induced Crossing (SLIC) dispersion and long-lived spin states of gaseous propane at low magnetic field (0.05T)

When parahydrogen reacts with propylene in low magnetic fields (e.g., 0.05T), the reaction product propane develops an overpopulation of pseudo-singlet nuclear spin states. We studied how the Spin-Lock Induced Crossing (SLIC) technique can be used to convert these pseudo-singlet spin states of hyperpolarized gaseous propane into observable magnetization and to detect ¹H NMR signal directly at 0.05T. The theoretical simulation and experimental study of the NMR signal dependence on B₁ power (SLIC amplitude) exhibits a well-resolved dispersion, which is induced by the spin-spin couplings in the eight-proton spin system of propane. We also measured the exponential decay time constants (T_LLSS or T_S) of these pseudo-singlet long-lived spin states (LLSS) by varying the time between hyperpolarized propane production and SLIC detection. We have found that, on average, T_S is approximately 3 times longer than the corresponding T₁ value under the same conditions in the range of pressures studied (up to 7.6atm). Moreover, T_S may exceed 13s at pressures above 7atm in the gas phase. These results are in agreement with the previous reports, and they corroborate a great potential of long-lived hyperpolarized propane as an inhalable gaseous contrast agent for lung imaging and as a molecular tracer to study porous media using low-field NMR and MRI.

NMR Hyperpolarization Techniques of Gases

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

NMR SLIC Sensing of Hydrogenation Reactions Using Parahydrogen in Low Magnetic Fields

Parahydrogen-induced polarization (PHIP) is an NMR hyperpolarization technique that increases nuclear spin polarization by orders of magnitude, and it is particularly well-suited to study hydrogenation reactions. However, the use of high-field NMR spectroscopy is not always possible, especially in the context of potential industrial-scale reactor applications. On the other hand, the direct low-field NMR detection of reaction products with enhanced nuclear spin polarization is challenging due to near complete signal cancellation from nascent parahydrogen protons. We show that hydrogenation products prepared by PHIP can be irradiated with weak (on the order of spin-spin couplings of a few hertz) alternating magnetic field (called Spin-Lock Induced Crossing or SLIC) and consequently efficiently detected at low magnetic field (e.g., 0.05 T used here) using examples of several types of organic molecules containing a vinyl moiety. The detected hyperpolarized signals from several reaction products at tens of millimolar concentrations were enhanced by 10000-fold, producing NMR signals an order of magnitude greater than the background signal from protonated solvents.

Efficient Batch-Mode Parahydrogen-Induced Polarization of Propane

We report on a simple approach for efficient NMR proton hyperpolarization of propane using the parahydrogen-induced polarization (PHIP) technique, which yielded ≈6.2 % proton polarization using ≈80 % parahydrogen, a record level achieved with any hyperpolarization technique for propane. Unlike in previously developed approaches designed for continuous-flow operation, where reactants (propene and parahydrogen) are simultaneously loaded for homogeneous or heterogeneous pairwise addition of parahydrogen, here a batch-mode method is applied: propene is first loaded into the catalyst-containing solution, which is followed by homogeneous hydrogenation via parahydrogen bubbling delivered at ≈7.1 atm. The achieved nuclear spin polarization of this contrast agent potentially useful for pulmonary imaging is approximately two orders of magnitude greater than that achieved in the continuous-flow homogeneous catalytic hydrogenation, and a factor of 3-10 more efficient compared to the typical results of heterogeneous continuous-flow hydrogenations.

The role of hyperpolarized 129xenon in MR imaging of pulmonary function

In the last two decades, functional imaging of the lungs using hyperpolarized noble gases has entered the clinical stage. Both helium (³He) and xenon (¹²⁹Xe) gas have been thoroughly investigated for their ability to assess both the global and regional patterns of lung ventilation. With advances in polarizer technology and the current transition towards the widely available ¹²⁹Xe gas, this method is ready for translation to the clinic. Currently, hyperpolarized (HP) noble gas lung MRI is limited to selected academic institutions; yet, the promising results from initial clinical trials have drawn the attention of the pulmonary medicine community. HP ¹²⁹Xe MRI provides not only 3-dimensional ventilation imaging, but also unique capabilities for probing regional lung physiology. In this review article, we aim to (1) provide a brief overview of current ventilation MR imaging techniques, (2) emphasize the role of HP ¹²⁹Xe MRI within the array of different imaging strategies, (3) discuss the unique imaging possibilities with HP ¹²⁹Xe MRI, and (4) propose clinical applications.

MR Imaging Biomarkers in Oncology Clinical Trials

The authors discuss eight areas of quantitative MR imaging that are currently used (RECIST, DCE-MR imaging, DSC-MR imaging, diffusion MR imaging) in clinical trials or emerging (CEST, elastography, hyperpolarized MR imaging, multiparameter MR imaging) as promising techniques in diagnosing cancer and assessing or predicting response of cancer to therapy. Illustrative applications of the techniques in the clinical setting are summarized before describing the current limitations of the methods.