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

MATRESHCA: Microtesla Apparatus for Transfer of Resonance Enhancement of Spin Hyperpolarization via Chemical Exchange and Addition

We present an integrated, open-source device for parahydrogen-based hyperpolarization processes in the microtesla field regime with a cost of components of less than $7000. The device is designed to produce a batch of 13C and 15N hyperpolarized (HP) compounds via hydrogenative or non-hydrogenative parahydrogen-induced polarization methods that employ microtesla magnetic fields for efficient polarization transfer of parahydrogen-derived spin order to X-nuclei (e.g., 13C and 15N). The apparatus employs a layered structure (reminiscent of a Russian doll "Matryoshka") that includes a nonmagnetic variable-temperature sample chamber, a microtesla magnetic field coil (operating in the range of 0.02-75 microtesla), a three-layered mu-metal shield (to attenuate the ambient magnetic field), and a magnetic shield degaussing coil placed in the overall device enclosure. The gas-handling manifold allows for parahydrogen-gas flow and pressure control (up to 9.2 bar of total parahydrogen pressure). The sample temperature can be varied either using a water bath or a PID-controlled heat exchanger in the range from -12 to 80 °C. This benchtop device measures 62 cm (length) × 47 cm (width) × 47 cm (height), weighs 30 kg, and requires only connections to a high-pressure parahydrogen gas supply and a single 110/220 VAC power source. The utility of the device has been demonstrated using an example of parahydrogen pairwise addition to form HP ethyl [1-13C]acetate (P13C = 7%, [c] = 1 M). Moreover, the Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) technique was employed to demonstrate efficient hyperpolarization of 13C and 15N spins in a wide range of biologically relevant molecules, including [1-13C]pyruvate (P13C = 14%, [c] = 27 mM), [1-13C]-α-ketoglutarate (P13C = 17%), [1-13C]ketoisocaproate (P13C = 18%), [15N3]metronidazole (P15N = 13%, [c] = 20 mM), and others. While the vast majority of the utility studies have been performed in standard 5 mm NMR tubes, the sample chamber of the device can accommodate a wide range of sample container sizes and geometries of up to 1 L sample volume. The device establishes an integrated, simple, inexpensive, and versatile equipment gateway needed to facilitate parahydrogen-based hyperpolarization experiments ranging from basic science to preclinical applications; indeed, detailed technical drawings and a bill of materials are provided to support the ready translation of this design to other laboratories.

Scanning Nuclear Spin Level Anticrossings by Constant-Adiabaticity Magnetic Field Sweeping of Parahydrogen-Induced 13C Polarization

The polarization transfer between ¹H protons and ¹³C heteronuclei is of central importance in the development of parahydrogen-based hyperpolarization techniques dedicated to the production of ¹³C-hyperpolarized molecular probes. Here we unveil the spin conversion efficiency in the polarization transfer between parahydrogen-derived protons and ¹³C nuclei of an ethyl acetate biomolecule, formed by the homogeneous hydrogenation of vinyl acetate with parahydrogen, obtained by applying constant-adiabaticity sweep profiles at ultralow magnetic fields. The experiments employed natural C-13 abundance. Spin level anticrossings can be detected experimentally using a scanning approach and are selected to improve the polarization transfer efficiency. ¹³C polarization of up to 12% is readily achieved on the carbonyl center. The results demonstrate the simplicity, reproducibility, and high conversion efficiency of the technique, opening the door for a refined manipulation of hyperpolarized spins in both basic science experiments (e.g., state-selected spectroscopy in the strong-coupling regime) and biomedical nuclear magnetic resonance applications.

Quasi-continuous production of highly hyperpolarized carbon-13 contrast agents every 15 seconds within an MRI system

Hyperpolarized contrast agents (HyCAs) have enabled unprecedented magnetic resonance imaging (MRI) of metabolism and pH in vivo. Producing HyCAs with currently available methods, however, is typically time and cost intensive. Here, we show virtually-continuous production of HyCAs using parahydrogen-induced polarization (PHIP), without stand-alone polarizer, but using a system integrated in an MRI instead. Polarization of ≈2% for [1-¹³C]succinate-d₂ or ≈19% for hydroxyethyl-[1-¹³C]propionate-d₃ was created every 15 s, for which fast, effective, and well-synchronized cycling of chemicals and reactions in conjunction with efficient spin-order transfer was key. We addressed these challenges using a dedicated, high-pressure, high-temperature reactor with integrated water-based heating and a setup operated via the MRI pulse program. As PHIP of several biologically relevant HyCAs has recently been described, this Rapid-PHIP technique promises fast preclinical studies, repeated administration or continuous infusion within a single lifetime of the agent, as well as a prolonged window for observation with signal averaging and dynamic monitoring of metabolic alterations.

Order-Unity 13 C Nuclear Polarization of [1-13 C]Pyruvate in Seconds and the Interplay of Water and SABRE Enhancement

Signal Amplification By Reversible Exchange in SHield Enabled Alignment Transfer (SABRE-SHEATH) is investigated to achieve rapid hyperpolarization of 13 C1 spins of [1-13 C]pyruvate, using parahydrogen as the source of nuclear spin order. Pyruvate exchange with an iridium polarization transfer complex can be modulated via a sensitive interplay between temperature and co-ligation of DMSO and H2 O. Order-unity 13 C (>50 %) polarization of catalyst-bound [1-13 C]pyruvate is achieved in less than 30 s by restricting the chemical exchange of [1-13 C]pyruvate at lower temperatures. On the catalyst bound pyruvate, 39 % polarization is measured using a 1.4 T NMR spectrometer, and extrapolated to >50 % at the end of build-up in situ. The highest measured polarization of a 30-mM pyruvate sample, including free and bound pyruvate is 13 % when using 20 mM DMSO and 0.5 M water in CD3 OD. Efficient 13 C polarization is also enabled by favorable relaxation dynamics in sub-microtesla magnetic fields, as indicated by fast polarization buildup rates compared to the T1 spin-relaxation rates (e. g., ∼0.2 s-1 versus ∼0.1 s-1 , respectively, for a 6 mM catalyst-[1-13 C]pyruvate sample). Finally, the catalyst-bound hyperpolarized [1-13 C]pyruvate can be released rapidly by cycling the temperature and/or by optimizing the amount of water, paving the way to future biomedical applications of hyperpolarized [1-13 C]pyruvate produced via comparatively fast and simple SABRE-SHEATH-based approaches.

Parahydrogen-Induced Polarization Relayed via Proton Exchange

The hyperpolarization of nuclear spins is a game-changing technology that enables hitherto inaccessible applications for magnetic resonance in chemistry and biomedicine. Despite significant advances and discoveries in the past, however, the quest to establish efficient and effective hyperpolarization methods continues. Here, we describe a new method that combines the advantages of direct parahydrogenation, high polarization (P), fast reaction, and low cost with the broad applicability of polarization transfer via proton exchange. We identified the system propargyl alcohol + pH₂ → allyl alcohol to yield ¹H polarization in excess of P ≈ 13% by using only 50% enriched pH₂ at a pressure of ≈1 bar. The polarization was then successfully relayed via proton exchange from allyl alcohol to various target molecules. The polarizations of water and alcohols (as target molecules) approached P ≈ 1% even at high molar concentrations of 100 mM. Lactate, glucose, and pyruvic acid were also polarized, but to a lesser extent. Several potential improvements of the methodology are discussed. Thus, the parahydrogen-induced hyperpolarization relayed via proton exchange (PHIP-X) is a promising approach to polarize numerous molecules which participate in proton exchange and support new applications for magnetic resonance.

PHIP hyperpolarized [1-13C]pyruvate and [1-13C]acetate esters via PH-INEPT polarization transfer monitored by 13C NMR and MRI

Parahydrogen-induced polarization of ¹³C nuclei by side-arm hydrogenation (PHIP-SAH) for [1-¹³C]acetate and [1-¹³C]pyruvate esters with application of PH-INEPT-type pulse sequences for ¹H to ¹³C polarization transfer is reported, and its efficiency is compared with that of polarization transfer based on magnetic field cycling (MFC). The pulse-sequence transfer approach may have its merits in some applications because the entire hyperpolarization procedure is implemented directly in an NMR or MRI instrument, whereas MFC requires a controlled field variation at low magnetic fields. Optimization of the PH-INEPT-type transfer sequences resulted in ¹³C polarization values of 0.66 ± 0.04% and 0.19 ± 0.02% for allyl [1-¹³C]pyruvate and ethyl [1-¹³C]acetate, respectively, which is lower than the corresponding polarization levels obtained with MFC for ¹H to ¹³C polarization transfer (3.95 ± 0.05% and 0.65 ± 0.05% for allyl [1-¹³C]pyruvate and ethyl [1-¹³C]acetate, respectively). Nevertheless, a significant ¹³C NMR signal enhancement with respect to thermal polarization allowed us to perform ¹³C MR imaging of both biologically relevant hyperpolarized molecules which can be used to produce useful contrast agents for the in vivo imaging applications.

Parahydrogen-Induced Radio Amplification by Stimulated Emission of Radiation

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

Pulse-Programmable Magnetic Field Sweeping of Parahydrogen-Induced Polarization by Side Arm Hydrogenation

Among the hyperpolarization techniques geared toward in vivo magnetic resonance imaging, parahydrogen-induced polarization (PHIP) shows promise due to its low cost and fast speed of contrast agent preparation. The synthesis of 13C-labeled, unsaturated precursors to perform PHIP by side arm hydrogenation has recently opened new possibilities for metabolic imaging owing to the biological compatibility of the reaction products, although the polarization transfer between the parahydrogen-derived protons and the 13C heteronucleus must yet be better understood, characterized, and eventually optimized. In this realm, a new experimental strategy incorporating pulse-programmable magnetic field sweeping and in situ detection has been developed. The approach is evaluated by measuring the 13C polarization of ethyl acetate-1-13C, i.e., the product of pairwise addition of parahydrogen to vinyl acetate-1-13C, resulting from zero-crossing magnetic field ramps of various durations, amplitudes, and step sizes. The results demonstrate (i) the profound effect these parameters have on the 1H to 13C polarization transfer efficiency and (ii) the high reproducibility of the technique.

In Situ SABRE Hyperpolarization with Earth's Field NMR Detection

Hyperpolarization methods, which increase the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), have the potential to expand the range of applications of these powerful analytical techniques and to enable the use of smaller and cheaper devices. The signal amplification by reversible exchange (SABRE) method is of particular interest because it is relatively low-cost, straight-forward to implement, produces high-levels of renewable signal enhancement, and can be interfaced with low-cost and portable NMR detectors. In this work, we demonstrate an in situ approach to SABRE hyperpolarization that can be achieved using a simple, commercially-available Earth’s field NMR detector to provide ¹H polarization levels of up to 3.3%. This corresponds to a signal enhancement over the Earth’s magnetic field by a factor of ε > 2 × 10⁸. The key benefit of our approach is that it can be used to directly probe the polarization transfer process at the heart of the SABRE technique. In particular, we demonstrate the use of in situ hyperpolarization to observe the activation of the SABRE catalyst, the build-up of signal in the polarization transfer field (PTF), the dependence of the hyperpolarization level on the strength of the PTF, and the rate of decay of the hyperpolarization in the ultra-low-field regime.

Hyperpolarizing Concentrated Metronidazole 15 NO2 Group over Six Chemical Bonds with More than 15 % Polarization and a 20 Minute Lifetime

The NMR hyperpolarization of uniformly ¹⁵ N-labeled [¹⁵ N₃ ]metronidazole is demonstrated by using SABRE-SHEATH. In this antibiotic, the ¹⁵ NO₂ group is hyperpolarized through spin relays created by ¹⁵ N spins in [¹⁵ N₃ ]metronidazole, and the polarization is transferred from parahydrogen-derived hydrides over six chemical bonds. In less than a minute of parahydrogen bubbling at approximately 0.4 μT, a high level of nuclear spin polarization (P_15N ) of around 16 % is achieved on all three ¹⁵ N sites. This product of ¹⁵ N polarization and concentration of ¹⁵ N spins is around six-fold better than any previous value determined for ¹⁵ N SABRE-derived hyperpolarization. At 1.4 T, the hyperpolarized state persists for tens of minutes (relaxation time, T₁ ≈10 min). A novel synthesis of uniformly ¹⁵ N-enriched metronidazole is reported with a yield of 15 %. This approach can potentially be used for synthesis of a wide variety of in vivo metabolic probes with potential uses ranging from hypoxia sensing to theranostic imaging.

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 (T_LLS) approximately 3-fold greater than the corresponding T₁ values. Both T_LLS 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 T_LLS value of HP propane gas at 1 atm is ~3 s. Deuteration of the substrate (propylene-d₆) yields hyperpolarized propane-d₆ gas with T_LLS 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 T_LLS 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 T_LLS, 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 T_LLS of liquefied HP propane reaches 14.7 seconds, which is greater than the T_LLS 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.

Metabolic and Molecular Imaging with Hyperpolarised Tracers

Since reaching the clinic, magnetic resonance imaging (MRI) has become an irreplaceable radiological tool because of the macroscopic information it provides across almost all organs and soft tissues within the human body, all without the need for ionising radiation. The sensitivity of MR, however, is too low to take full advantage of the rich chemical information contained in the MR signal. Hyperpolarisation techniques have recently emerged as methods to overcome the sensitivity limitations by enhancing the MR signal by many orders of magnitude compared to the thermal equilibrium, enabling a new class of metabolic and molecular X-nuclei based MR tracers capable of reporting on metabolic processes at the cellular level. These hyperpolarised (HP) tracers have the potential to elucidate the complex metabolic processes of many organs and pathologies, with studies so far focusing on the fields of oncology and cardiology. This review presents an overview of hyperpolarisation techniques that appear most promising for clinical use today, such as dissolution dynamic nuclear polarisation (d-DNP), parahydrogen-induced hyperpolarisation (PHIP), Brute force hyperpolarisation and spin-exchange optical pumping (SEOP), before discussing methods for tracer detection, emerging metabolic tracers and applications and progress in preclinical and clinical application.

Effects of Deuteration of 13C-Enriched Phospholactate on Efficiency of Parahydrogen-Induced Polarization by Magnetic Field Cycling

We report herein a large-scale (>10 g) synthesis of isotopically enriched 1-¹³C-phosphoenolpyruvate and 1-¹³C-phosphoenolpyruvate-d₂ for application in hyperpolarized imaging technology. The 1-¹³C-phosphoenolpyruvate-d₂ was synthesized with 57% overall yield (over two steps), and >98% ²H isotopic purity, representing an improvement over the previous report. The same outcome was achieved for 1-¹³C-phosphoenolpyruvate. These two unsaturated compounds with C=C bonds were employed for parahydrogen-induced polarization via pairwise parahydrogen addition in aqueous medium. We find that deuteration of 1-¹³C-phosphoenolpyruvate resulted in overall increase of ¹H T₁ of nascent hyperpolarized protons (4.30 ± 0.04 s versus 2.06 ± 0.01 s) and ¹H polarization (~2.5% versus ~0.7%) of the resulting hyperpolarized 1-¹³C-phospholactate. The nuclear spin polarization of nascent parahydrogen-derived protons was transferred to 1-¹³C nucleus via magnetic field cycling procedure. The proton T₁ increase in hyperpolarized deuterated 1-¹³C-phospholactate yielded approximately 30% better ¹³C polarization compared to non-deuterated hyperpolarized 1-¹³C-phospholactate. Analysis of T₁ relaxation revealed that deuteration of 1-¹³C-phospholactate may have resulted in approximately 3-fold worse H→¹³C polarization transfer efficiency via magnetic field cycling. Since magnetic field cycling is a key polarization transfer step in the Side-Arm Hydrogenation approach, the presented findings may guide more rationale design of contrast agents using parahydrogen polarization of a broad range of ¹³C hyperpolarized contrast agents for molecular imaging employing ¹³C MRI. The hyperpolarized 1-¹³C-phospholactate-d₂ is of biomedical imaging relevance because it undergoes in vivo dephosphorylation and becomes ¹³C hyperpolarized lactate, which as we show can be detected in the brain using ¹³C hyperpolarized MRI; an implication for future imaging of neurodegenerative diseases and dementia.

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.

SABRE hyperpolarization enables high-sensitivity 1H and 13C benchtop NMR spectroscopy

Benchtop NMR spectrometers operating with low magnetic fields of 1-2 T at sub-ppm resolution show great promise as analytical platforms that can be used outside the traditional laboratory environment for industrial process monitoring. One current limitation that reduces the uptake of benchtop NMR is associated with the detection fields' reduced sensitivity. Here we demonstrate how para-hydrogen (p-H2) based signal amplification by reversible exchange (SABRE), a simple to achieve hyperpolarization technique, enhances agent detectability within the environment of a benchtop (1 T) NMR spectrometer so that informative 1H and 13C NMR spectra can be readily recorded for low-concentration analytes. SABRE-derived 1H NMR signal enhancements of up to 17 000-fold, corresponding to 1H polarization levels of P = 5.9%, were achieved for 26 mM pyridine in d4-methanol in a matter of seconds. Comparable enhancement levels can be achieved in both deuterated and protio solvents but now the SABRE-enhanced analyte signals dominate due to the comparatively weak thermally-polarized solvent response. The SABRE approach also enables the acquisition of 13C NMR spectra of analytes at natural isotopic abundance in a single scan as evidenced by hyperpolarized 13C NMR spectra of tens of millimolar concentrations of 4-methylpyridine. Now the associated signal enhancement factors are up to 45 500 fold (P = 4.0%) and achieved in just 15 s. Integration of an automated SABRE polarization system with the benchtop NMR spectrometer framework produces renewable and reproducible NMR signal enhancements that can be exploited for the collection of multi-dimensional NMR spectra, exemplified here by a SABRE-enhanced 2D COSY NMR spectrum.

Synthesis of Unsaturated Precursors for Parahydrogen-Induced Polarization and Molecular Imaging of 1-13C-Acetates and 1-13C-Pyruvates via Side Arm Hydrogenation

Hyperpolarized forms of 1-¹³C-acetates and 1-¹³C-pyruvates are used as diagnostic contrast agents for molecular imaging of many diseases and disorders. Here, we report the synthetic preparation of 1-¹³C isotopically enriched and pure from solvent acetates and pyruvates derivatized with unsaturated ester moiety. The reported unsaturated precursors can be employed for NMR hyperpolarization of 1-¹³C-acetates and 1-¹³C-pyruvates via parahydrogen-induced polarization (PHIP). In this PHIP variant, Side arm hydrogenation (SAH) of unsaturated ester moiety is followed by the polarization transfer from nascent parahydrogen protons to ¹³C nucleus via magnetic field cycling procedure to achieve hyperpolarization of ¹³C nuclear spins. This work reports the synthesis of PHIP-SAH precursors: vinyl 1-¹³C-acetate (55% yield), allyl 1-¹³C-acetate (70% yield), propargyl 1-¹³C-acetate (45% yield), allyl 1-¹³C-pyruvate (60% yield), and propargyl 1-¹³C-pyruvate (35% yield). Feasibility of PHIP-SAH ¹³C hyperpolarization was verified by ¹³C NMR spectroscopy: hyperpolarized allyl 1-¹³C-pyruvate was produced from propargyl 1-¹³C-pyruvate with ¹³C polarization of ∼3.2% in CD₃OD and ∼0.7% in D₂O. ¹³C magnetic resonance imaging is demonstrated with hyperpolarized 1-¹³C-pyruvate in aqueous medium.

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 highly versatile automatized setup for quantitative measurements of PHIP enhancements

The design and application of a versatile and inexpensive experimental extension to NMR spectrometers is described that allows to carry out highly reproducible PHIP experiments directly in the NMR sample tube, i.e. under PASADENA condition, followed by the detection of the NMR spectra of hyperpolarized products with high spectral resolution. Employing this high resolution it is feasible to study kinetic processes in the solution with high accuracy. As a practical example the dissolution of hydrogen gas in the liquid and the PHIP kinetics during the hydrogenation reaction of Fmoc-O-propargyl-l-tyrosine in acetone-d₆ are monitored. The timing of the setup is fully controlled by the pulse-programmer of the NMR spectrometer. By flushing with an inert gas it is possible to efficiently quench the hydrogenation reaction in a controlled fashion and to detect the relaxation of hyperpolarization without a background reaction. The proposed design makes it possible to carry out PHIP experiments in an automatic mode and reliably determine the enhancement of polarized signals.