Proton magnetization enhancement of solvents with hyperpolarized xenon in very low-magnetic fields

Background-Free Detection of Spin-Exchange Dynamics at Ultra-Low Magnetic Field

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) inherently suffer from a low signal-to-noise ratio due to the small thermal polarization of nuclear spins. Transfer of polarization from a pre-polarized spin system to a thermally polarized spin system via the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) could potentially be used to overcome this limitation. SPINOE is particularly advantageous at ultra-low magnetic field, where the transferred polarization can be several orders of magnitude higher than thermal polarization. Here we demonstrate direct detection of polarization transfer from highly polarized 129 Xe gas spins to 1 H spins in solution via SPINOE. At ultra-low field, where thermal nuclear spin polarization is close to background noise levels and where different nuclei can be simultaneously detected in a single spectrum, the dynamics of the polarization transfer can be observed in real time. We show that by simply bubbling hyperpolarized 129 Xe into solution, we can enhance 1 H polarization levels by a factor of up to 151-fold. While our protocol leads to lower enhancements than those previously reported under extreme Xe gas pressures, the methodology is easily repeatable and allows for on-demand enhanced spectroscopy. SPINOE at ultra-low magnetic field could also be employed to study 129 Xe interactions in solutions.

Spin Hyperpolarization in Modern Magnetic Resonance

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

Detection of natural abundance 13C J-couplings at Earth's magnetic field for spin system differentiation of small organic molecules

Nuclear magnetic resonance (NMR) spectroscopy routinely characterizes the unique spin systems of molecules using a combination of chemical shift and J-coupling interactions for the ¹H and ¹³C nuclei. However, at Earth's magnetic field, chemical shifts are unresolvable and the ability to characterize structure relies solely on the J-couplings. Fortuitously, the J-couplings at Earth's field provides the same spin system information as high field, but only requires detection of the ¹H nucleus. We report the first identification of the multiple natural abundance ¹H-¹³C spin systems on organic molecules detected at Earth's magnetic field. The results clearly demonstrate the feasibility of Earth's field NMR to characterize small organic molecules without costly enrichment strategies.

Quantitation of Nuclear Magnetic Resonance Spectra at Earth's Magnetic Field

The inherently quantitative nature of nuclear magnetic resonance (NMR) spectroscopy is one of the most attractive aspects of this analytical technique. Quantitative NMR analyses have typically been limited to high-field (>1 T) applications. The aspects for quantitation at low magnetic fields (<1 mT) have not been thoroughly investigated and are shown to be impacted by the complex signatures that arise at these fields from strong heteronuclear J-couplings. This study investigates quantitation at Earth's magnetic field (∼50 μT) for a variety of samples in strongly, weakly, and uncoupled spin systems. To achieve accurate results in this regime, the instrumentation, experimental acquisition, processing, and theoretical aspects must be considered and reconciled. Of particular note is the constant field nuclear receptivity equation, which has been re-derived in this study to account for strong coupling and quality factor effects. The results demonstrate that the quantitation of homonuclear molecular groups, determination of heteronuclear pseudoempirical formulas, and mixture analysis are all feasible at Earth's magnetic field in a greatly simplified experimental system.

An open-source, low-cost NMR spectrometer operating in the mT field regime

In recent years, low field and ultra-low field NMR spectrometers have gained interest due to their portability, lower cost, and reduced subject-induced magnetic field inhomogeneities. Here, we describe the design of a low-cost multinuclear NMR spectrometer operating in the ultra-low field regime (ULF), which possesses high spectral resolution and enables arbitrary pulse programming. An inexpensive multifunction input/output (I/O) device is used to handle waveform generation and digitization in the kHz operating range. A home-built radio frequency (RF) mixing circuit is used to down-mix the NMR signals, allowing for the slower sampling rates and lower memory requirements needed to enable minute-long acquisitions using a standard Windows PC. The LabVIEW code, along with a bill of materials for all components used in the spectrometer, is included. As proof of concept, ¹H relaxation measurements and the simultaneous detection of ¹H with gas phase and dissolved ¹²⁹Xe frequencies using the described low field NMR spectrometer are demonstrated.

Low-cost low-field NMR and MRI: Instrumentation and applications

While NMR and MRI are often thought of as expensive techniques requiring large institutional investment, opportunities for low-cost, low-field NMR and MRI abound. We discuss a number of approaches to performing magnetic resonance experiments with inexpensive, easy to find or build components, aimed at applications in industry, education, and research. Opportunities that aim to make NMR accessible to a broad community are highlighted. We describe and demonstrate some projects from our laboratory, including a new prototype instrument for measurements at frequencies up to ∼200 kHz and demonstrate its application to the study of the rapidly advancing technique known as inhomogeneous magnetization transfer imaging, a promising method for characterizing myelin in vivo.

Design and implementation of a J-coupled spectrometer for multidimensional structure and relaxation detection at low magnetic fields

In recent years, it has been realized that low and ultra-low field (mT-nT magnetic field range) nuclear magnetic resonance spectroscopy can be used for molecular structural analysis. However, spectra are often hindered by lengthy acquisition times or require large sample volumes and high concentrations. Here, we report a low field (50 μT) instrument that employs a linear actuator to shuttle samples between a 1 T prepolarization field and a solenoid detector in a laboratory setting. The current experimental setup is benchmarked using water and ¹³C-methanol with a single scan detection limit of 2 × 10²⁰ spins (3 µl, 55M H₂O) and detection limit of 2.9 × 10¹⁹ (200 µl, 617 mM ¹³C-methanol) spins with signal averaging. The system has a dynamic range of >3 orders of magnitude. Investigations of room-temperature relaxation dynamics of ¹³C-methanol show that sample dilution can be used in lieu of sample heating to acquire spectra with linewidths comparable to high-temperature spectra. These results indicate that the T₁ and T₂ mechanisms are governed by both the proton exchange rate and the dissolved oxygen in the sample. Finally, a 2D correlation spectroscopy experiment is reported, performed in the strong coupling regime that resolves the multiple resonances associated with the heteronuclear J-coupling. The spectrum was collected using 10 times less sample and in less than half the time from previous reports in the strong coupling limit.

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.

Earth field NMR with chemical shift spectral resolution: theory and proof of concept

A new method for obtaining an NMR signal in the Earth's magnetic field (EF) is presented. The method makes use of a simple pulse sequence with only DC fields which is much less demanding than previous approaches in terms of the pulses' rise and fall times. Furthermore, it offers the possibility of obtaining NMR data with enough spectral resolution to allow retrieving high resolution molecular chemical shift (CS) information - a capability that was not considered possible in EF NMR until now. Details of the pulse sequence, the experimental system, and our specially tailored EF NMR probe are provided. The experimental results demonstrate the capability to differentiate between three types of samples made of common fluorine compounds, based on their CS data.

Studies of ⁶Li-NMR properties in different salt solutions in low magnetic fields

In this article we report the longitudinal relaxation times (T(1)) of various (6)Li salts ((6)LiI, (6)LiCl and (6)LiNO(3)) in D(2)O and H(2)O, measured in low magnetic fields (B(0)=3.5mT). This investigation serves the purpose of clarifying the relaxation behavior of different (6)Li solutions and different concentrations. The measurement were undertaken to establish a framework for future applications of hyperpolarized (6)Li in medical imaging, biological studies and investigations of lithium ion batteries. Time will pass during the transport of hyperpolarized lithium ions to the sample, which leads to a polarization loss. In order to store polarization as long as possible, it is necessary to examine which (6)Li salt solution has the longest relaxation time T(1). Longitudinal relaxation times of (6)Li salts in D(2)O and H(2)O were investigated as a function of concentration and the most extended T(1) was found for (6)LiI in D(2)O and H(2)O. In agreement with the theory the relaxation time T(1) of all (6)Li salts increase with decreasing concentration. In the case of (6)LiI in H(2)O an inverse behavior was observed. We assume that the prolonged T(1) times occur due to formation of (6)LiOH upon the solution of (6)LiI in H(2)O, which settles as a precipitate. By diluting the solution, the precipitate continuously dissolves and approaches T(1) of (6)LiOH (T(1)∼28s), leading to a shorter T(1) relaxation time.

Selective drug trace detection with low-field NMR

Advances with para-hydrogen induced polarization open up new fields of applications for portable low-field NMR. Here we report the possibility of tracing drugs down to the micromolar regime. We could selectively polarize nicotine quantities similar to those found in one cigarette. Also less than 1 mg of harmine, a drug used for treatment of Parkinson's disease, and morphine extracted from an opium solution were detectable after polarization with para-hydrogen in single-scan (1)H-experiments. Moreover, we demonstrate the possibility to selectively enhance and detect the (1)H-signal of drug molecules with PHIP in proton rich standard solutions that would otherwise mask the (1)H NMR signal of the drug.

Trace analysis by low-field NMR: breaking the sensitivity limit

Sensitivity poses a persistent challenge to NMR spectroscopy and magnetic resonance imaging (MRI). Nonhydrogenative para-hydrogen induced polarization (NH-PHIP) has recently emerged as an efficient method to substantially increase the sensitivity of high-field NMR. Here, we report the feasibility of applying NH-PHIP in the low-field NMR. A trace amount of pyridine of just a few nanoliters ( approximately 12 nmol) in a 0.4 mL NMR sample (a concentration of 31 microM or 10(16)/cm(3)) could be measured in a single scan by NH-PHIP. There is a striking difference in the signal-to-noise ratio (SNR) between thermal prepolarization and NH-PHIP: The SNR of the prepolarized (1)H NMR signal decreases linearly with decreasing (1)H concentration ([(1)H]) while the SNR in NH-PHIP experiments first increases with decreasing [(1)H], then remains constant over 2 orders of magnitude, and finally decreases linearly with decreasing [(1)H]. A hitherto unknown potential opens up for trace analysis by low-field NMR in the bio-, chemical, and material sciences.

Quantitative estimation of SPINOE enhancement in solid state

A theoretical approach to quantitatively estimate the spin polarization enhancement via spin polarization-induced nuclear Overhauser effect (SPINOE) in solid state is presented. We show that theoretical estimates from the model are in good agreement with published experimental results. This method provides a straightforward way to predict the enhanced factor of nuclear magnetic resonance signals in solid state experiments.

A dynamic nuclear polarization strategy for multi-dimensional Earth's field NMR spectroscopy

Dynamic nuclear polarization (DNP) is introduced as a powerful tool for polarization enhancement in multi-dimensional Earth's field NMR spectroscopy. Maximum polarization enhancements, relative to thermal equilibrium in the Earth's magnetic field, are calculated theoretically and compared to the more traditional prepolarization approach for NMR sensitivity enhancement at ultra-low fields. Signal enhancement factors on the order of 3000 are demonstrated experimentally using DNP with a nitroxide free radical, TEMPO, which contains an unpaired electron which is strongly coupled to a neighboring (14)N nucleus via the hyperfine interaction. A high-quality 2D (19)F-(1)H COSY spectrum acquired in the Earth's magnetic field with DNP enhancement is presented and compared to simulation.

Surface relaxation of polarized Xe atoms dissolved in deuterated ethanol

We measured the spin relaxation of polarized xenon atoms dissolved in deuterated ethanol. Surface relaxation was suppressed by coating the cell walls with deuterated eicosane. From the dependence of the decay rate on temperature and static magnetic field, we obtained the correlation time of random fluctuations of the local field at the liquid-solid interface. By varying the cell volume, the wall coating, and the surface area of the eicosane, we measured the contribution of the spin-rotation interaction to the relaxation. The use of both deuterated molecules enables us to distinguish surface relaxation from the magnetic dipole-dipole and spin-rotation interactions in solution.

Mobile high resolution xenon nuclear magnetic resonance spectroscopy in the earth's magnetic field

Conventional high resolution nuclear magnetic resonance (NMR) spectra are usually measured in homogeneous, high magnetic fields (>1 T), which are produced by expensive and immobile superconducting magnets. We show that chemically resolved xenon (Xe) NMR spectroscopy of liquid samples can be measured in the Earth's magnetic field (5 x 10(-5) T) with a continuous flow of hyperpolarized Xe gas. It was found that the measured normalized Xe frequency shifts are significantly modified by the Xe polarization density, which causes different dipolar magnetic fields in the liquid and in the gas phases.

Substrate and Field Dependence of the SPINOE Transfer to Surface 13C from Hyperpolarized 129Xe

The substrate and field dependencies of surface SPINOE enhancements using optical pumping and magic angle spinning NMR were monitored. Relaxation rates and enhancements were examined to gain an understanding of the parameters that determine the SPINOE enhancement. (13)C-labeled deuterated methanol was adsorbed on three different substrates (SnO(2), TiO(2), Ti/SiO(2)) with heats of adsorption for xenon ranging from 14.2 to 22.6 kJ/mol. The different heats of adsorption led to a range of xenon coverages and xenon relaxation rates. Using a simple model along with experimental values for the xenon surface polarization and cross- and self-relaxation rates, the (13)C signal enhancement could be predicted and compared with experimental enhancement values. Magnetic field dependence studies were also made by monitoring the (13)C enhancements via SPINOE from hyperpolarized xenon at fields of 0.075, 4.7, and 9.4 T. The pertinent parameters necessary to achieve maximum SPINOE enhancement are discussed.

Hyperpolarized xenon in NMR and MRI

Hyperpolarized gases have found a steadily increasing range of applications in nuclear magnetic resonance (NMR) and NMR imaging (MRI). They can be regarded as a new class of MR contrast agent or as a way of greatly enhancing the temporal resolution of the measurement of processes relevant to areas as diverse as materials science and biomedicine. We concentrate on the properties and applications of hyperpolarized xenon. This review discusses the physics of producing hyperpolarization, the NMR-relevant properties of 129Xe, specific MRI methods for hyperpolarized gases, applications of xenon to biology and medicine, polarization transfer to other nuclear species and low-field imaging.

Magnetic resonance imaging of spin-polarization transfer of polarized Xe atoms dissolving into ethanol

We detect the free-induction signals of xenon atoms polarized by spin-exchange optical pumping. The temperature dependence of dissolution and spin-polarization transfer of xenon atoms to ethanol is measured by simultaneous detection of both xenon and proton signals. The polarization of proton is efficiently enhanced in the xenon-saturated solution at low magnetic fields. The large polarization and chemical shift enable us to obtain clearly the distribution image of xenon atoms near the gas-liquid and liquid-liquid boundaries. Therefore the localization of polarized xenon atoms is observed near the surface. By time-resolved magnetic resonance imaging of polarized xenon and polarization-enhanced proton, the spin dynamics is qualitatively studied for the nuclear spins interacting with each other in a dense solution.

Time resolved spectroscopic NMR imaging using hyperpolarized 129Xe

We have visualized the melting and dissolution processes of xenon (Xe) ice into different solvents using the methods of nuclear magnetic resonance (NMR) spectroscopy, imaging, and time resolved spectroscopic imaging by means of hyperpolarized 129Xe. Starting from the initial condition of a hyperpolarized solid Xe layer frozen on top of an ethanol (ethanol/water) ice block we measured the Xe phase transitions as a function of time and temperature. In the pure ethanol sample, pieces of Xe ice first fall through the viscous ethanol to the bottom of the sample tube and then form a thin layer of liquid Xe/ethanol. The xenon atoms are trapped in this liquid layer up to room temperature and keep their magnetization over a time period of 11 min. In the ethanol/water mixture (80 vol%/20%), most of the polarized Xe liquid first stays on top of the ethanol/water ice block and then starts to penetrate into the pores and cracks of the ethanol/water ice block. In the final stage, nearly all the Xe polarization is in the gas phase above the liquid and trapped inside the pores. NMR spectra of homogeneous samples of pure ethanol containing thermally polarized Xe and the spectroscopic images of the melting process show that very high concentrations of hyperpolarized Xe (about half of the density of liquid Xe) can be stored or delivered in pure ethanol.

Enhancement of SQUID-detected NMR signals with hyperpolarized liquid 129Xe in a 1 microT magnetic field

We report an enhancement of proton NMR signals by a factor of 10(6) by cross polarization with hyperpolarized liquid 129Xe in an ultralow magnetic field of 1 microT. The NMR signals from cyclopentane, acetone, and methanol are detected using a commercial high-T(c) SQUID magnetometer with a signal-to-noise ratio of up to 1000 from a single 90 degrees tipping pulse. This technique allows a wide range of low-field NMR measurements and is promising for the detection of intermolecular scalar spin-spin couplings. Scalar intermolecular couplings can produce a shift of the average NMR frequency in a hyperpolarized sample even in the presence of rapid chemical exchange.