Non-linear liquid-state NMR

Radiation damping at clinical field strength: Characterization and compensation in quantitative measurements

PURPOSE

In any MR experiment, the bulk magnetization acts on itself, caused by the induced current in the RF receiver circuit that generates an oscillating damping field. This effect, known as "radiation damping" (RD), is usually weak and, therefore, unconsidered in MRI, but can affect quantitative studies performed with dedicated coils that provide a high SNR. The current work examined RD in a setup for investigations of small tissue specimens including a quantitative characterization of the spin-coil system.

THEORY AND METHODS

A custom-made Helmholtz coil (radius and spacing 16 mm) was interfaced to a transmit-receive (Tx/Rx) switch with integrated passive feedback for modulation or suppression of RD similar to preamplifier decoupling. Pulse sequences included pulse-width arrays to demonstrate the absence/ presence of RD and difference techniques employing gradient pulses or composite RF pulses to quantify RD effects during free precession and transmission, respectively. Experiments were performed at 3T in small samples of MnCl2 solution.

RESULTS

Significant RD effects may impact RF pulse application and evolution periods. Effective damping time constants were comparable to typical T2 * times or echo spacings in multi-echo sequences. Measurements of the phase relation showed that deviations from the commonly assumed 90° angle between the damping field and the transverse magnetization may occur.

CONCLUSION

Radiation damping may affect the accuracy of quantitative MR measurements performed with dedicated RF coils. Efficient mitigation can be achieved hardware-based or by appropriate consideration in the pulse sequence.

Various facets of intermolecular transfer of phase coherence by nuclear dipolar fields

It has long been recognized that dipolar fields can mediate intermolecular transfer of phase coherence from abundant solvent to sparse solute spins. Generally, the dipolar field has been considered while acting during prolonged free-precession delays. Recently, we have shown that transfer can also occur during suitable uninterrupted radio frequency pulse trains that are used for total correlation spectroscopy. Here, we will expand upon the latter work. First, analytical expressions for the evolution of the solvent magnetization under continuous irradiation and the influence of the dipolar field are derived. These expressions facilitate the simulations of the transfer process. Then, a pulse sequence for the retrieval of high-resolution spectra in inhomogeneous magnetic fields is described, along with another sequence to detect a transfer from an intermolecular double-quantum coherence. Finally, various schemes are discussed where the magnetization is modulated by a combination of multiple selective radio frequency pulses and pulsed field gradients in different directions. In these schemes, the magnetization is manipulated in such a way that the dipolar field, which originates from a single-spin species, can be decomposed into two components. Each component originates from a part of the magnetization that is modulated in a different direction. Both can independently, but simultaneously, mediate an intermolecular transfer of phase coherence.

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.

Spin noise gradient echoes

Nuclear spin noise spectroscopy in the absence of radio frequency pulses was studied under the influence of pulsed field gradients (PFGs) on pure and mixed liquids. Under conditions where the radiation-damping-induced line broadening is smaller than the gradient-dependent inhomogeneous broadening, echo responses can be observed in difference spectra between experiments employing pulsed field gradient pairs of the same and opposite signs. These observed spin noise gradient echoes (SNGEs) were analyzed through a simple model to describe the effects of transient phenomena. Experiments performed on high-resolution nuclear magnetic resonance (NMR) probes demonstrate how refocused spin noise behaves and how it can be exploited to determine sample properties. In bulk liquids and their mixtures, transverse relaxation times and translational diffusion constants can be determined from SNGE spectra recorded following tailored sequences of magnetic field gradient pulses.

An Optimized NMR Stripline for Sensitive Supercritical Fluid Chromatography-Nuclear Magnetic Resonance of Microliter Sample Volumes

To optimize sensitivity, there has been an increasing interest in the miniaturization of NMR detectors. In our lab, a stripline NMR detector has been developed, which provides high resolution and is scalable to a large range of sample volumes. These features make it an ideal detector for hyphenated techniques. In this manuscript, we demonstrate a stripline probe, which is designed for combining supercritical fluid chromatography (SFC) experiments with NMR. It features a novel stripline chip, designed to reduce the signal from the contact pads, which results in an improved lineshape. An external lock circuit provides stability over time to perform signal averaging or multidimensional experiments. As proof of concept, we demonstrate the SFC-NMR technique with this stripline probe using a mixture of cholesterol and cholestanol, which is relevant for studying cerebrotendinous xanthomatosis. Additionally, this probe makes it possible to record high-resolution spectra of samples with a high spin density. This means that it is possible to directly observe shifts due to the nuclear demagnetizing field in the “homomolecular” case, which is challenging using conventional probes due to broadening effects from radiation damping.

Continuous Radio Amplification by Stimulated Emission of Radiation using Parahydrogen Induced Polarization (PHIP-RASER) at 14 Tesla

Nuclear Magnetic Resonance (NMR) is an intriguing quantum-mechanical effect that is used for routine medical diagnostics and chemical analysis alike. Numerous advancements have contributed to the success of the technique, including hyperpolarized contrast agents that enable real-time imaging of metabolism in vivo. Herein, we report the finding of an NMR radio amplification by stimulated emission of radiation (RASER), which continuously emits 1 H NMR signal for more than 10 min. Using parahydrogen induced hyperpolarization (PHIP) with 50 % para-hydrogen, we demonstrated the effect at 600 MHz but expect that it is functional across a wide range of frequencies, e.g. 101 -103  MHz. PHIP-RASER occurs spontaneously or can be triggered with a standard NMR excitation. Full chemical shift resolution was maintained, and a linewidth of 0.6 ppb was achieved. The effect was reproduced by simulations using a weakly coupled, two spin- 1 / 2 system. All devices used were standard issue, such that the effect can be reproduced by any NMR lab worldwide with access to liquid nitrogen for producing parahydrogen.

Signal feedback applications in low-field NMR and MRI

Tuned pick-up coils with high quality factors Q are used in NMR and MRI for high-sensitivity and low-noise detection. However, large Q-factors introduce bandwidth issues at low frequency and the associated enhanced currents may cause significant radiation damping effects, especially with hyperpolarised samples. Signal feedback can be used to actively control these currents and adjust the detection bandwidth without resistive losses. Capacitive and inductive coupling methods are compared using detailed models and the operating conditions for efficient feedback with negligible noise penalty are discussed. Several high-impedance commercial preamplifiers have been found to affect the resonance characteristics of tuned coils in a gain-dependent way, or could not be used in low-frequency NMR because of oscillations at large positive gain. This is attributed to an undocumented internal feedback, and could be neutralised using external feedback. The implementation of an inductive coupling scheme to feed a suitably amplified phase-adjusted signal back into the PU coils of low-field NMR systems is described, and three experimental applications are reported. One system is used for NMR studies of distant dipolar field effects in highly polarised liquid ³He without or with radiation damping. The moderate intrinsic Q-factor (≈7) could be reduced (down to 1) or increased (up to 100) to control transient maser oscillations. Another system was used for MRI of water samples around 2 mT with Q≈190 litz-wire detection coils. The detection bandwidth was increased by actively reducing the Q-factor to obtain uniform sensitivities in images and avoid artifacts introduced by intensity corrections. Finally, parallel acquisition in MRI was performed using two separately tuned detection coils placed above and below the sample. They were actively decoupled using two feedback systems. For an imaging field of view smaller than the sample, artifact-free unfolded images demonstrate the efficiency of this active coil decoupling scheme.

From LASER physics to the para-hydrogen pumped RASER

The properties of the LASER with respect to self-organization are compared with the key features of the p-H₂ pumped RASER. According to LASER theory the equations of motion for the LASER can be derived from the enslaving principle, i.e. the slowest-changing order parameter (the light field in the resonator) enslaves the rapidly relaxing atomic degrees of freedom. Likewise, it is shown here that the equations of motion for the p-H₂ pumped RASER result from a set of order parameters, where the transverse magnetization of the RASER-active spin states enslaves the electromagnetic modes. The consequences are striking for nuclear magnetic resonance (NMR) spectroscopy, since long-lasting multi-mode RASER oscillations enable unprecedented spectroscopic resolution down to the micro-Hertz regime. Based on the theory for multi-mode RASER operation we analyze the conditions that reveal either the collapse of the entire NMR spectrum, the occurrence of self-organized frequency-combs, or RASER spectra which reflect the J-coupled network of the molecule. Certain RASER experiments involving the protons of ¹⁵N pyridine or 3-picoline molecules pumped with p-H₂ via SABRE (Signal Amplification By Reversible Exchange) show either a single RASER oscillation in the time domain, giant RASER pulses or a complex RASER beat pattern. The corresponding ¹H spectra consist of one narrow line, equidistant narrow lines (frequency-comb), or highly resolved lines reporting NMR properties, respectively. Numerous applications in the areas of material sciences, fundamental physics and medicine involving high precision sensors for magnetic fields, rotational motions or molecular structures become feasible.

Enhancement of the Nuclear Spin Noise Signal Using Wavelet Transform

Spin noise spectroscopy has attracted considerable attention recently owing partly to intrinsic interest in the phenomenon and partly to its significant application potential. Here, we address the inherent problem of low sensitivity of nuclear spin noise and examine the utility of wavelet transform to mitigate this problem by distinguishing real peaks from the noise contaminated data. Suppression of the random circuit noise and the consequent enhancement of the correlated nuclear spin noise signal have been demonstrated with discrete wavelet transform. Spectra of both ¹ H and ¹³ C nuclear spins have been considered and significant signal enhancements in both the cases have been observed. A detailed analysis of several possible wavelet, thresholding and decomposition solutions have been made to obtain the optimum condition for signal enhancement. It is observed that the application of wavelet transform leaves the spin noise signal line shape essentially unchanged, which is an advantage for several applications involving spin noise spectra.

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.

Optically polarized 3He

This article reviews the physics and technology of producing large quantities of highly spin-polarized 3He nuclei using spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping. Both technical developments and deeper understanding of the physical processes involved have led to substantial improvements in the capabilities of both methods. For SEOP, the use of spectrally narrowed lasers and K-Rb mixtures has substantially increased the achievable polarization and polarizing rate. For MEOP nearly lossless compression allows for rapid production of polarized 3He and operation in high magnetic fields has likewise significantly increased the pressure at which this method can be performed, and revealed new phenomena. Both methods have benefitted from development of storage methods that allow for spin-relaxation times of hundreds of hours, and specialized precision methods for polarimetry. SEOP and MEOP are now widely applied for spin-polarized targets, neutron spin filters, magnetic resonance imaging, and precision measurements.

Accelerating two-dimensional nuclear magnetic resonance correlation spectroscopy via selective coherence transfer

Nuclear magnetic resonance (NMR) spectroscopy serves as an important tool for both qualitative and quantitative analyses of various systems in chemistry, biology, and medicine. However, applications of one-dimensional ¹H NMR are often restrained by the presence of severe overlap among different resonances. The advent of two-dimensional (2D) ¹H NMR constitutes a promising alternative by extending the crowded resonances into a plane and thereby alleviating the spectral congestions. However, the enhanced ability in discriminating resonances is achieved at the cost of extended experimental duration due to necessity of various scans with progressive delays to construct the indirect dimension. Therefore, in this study, we propose a selective coherence transfer (SECOT) method to accelerate acquisitions of 2D correlation spectroscopy by converting chemical shifts into spatial positions within the effective sample length and then performing an echo planar spectroscopic imaging module to record the spatial and spectral information, which generates 2D correlation spectrum after 2D Fourier transformation. The feasibility and effectiveness of SECOT have been verified by a set of experiments under both homogeneous and inhomogeneous magnetic fields. Moreover, evaluations of SECOT for quantitative analyses are carried out on samples with a series of different concentrations. Based on these experimental results, the SECOT may open important perspectives for fast, accurate, and stable investigations of various chemical systems both qualitatively and quantitatively.

Nonlinear detection of secondary isotopic chemical shifts in NMR through spin noise

The detection of minor species in the presence of large amounts of similar main components remains a key challenge in analytical chemistry, for instance, to obtain isotopic fingerprints. As an alternative to the classical NMR scheme based on coherent excitation and detection, here we introduce an approach based on spin-noise detection. Chemical shifts and transverse relaxation rates are determined using only the detection circuit. Thanks to a nonlinear effect in mixtures with small chemical shift dispersion, small signals on top of a larger one can be observed with increased sensitivity as bumps on a dip; the latter being the signature of the main magnetization. Experimental observations are underpinned by an analytical theory: the coupling between the magnetization and the coil provides an amplified detection capability of both small static magnetic field inhomogeneities and small NMR signals. This is illustrated by two-bond ¹²C/¹³C isotopic measurements.

Nuclear spin noise allows passive monitoring of magnetization using the sole NMR detection circuit. Here, the authors report spectroscopic signatures of low abundance molecules and of weak magnetic field gradients which are nonlinearly amplified by cooled-coil probes and large overlapping signals.

Nuclear spin noise in NMR revisited

The theoretical shapes of nuclear spin-noise spectra in NMR are derived by considering a receiver circuit with finite preamplifier input impedance and a transmission line between the preamplifier and the probe. Using this model, it becomes possible to reproduce all observed experimental features: variation of the NMR resonance linewidth as a function of the transmission line phase, nuclear spin-noise signals appearing as a "bump" or as a "dip" superimposed on the average electronic noise level even for a spin system and probe at the same temperature, pure in-phase Lorentzian spin-noise signals exhibiting non-vanishing frequency shifts. Extensive comparisons to experimental measurements validate the model predictions, and define the conditions for obtaining pure in-phase Lorentzian-shape nuclear spin noise with a vanishing frequency shift, in other words, the conditions for simultaneously obtaining the spin-noise and frequency-shift tuning optima.

On the tuning of high-resolution NMR probes

Three optimum conditions for the tuning of NMR probes are compared: the conventional tuning optimum, which is based on radio-frequency pulse efficiency, the spin noise tuning optimum based on the line shape of the spin noise signal, and the newly introduced frequency shift tuning optimum, which minimizes the frequency pushing effect on strong signals. The latter results if the radiation damping feedback field is not in perfect quadrature to the precessing magnetization. According to the conventional RLC (resistor-inductor-capacitor) resonant circuit model, the optima should be identical, but significant deviations are found experimentally at low temperatures, in particular on cryogenically cooled probes. The existence of different optima with respect to frequency pushing and spin noise line shape has important consequences on the nonlinearity of spin dynamics at high polarization levels and the implementation of experiments on cold probes.

High-resolution heteronuclear multi-dimensional NMR spectroscopy in magnetic fields with unknown spatial variations

Heteronuclear NMR spectroscopy is an extremely powerful tool for determining the structures of organic molecules and is of particular significance in the structural analysis of proteins. In order to leverage the method's potential for structural investigations, obtaining high-resolution NMR spectra is essential and this is generally accomplished by using very homogeneous magnetic fields. However, there are several situations where magnetic field distortions and thus line broadening is unavoidable, for example, the samples under investigation may be inherently heterogeneous, and the magnet's homogeneity may be poor. This line broadening can hinder resonance assignment or even render it impossible. We put forth a new class of pulse sequences for obtaining high-resolution heteronuclear spectra in magnetic fields with unknown spatial variations based on distant dipolar field modulations. This strategy's capabilities are demonstrated with the acquisition of high-resolution 2D gHSQC and gHMBC spectra. These sequences' performances are evaluated on the basis of their sensitivities and acquisition efficiencies. Moreover, we show that by encoding and decoding NMR observables spatially, as is done in ultrafast NMR, an extra dimension containing J-coupling information can be obtained without increasing the time necessary to acquire a heteronuclear correlation spectrum. Since the new sequences relax magnetic field homogeneity constraints imposed upon high-resolution NMR, they may be applied in portable NMR sensors and studies of heterogeneous chemical and biological materials.

Dependence of NMR noise line shapes on tuning, matching, and transmission line properties

The tuning and matching conditions of rf circuits, as well as the properties of the transmission lines connecting these to the preamplifier, have direct consequences for NMR probe sensitivity and as for the optimum delivery of rf power to the sample. In addition, tuning/matching conditions influence radiation damping effects, which manifest themselves as fast signal flip-back and line broadening effects, and can lead to concentration-dependent frequency shifts. Previous studies have also shown that the appearance of spin-noise and absorbed circuit noise signals heavily depended on tuning settings. Consequently, all these phenomena are linked together. The mutual connections and interdependences of these effects are highlighted and reviewed here.

Collective effects due to dipolar fields as the origin of the extremely random behavior in hyperpolarized NMR maser: a theoretical and numerical study

Numerical simulations based on microscopic approach are used to explore the spin dynamics encountered in the recently reported hyperpolarized (129)Xe NMR maser [D. J. Y. Marion, G. Huber, P. Berthault, and H. Desvaux, ChemPhysChem 9, 1395-1401 (2008)] where series of amplitude modulated rf emissions are observed. The integration of the dynamic features of the electronic detection circuit in the present simulations, based on non-linear Maxwell-Bloch differential equations with dipole-dipole interactions, allows us to prove that the experimentally observed extremely random amplitude modulations crucially require the long-distance dipolar couplings between the nuclear spins with the feedback field acting as an amplifier. The massive dipolar couplings act, when the magnetization is largely tilted off the longitudinal axis, as an apparent transverse self-relaxation mechanism which destroys coherence. This, in particular, explains why the final magnetization after emissions can still be opposite to the magnetic field direction, i.e., being in an unstable state.