Exploiting adiabatically switched RF-field for manipulating spin hyperpolarization induced by parahydrogen

Radio-Frequency Sweeps at Microtesla Fields for Parahydrogen-Induced Polarization of Biomolecules

Magnetic resonance imaging of ¹³C-labeled metabolites enhanced by parahydrogen-induced polarization (PHIP) enables real-time monitoring of processes within the body. We introduce a robust, easily implementable technique for transferring parahydrogen-derived singlet order into ¹³C magnetization using adiabatic radio frequency sweeps at microtesla fields. We experimentally demonstrate the applicability of this technique to several molecules, including some molecules relevant for metabolic imaging, where we show significant improvements in the achievable polarization, in some cases reaching above 60% nuclear spin polarization. Furthermore, we introduce a site-selective deuteration scheme, where deuterium is included in the coupling network of a pyruvate ester to enhance the efficiency of the polarization transfer. These improvements are enabled by the fact that the transfer protocol avoids relaxation induced by strongly coupled quadrupolar nuclei.

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.

Robust transformation of singlet order into heteronuclear magnetisation over an extended coupling range

Several important NMR procedures involve the conversion of nuclear singlet order into heteronuclear magnetisation, including some experiments involving long-lived spin states and parahydrogen-induced hyperpolarisation. However most existing sequences suffer from a limited range of validity or a lack of robustness against experimental imperfections. We present a new radio-frequency scheme for the transformation of the singlet order of a chemically-equivalent homonuclear spin pair into the magnetisation of a heteronuclear coupling partner. The proposed radio-frequency (RF) scheme is called gS2hM (generalized singlet-to-heteronuclear magnetisation) and has good compensation for common experimental errors such as RF and static field inhomogeneities. The sequence retains its robustness for homonuclear spin pairs in the intermediate coupling regime, characterised by the in-pair coupling being of the same order of magnitude as the difference between the out-of-pair couplings. This is a substantial improvement to the validity range of existing sequences. Analytical solutions for the pulse sequence parameters are provided. Experimental results are shown for two test cases.

Long live the singlet state!

The field of long-lived states in NMR is reviewed. The relationship of long-lived-state phenomena to those associated with spin isomerism is discussed. A brief overview is given of key developments in the field of long-lived states, including chemical symmetry-switching, the role of magnetic equivalence and magnetic inequivalence, long-lived coherences, hyperpolarized NMR involving long-lived states, quantum-rotor-induced polarization, and parahydrogen-induced hyperpolarization. Current application areas of long-lived states are reviewed, and a peer into the crystal ball reveals future developments in the field.

Proton Relaxometry of Long-Lived Spin Order

A study of long-lived spin order in chlorothiophene carboxylates at both high and low magnetic fields is presented. Careful sample preparation (removal of dissolved oxygen in solution, chelating of paramagnetic impurities, reduction of convection) allows one to obtain very long-lived singlet order of the two coupled protons in chlorothiophene derivatives, having lifetimes of about 130 s in D₂ O and 240 s in deuterated methanol, which are much longer than the T₁ -relaxation times (18 and 30 s, respectively, at a field B 0 =9.4 T). In protonated solvents the relaxation times become shorter, but the lifetime is still substantially longer than T 1 . In addition, long-lived coherences are shown to have lifetimes as long as 30 s. Thiophene derivatives can be used as molecular tags to study slow transport, slow dynamics and slow chemical processes, as has been shown in recent years.

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.

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.

Singlet NMR methodology in two-spin-1/2 systems

This paper discusses methodology developed over the past 12years in order to access and manipulate singlet order in systems comprising two coupled spin-1/2 nuclei in liquid-state nuclear magnetic resonance. Pulse sequences that are valid for different regimes are discussed, and fully analytical proofs are given using different spin dynamics techniques that include product operator methods, the single transition operator formalism, and average Hamiltonian theory. Methods used to filter singlet order from byproducts of pulse sequences are also listed and discussed analytically. The theoretical maximum amplitudes of the transformations achieved by these techniques are reported, together with the results of numerical simulations performed using custom-built simulation code.

Singlet order conversion and parahydrogen-induced hyperpolarization of 13C nuclei in near-equivalent spin systems

We have demonstrated two radiofrequency pulse methods which convert the nuclear singlet order of proton spin pairs into the magnetisation of nearby ¹³C nuclei. These irradiation schemes work well in the near-equivalence regime of the three-spin system, which applies when the difference in the two ¹H-¹³C couplings is much smaller than the ¹H-¹H coupling. We use pulse sequences to generate thermally polarized singlet states in a reproducible manner, and study the singlet-to-magnetisation transfer step. Preliminary results demonstrate a parahydrogen-enhanced ¹³C polarization level of at least 9%, providing a signal enhancement factor of more than 9000, using 50% enriched parahydrogen.

13 C MR Hyperpolarization of Lactate by Using ParaHydrogen and Metabolic Transformation in Vitro

Hyperpolarization of the ¹³ C magnetic resonance signal of l-[1-¹³ C]lactate has been obtained using the chemically based, cost-effective method called parahydrogen-induced polarization by means of side-arm hydrogenation (PHIP-SAH). Two ester derivatives of lactate were tested and the factors that determine the polarization level on the product have been investigated in detail. The metabolic conversion of hyperpolarized l-[1-¹³ C]lactate into pyruvate has been observed in vitro using lactate dehydrogenase (LDH) and in a cells lysate. From the acquisition of a series of ¹³ C NMR spectra, the metabolic build-up of the [1-¹³ C]pyruvate signal has been observed. These studies demonstrate that, even if the experimental set-up used for these PHIP-SAH hyperpolarization studies is still far from optimal, the attained polarization level is already sufficient to carry out in vitro metabolic studies.

Robust conversion of singlet spin order in coupled spin-1/2 pairs by adiabatically ramped RF-fields

We propose a robust and highly efficient NMR technique to create singlet spin order from longitudinal spin magnetization in coupled spin-½ pairs and to perform backward conversion (singlet order)→magnetization. In this method we exploit adiabatic ramping of an RF-field in order to drive transitions between the singlet state and the T_± triplet states of a spin pair under study. We demonstrate that the method works perfectly for both strongly and weakly coupled spin pairs, providing a conversion efficiency between the singlet spin order and magnetization, which is equal to the theoretical maximum. We anticipate that the proposed technique is useful for generating long-lived singlet order, for preserving spin hyperpolarization and for analyzing singlet spin order in nearly equivalent spin pairs in specially designed molecules and in low-field NMR studies.

SABRE Hyperpolarization of Bipyridine Stabilized Ir-Complex at High, Low and Ultralow Magnetic Fields

A strong limitation of nuclear magnetic resonance is its low inherent sensitivity that can be overcome by using an appropriate hyperpolarization technique. Presently, dynamic nuclear polarization and spin-exchange optical pumping are the only hyperpolarization techniques that are used in applied medicine. However, both are relatively complex in use and expensive. Here we present a modification of the signal amplification by reversible exchange (SABRE) hyperpolarization method – SABRE on stabilized Ir-complexes. A stabilized Ir-complex (here we used bipyridine for stabilization) can be hyperpolarized in a wide range of magnetic fields from a few μT upto 10 T with 15N polarization of about 1–3%. Moreover, the investigated complex can be incorporated into biomolecules or other bulky molecules; in this situation exchange with para-hydrogen will allow one to continuously generate hyperpolarization.