High-Resolution NMR Spectroscopy with a Portable Single-Sided Sensor

NMR Spectral Editing, Water Suppression, and Dipolar Decoupling in Low-Field NMR Spectroscopy Using Optimal Control Pulses and Multiple-Pulse Sequence

Spectral dispersion in low-field nuclear magnetic resonance (NMR) can significantly affect NMR spectral analysis, particularly when studying complex mixtures like metabolic profiling of biological samples. To address signal superposition in these spectra, we employed spectral editing with selective excitation pulses, proving it to be a suitable approach. Optimal control pulses were implemented in low-field NMR and demonstrated their capability to selectively excite and eliminate specific amino acids, such as phenylalanine and taurine, either individually or simultaneously. The broadening of NMR signals in viscous samples, like bio samples, due to homonuclear dipolar coupling often leads to loss of spectral details, impacting spectral assignments. Therefore, in this work, the multiple-pulse WAHUHA sequence at both high and low field NMR was employed resulting in approximately 63 and 25% reduction in line widths respectively, evident from line width changes in the NMR spectra. The effectiveness of this process was validated by comparing its performance with that of magic angle spinning NMR. Additionally, water suppression was achieved through selective excitation by adding a term representing the water signal to the overall Hamiltonian, expressing the water signal peak frequency, and covering adjacent frequencies on both sides of the water peak within the water signal.

Restore High-Resolution Nuclear Magnetic Resonance Spectra from Inhomogeneous Magnetic Fields Using a Neural Network

High-resolution nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool with wide applications. However, the conventional shim technique may not guarantee the homogeneity of the magnetic field when the experimental conditions are unfavorable. In this study, we proposed a data postprocessing method called Restore High-resolution Unet (RH-Unet), which uses a convolutional neural network to restore distorted NMR spectra that have been acquired in inhomogeneous magnetic fields. The method generates feature-label pairs from singlet peak regions and ideal Lorentzian line shapes and trains a RH-Unet model to map low-resolution spectra to high-resolution spectra. The method was applied to different samples and showed superior performance than the reference deconvolution method incorporated in Bruker Topspin software. The proposed method provides a simple and fast way to obtain high-resolution NMR spectra in inhomogeneous fields that can facilitate the application of NMR spectroscopy in various fields.

Inline NMR via a Dedicated V-Shaped Sensor

Process monitoring and control require dedicated and reliable measures which reflect the status of the process under investigation. Although nuclear magnetic resonance is known to be a versatile analytical technique, it is only seldomly found in process monitoring. Single-sided nuclear magnetic resonance is one well known approach for being applied in process monitoring. The dedicated V-sensor is a recent approach that allows the inline investigation of materials in a pipe non-destructively and non-invasively. An open geometry of the radiofrequency unit is realized using a tailored coil, enabling the sensor to be applied for manifold mobile applications in in-line process monitoring. Stationary liquids were measured, and their properties were integrally quantified as the basis for successful process monitoring. The sensor, in its inline version, is presented along with its characteristics. An exemplary field of application is battery production in terms of anode slurries; thus, the first results on graphite slurries will demonstrate the added value of the sensor in process monitoring.

When the MOUSE leaves the house

Change is inherent to time being transient. With the NMR-MOUSE (MObile Universal Surface Explorer) having matured into an established NMR tool for nondestructive testing of materials, this forward-looking retrospective assesses the challenges the NMR-MOUSE faced when deployed outside a protected laboratory and how its performance quality can be maintained and improved when operated under adverse conditions in foreign environments. This work is dedicated to my dear colleague and friend Geoffrey Bodenhausen on the occasion of his crossing an honorable timeline in appreciation of his ever-continuing success of fueling the dynamics of magnetic resonance.

Rapid parameter determination of discrete damped sinusoidal oscillations

We present different computational approaches for the rapid extraction of the signal parameters of discretely sampled damped sinusoidal signals. We compare time- and frequency-domain-based computational approaches in terms of their accuracy and precision and computational time required in estimating the frequencies of such signals, and observe a general trade-off between precision and speed. Our motivation is precise and rapid analysis of damped sinusoidal signals as these become relevant in view of the recent experimental developments in cavity-enhanced polarimetry and ellipsometry, where the relevant time scales and frequencies are typically within the ∼1 - 10 µs and ∼1 - 100 MHz ranges, respectively. In such experimental efforts, single-shot analysis with high accuracy and precision becomes important when developing experiments that study dynamical effects and/or when developing portable instrumentations. Our results suggest that online, running-fashion, microsecond-resolved analysis of polarimetric/ellipsometric measurements with fractional uncertainties at the 10^-6 levels, is possible, and using a proof-of-principle experimental demonstration we show that using a frequency-based analysis approach we can monitor and analyze signals at kHz rates and accurately detect signal changes at microsecond time-scales.

Microfluidic Overhauser DNP chip for signal-enhanced compact NMR

Nuclear magnetic resonance at low field strength is an insensitive spectroscopic technique, precluding portable applications with small sample volumes, such as needed for biomarker detection in body fluids. Here we report a compact double resonant chip stack system that implements in situ dynamic nuclear polarisation of a 130 nL sample volume, achieving signal enhancements of up to - 60 w.r.t. the thermal equilibrium level at a microwave power level of 0.5 W. This work overcomes instrumental barriers to the use of NMR detection for point-of-care applications.

Generation of acoustic-Brownian noise in nuclear magnetic resonance under non-equilibrium thermal fluctuations

We present an analytical study on generation of acoustic-Brownian noise in nuclear magnetic resonance (NMR) induced as a result of thermal fluctuations of the magnetic moments under non-equilibrium thermal interactions which has not been explored independent of Nyquist–Johnson noise until now. The mechanism of physical coupling between non-equilibrium thermal fluctuations and magnetic moments is illustrated using Lighthill’s formulation on suspension dynamics. We discover that unlike Nyquist–Johnson noise which has a uniform spectral density across a range of frequencies, the spectral dependence of acoustic-Brownian noise decreases with an increase in frequency and resembles Brownian noise associated with a particle in a potential well. The results have applications in the field of image enhancement algorithm as well as noise reduction instrumentation in NMR systems.

Low-cost and portable MRI

Research in MRI technology has traditionally expanded diagnostic benefit by developing acquisition techniques and instrumentation to enable MRI scanners to "see more." This typically focuses on improving MRI's sensitivity and spatiotemporal resolution, or expanding its range of biological contrasts and targets. In complement to the clear benefits achieved in this direction, extending the reach of MRI by reducing its cost, siting, and operational burdens also directly benefits healthcare by increasing the number of patients with access to MRI examinations and tilting its cost-benefit equation to allow more frequent and varied use. The introduction of low-cost, and/or truly portable scanners, could also enable new point-of-care and monitoring applications not feasible for today's scanners in centralized settings. While cost and accessibility have always been considered, we have seen tremendous advances in the speed and spatial-temporal capabilities of general-purpose MRI scanners and quantum leaps in patient comfort (such as magnet length and bore diameter), but only modest success in the reduction of cost and siting constraints. The introduction of specialty scanners (eg, extremity, brain-only, or breast-only scanners) have not been commercially successful enough to tilt the balance away from the prevailing model: a general-purpose scanner in a centralized healthcare location. Portable MRI scanners equivalent to their counterparts in ultrasound or even computed tomography have not emerged and MR monitoring devices exist only in research laboratories. Nonetheless, recent advances in hardware and computational technology as well as burgeoning markets for MRI in the developing world has created a resurgence of interest in the topic of low-cost and accessible MRI. This review examines the technical forces and trade-offs that might facilitate a large step forward in the push to "jail-break" MRI from its centralized location in healthcare and allow it to reach larger patient populations and achieve new uses. Level of Evidence: 5 Technical Efficacy Stage: 6 J. Magn. Reson. Imaging 2019. J. Magn. Reson. Imaging 2020;52:686-696.

Portable NMR with Parallelism

Portable NMR combining a permanent magnet and a complementary metal-oxide-semiconductor (CMOS) integrated circuit has recently emerged to offer the long desired online, on-demand, or in situ NMR analysis of small molecules for chemistry and biology. Here we take this cutting-edge technology to the next level by introducing parallelism to a state-of-the-art portable NMR platform to accelerate its experimental throughput, where NMR is notorious for inherently low throughput. With multiple (N) samples inside a single magnet, we perform simultaneous NMR analyses using a single silicon electronic chip, going beyond the traditional single-sample-per-magnet paradigm. We execute the parallel analyses via either time-interleaving or magnetic resonance imaging (MRI). In the time-interleaving method, the N samples occupy N separate NMR coils: we connect these N NMR coils to the single silicon chip one after another and repeat these sequential NMR scans. This time-interleaving is an effective parallelization, given a long recovery time of a single NMR scan. To demonstrate this time-interleaved parallelism, we use N = 2 for high-resolution multidimensional spectroscopy such as J-coupling resolved free induction decay spectroscopy and correlation spectroscopy (COSY) with the field homogeneity carefully optimized (<0.16 ppm) and N = 4 for multidimensional relaxometry such as diffusion-edited T₂ mapping and T₁-T₂ correlation mapping, expediting the throughput by 2-4 times. In the MRI technique, the N samples (N = 18 in our demonstration) share 1 NMR coil connected to the single silicon chip and are imaged all at once multiple times, which reveals the relaxation time of all N samples simultaneously. This imaging-based approach accelerates the relaxation time measurement by 4.5 times, and it could be by 18 times if the signal-to-noise were not limited. Overall, this work demonstrates the first portable high-resolution multidimensional NMR with throughput-accelerating parallelism.

MR to go

In this paper, we provide a review of the recent advances in miniaturizing nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectrometers for portable magnetic resonance (MR) applications. We focus the discussion on the application of integrated circuit technology for the miniaturization of the NMR and EPR spectrometer hardware and/or the detector and we will briefly touch on magnet technology. Finally, we will summarize current challenges of chip-integrated spectrometers and give an outlook on future applications of mobile MR spectrometers.

Benchtop NMR for the monitoring of bioprocesses

This mini-review highlights the potential of benchtop nuclear magnetic resonance (NMR) for the monitoring of bioprocesses. It describes recent perspectives opened by the reduced size of devices in relaxometry, magnetic resonance imaging and NMR spectroscopy. In particular, the recent emergence of the benchtop NMR spectroscopy gives access to many applications thanks to the implementation of advanced experiments.

Visualizing the detection area of a unilateral NMR sensor using deconvolution and back-projection

Understanding the detection volume of a unilateral NMR sensor is crucial to interpret acquired data appropriately. Whereas this is easily done in the sensor's axial dimension by running a depth profile on a well-defined sample, the lateral dimension is commonly characterized with very small samples, where each position along a regular grid is scanned individually, typically resulting in measurement times of several days and a resolution that is limited to the dimensions of the sample. Here we apply two mathematical procedures known from image processing that employ samples larger than the pixel size to characterize the lateral detection area. One procedure uses deconvolution algorithms to account for blurring effects caused by a larger sample while the other utilizes back-projection of radial field profiles. Both approaches are demonstrated with a Profile NMR-MOUSE® (PM5). They yield field maps in good agreement with those acquired with pixel-size test samples but save about one order of magnitude in scanning time.

Single-Scan High-Resolution 2-D $J$ -Resolved Spectroscopy in Inhomogeneous Magnetic Fields

OBJECTIVE

A method is proposed to obtain high-resolution 2-D -resolved nuclear magnetic resonance (NMR) spectra in inhomogeneous magnetic fields.

METHODS

The proposed experiment enables the acquisition of an entire 2-D spectrum in a single scan by utilizing intermolecular double-quantum coherences and the spatial encoding of NMR observables.

RESULTS

Chemical shifts, coupling constants, and multiplet patterns are recovered even when field inhomogeneities are severe enough to completely obscure conventional NMR spectra. After intentional deshimming to yield inhomogeneous magnetic fields, the method was demonstrated on ethyl 3-bromoproprionate in acetone and on a complex mixture of organic compounds. To illustrate the technique's applicability to biological samples with intrinsic magnetic field inhomogeneities arising from macroscopic magnetic susceptibility variations, we performed the experiment on a pig bone marrow sample.

CONCLUSION

Our results show that the new method is a fast and effective tool for studying complex chemical mixtures and biological tissues.

SIGNIFICANCE

The method could potentially be useful for real-time in vivo NMR studies.

Unilateral NMR with a barrel magnet

Unilateral NMR can examine samples without regard to sample size. It is also an easy path to mobile or portable NMR as well as inexpensive NMR. The objective of this work was to develop unilateral NMR with an improved performance in a sample region that was remote from the apparatus. This was accomplished with the creation of a saddle point where all second derivatives of the main component of the field were nulled. A ∼10cm diameter ∼5cm thick magnet combined with a gradiometer coil on the surface detected signals from a sensitive region that extended ∼2cm from the magnet. The relatively homogeneous field of these unilateral NMR devices allows the measurement of rapidly diffusing spins as well as the use of smaller RF amplifiers, which enhances system mobility.

A simple and low-cost permanent magnet system for NMR

We have developed a simple, easy to build, and low-cost magnet system for NMR, of which homogeneity is about 4×10^-4 at 57mT, with a pair of two commercially available ferrite magnets. This homogeneity corresponds to about 90Hz spectral resolution at 2.45MHz of the hydrogen Larmor frequency. The material cost of this NMR magnet system is little more than $100. The components can be printed by a 3D printer.

Ultrafast Multidimensional Laplace NMR Using a Single-Sided Magnet

Laplace NMR (LNMR) consists of relaxation and diffusion measurements providing detailed information about molecular motion and interaction. Here we demonstrate that ultrafast single- and multidimensional LNMR experiments, based on spatial encoding, are viable with low-field, single-sided magnets with an inhomogeneous magnetic field. This approach shortens the experiment time by one to two orders of magnitude relative to traditional experiments, and increases the sensitivity per unit time by a factor of three. The reduction of time required to collect multidimensional data opens significant prospects for mobile chemical analysis using NMR. Particularly tantalizing is future use of hyperpolarization to increase sensitivity by orders of magnitude, allowed by single-scan approach.

High-resolution NMR spectroscopy in inhomogeneous fields

High-resolution NMR spectroscopy, providing information on chemical shifts, J coupling constants, multiplet patterns, and relative peak areas, is a mainstream tool for analysis of molecular structures, conformations, compositions, and dynamics. Generally, a homogeneous magnetic field is a prerequisite for obtaining high-resolution NMR information. Magnetic field inhomogeneity, whether from non-ideal experimental conditions or from intrinsic magnetic susceptibility discontinuities in samples, represents a hurdle for applications of high-resolution NMR. Numerous techniques have been proposed for measuring high-resolution NMR spectra free from the influence of inhomogeneous magnetic fields. Besides developments and improvements in NMR instrumentation, various types of experimental approaches have been established for recovering NMR information in inhomogeneous magnetic fields. Three main types are systematically described in this review. In addition, other high-resolution NMR approaches or data processing methods are also briefly described. All high-resolution NMR approaches covered in this review have individual advantages and disadvantages in practical applications, and no one technique is applicable to all practical circumstances. Hence, they are complementary for high-resolution NMR applications in inhomogeneous fields. The underlying mechanisms of these approaches are presented, together with analyses of their applicability and efficiency.

Miniaturized, biopsy-implantable chemical sensor with wireless, magnetic resonance readout

Biopsy is an important diagnostic tool for a broad range of conditions. Cancer diagnoses, for example, are confirmed using tissue explanted with biopsy. Here we demonstrate a miniaturized wireless sensor that can be implanted during a biopsy procedure and return chemical information from within the body. Power and readout are wireless via weak magnetic resonant coupling to an external reader. The sensor is filled with responsive nuclear magnetic resonance (NMR) contrast agents for chemical sensitivity, and on-board circuitry constrains the NMR measurement to the contents. This sensor enables longitudinal monitoring of the same location, and its simple readout mechanism is ideal for applications not requiring the spatial information available through imaging techniques. We demonstrated the operation of this sensor by measuring two metabolic markers, both in vitro and in vivo: pH in flowing fluid for over 25 days and in a xenograft tumor model in mice, and oxygen in flowing gas and in a rat hind-limb constriction experiment. The results suggest that this in vivo sensing platform is generalizable to other available NMR contrast agents. These sensors have potential for use in biomedicine, environmental monitoring and quality control applications.

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.

NMR–DMF: a modular nuclear magnetic resonance–digital microfluidics system for biological assays

We present a modular nuclear magnetic resonance–digital microfluidics (NMR–DMF) system as a portable diagnostic platform for miniaturized biological assays.

Low-field permanent magnets for industrial process and quality control

In this review we focus on the technology associated with low-field NMR. We present the current state-of-the-art in low-field NMR hardware and experiments, considering general magnet designs, rf performance, data processing and interpretation. We provide guidance on obtaining the optimum results from these instruments, along with an introduction for those new to low-field NMR. The applications of lowfield NMR are now many and diverse. Furthermore, niche applications have spawned unique magnet designs to accommodate the extremes of operating environment or sample geometry. Trying to capture all the applications, methods, and hardware encompassed by low-field NMR would be a daunting task and likely of little interest to researchers or industrialists working in specific subject areas. Instead we discuss only a few applications to highlight uses of the hardware and experiments in an industrial environment. For details on more particular methods and applications, we provide citations to specialized review articles.

Magic-angle spinning solid-state multinuclear NMR on low-field instrumentation

Mobile and cost-effective NMR spectroscopy exploiting low-field permanent magnets is a field of tremendous development with obvious applications for arrayed large scale analysis, field work, and industrial screening. So far such demonstrations have concentrated on relaxation measurements and lately high-resolution liquid-state NMR applications. With high-resolution solid-state NMR spectroscopy being increasingly important in a broad variety of applications, we here introduce low-field magic-angle spinning (MAS) solid-state multinuclear NMR based on a commercial ACT 0.45 T 62 mm bore Halbach magnet along with a homebuilt FPGA digital NMR console, amplifiers, and a modified standard 45 mm wide MAS probe for 7 mm rotors. To illustrate the performance of the instrument and address cases where the low magnetic field may offer complementarity to high-field NMR experiments, we demonstrate applications for (23)Na MAS NMR with enhanced second-order quadrupolar coupling effects and (31)P MAS NMR where reduced influence from chemical shift anisotropy at low field may facilitate determination of heteronuclear dipole-dipole couplings.

Magnetic field shimming of a permanent magnet using a combination of pieces of permanent magnets and a single-channel shim coil for skeletal age assessment of children

We adopted a combination of pieces of permanent magnets and a single-channel (SC) shim coil to shim the magnetic field in a magnetic resonance imaging system dedicated for skeletal age assessment of children. The target magnet was a 0.3-T open and compact permanent magnet tailored to the hand imaging of young children. The homogeneity of the magnetic field was first improved by shimming using pieces of permanent magnets. The residual local inhomogeneity was then compensated for by shimming using the SC shim coil. The effectiveness of the shimming was measured by imaging the left hands of human subjects and evaluating the image quality. The magnetic resonance images for the child subject clearly visualized anatomical structures of all bones necessary for skeletal age assessment, demonstrating the usefulness of combined shimming.

Stacked planar micro coils for single-sided NMR applications

This paper describes planar micro structured coils fabricated in a novel multilayer assembly for single-sided NMR experiments. By arranging the coil's turns in both lateral and vertical directions, all relevant coil parameters can be tailored to a specific application. To this end, we implemented an optimization algorithm based on simulations applying finite element methods (FEMs), which maximizes the coil's sensitivity and thus signal-to-noise ratio (SNR) while incorporating boundary conditions such as the coil's electrical properties and a localized sensitivity needed for single-sided applications. Utilizing thin-film technology and microstructuring techniques, the planar character is kept by a sub-millimeter overall thickness. The coils are adapted to the Profile NMR-MOUSE® magnet with a homogeneous slice of about 200 μm in height and a uniform depth gradient of about 20T/m. The final design of a coil with 20 turns, separated in four layers with five turns each, and an outer dimension of 4×4 mm(2) is able to measure a sample volume almost five times smaller than that of a state-of-the-art 14×16 mm(2) Profile NMR-MOUSE® coil with the same SNR. This allows for volume-limited measurements with high SNR and enables different future developments. The minimal dead time of 4 μs facilitates further improvements of the SNR by echo adding techniques and the characterization of samples with short T2 relaxation times. Measurements on solid polymers like polyethylene (PE) and polypropylene (PP) with T2 components as short as 200 μs approve the overall beneficial coil properties. Furthermore the ability to perform depth profiling with microscopic resolution is demonstrated.

Spatially encoded ultrafast high-resolution 2D homonuclear correlation spectroscopy in inhomogeneous fields

Two-dimensional (2D) NMR spectroscopy shows strong vitality and unsubstitutable significance among NMR techniques. In many cases, however, it is virtually impossible to obtain high-resolution 2D spectra in inhomogeneous fields. Furthermore, conventional 2D NMR spectroscopy suffers from long acquisition time. In this paper, two new pulse sequences tracking the differences of the precession frequencies of two coupled spins are proposed to ultrafast achieve high-resolution 2D correlation spectroscopy (COSY and TOCSY) in inhomogeneous fields in a single scan. The spectral width of the indirect dimension can be reduced to improve the spectral resolution without loss of the correlative information for reconstructing the COSY and TOCSY spectra with mathematical manipulations. The theoretical analysis was given and experiments were performed to verify theoretical predictions. The results show that the proposed sequences can give correct 2D COSY and TOCSY spectra if the field inhomogeneity is linear along the orientation of encoding and decoding gradients.

Unmanned platform for long-range remote analysis of volatile compounds in air samples

This paper describes a long-range remotely controlled CE system built on an all-terrain vehicle. A four-stroke engine and a set of 12-V batteries were used to provide power to a series of subsystems that include drivers, communication, computers, and a capillary electrophoresis module. This dedicated instrument allows air sampling using a polypropylene porous tube, coupled to a flow system that transports the sample to the inlet of a fused-silica capillary. A hybrid approach was used for the construction of the analytical subsystem combining a conventional fused-silica capillary (used for separation) and a laser machined microfluidic block, made of PMMA. A solid-state cooling approach was also integrated in the CE module to enable controlling the temperature and therefore increasing the useful range of the robot. Although ultimately intended for detection of chemical warfare agents, the proposed system was used to analyze a series of volatile organic acids. As such, the system allowed the separation and detection of formic, acetic, and propionic acids with signal-to-noise ratios of 414, 150, and 115, respectively, after sampling by only 30 s and performing an electrokinetic injection during 2.0 s at 1.0 kV.

Ultrahigh-resolution magnetic resonance in inhomogeneous magnetic fields: two-dimensional long-lived-coherence correlation spectroscopy

A half-century quest for improving resolution in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) has enabled the study of molecular structures, biological interactions, and fine details of anatomy. This progress largely relied on the advent of sophisticated superconducting magnets that can provide stable and homogeneous fields with temporal and spatial variations below ΔB(0)/B(0)<0.01 ppm. In many cases however, inherent properties of the objects under investigation, pulsating arteries, breathing lungs, tissue-air interfaces, surgical implants, etc., lead to fluctuations and losses of local homogeneity. A new method dubbed "long-lived-coherence correlation spectroscopy" (LLC-COSY) opens the way to overcome both inhomogeneous and homogeneous broadening, which arise from local variations in static fields and fluctuating dipole-dipole interactions, respectively. LLC-COSY makes it possible to obtain ultrahigh resolution two-dimensional spectra, with linewidths on the order of Δν=0.1 to 1 Hz, even in very inhomogeneous fields (ΔB(0)/B(0)>10 ppm or 5000 Hz at 9.7 T), and can improve resolution by a factor up to 9 when the homogeneous linewidths are determined by dipole-dipole interactions. The resulting LLC-COSY spectra display chemical shift differences and scalar couplings in two orthogonal dimensions, like in "J spectroscopy." LLC-COSY does not require any sophisticated gradient switching or frequency-modulated pulses. Applications to in-cell NMR and to magnetic resonance spectroscopy (MRS) of selected volume elements in MRI appear promising, particularly when susceptibility variations tend to preclude high resolution.

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.

Low-gradient single-sided NMR sensor for one-shot profiling of human skin

This paper describes a shimming approach useful to reduce the gradient strength of the magnetic field generated by single-sided sensors simultaneously maximizing its uniformity along the lateral directions of the magnet. In this way, the thickness of the excited sensitive volume can be increased without compromising the depth resolution of the sensor. By implementing this method on a standard U-shaped magnet, the gradient strength was reduced one order of magnitude. In the presence of a gradient of about 2 T/m, slices of 2mm could be profiled with a resolution that ranges from 25 μm at the center of the slice to 50 μm at the borders. This sensor is of particular advantage for applications, where the scanning range is of the order of the excited slice. In those cases, the full profile is measured in a single excitation experiment, eliminating the need for repositioning the excited slice across the depth range to complete the profile as occurs with standard high gradient sensors. Besides simplifying the experimental setup, the possibility to move from a point-by-point measurement to the simultaneous acquisition of the full profile led to the shortening of the experimental time. A further advantage of performing the experiment under a smaller static gradient is a reduction of the diffusion attenuation affecting the signal decay measured with a CPMG sequence, making it possible to measure the T(2) of samples with high diffusivity (comparable to the water diffusivity). The performance of the sensor in terms of resolution and sensitivity is first evaluated and compared with conventional singled-sided sensors of higher gradient strength using phantoms of known geometry and relaxation times. Then, the device is used to profile the structure of human skin in vivo. To understand the contrast between the different skin layers, the distribution of relaxation times T(2) and diffusion coefficients is spatially resolved along the depth direction.

High-resolution absorptive intermolecular multiple-quantum coherence NMR spectroscopy under inhomogeneous fields

Intermolecular multiple-quantum coherence (iMQC) is capable of improving NMR spectral resolution using a 2D shearing manipulation method. A pulse sequence termed CT-iDH, which combines intermolecular double-quantum filter (iDQF) with a modified constant-time (CT) scheme, is designed to achieve fast acquisition of high-resolution intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) spectra without strong coupling artifacts. Furthermore, double-absorption lineshapes are first realized in 2D intermolecular multi-quantum coherences (iMQCs) spectra under inhomogeneous fields through a combination of iZQC and iDQC signals to double the resolution without loss of sensitivity. Theoretically the spectral linewidth can be further reduced by half compared to original iMQC high-resolution spectra. Several experiments were performed to test the feasibility of the new method and the improvements are evaluated quantitatively. The study suggests potential applications for in vivo spectroscopy.

High-resolution MR spectroscopy via intermolecular double-quantum coherences in inhomogeneous B0 and B1 fields

Inhomogeneity in static field B0 and/or RF field B1 is inevitable under some circumstances. In this work, a method based on intermolecular double-quantum coherences is employed for high-resolution 1D MR spectroscopy via 2D acquisition under such a condition. High-resolution information on chemical shifts, multiplet patterns, J coupling constants and relative peak areas can be retained in the resulting 1D projected spectra, as shown with results from a narrow-bore NMR spectrometer and a whole-body clinical scanner.

A portable Halbach magnet that can be opened and closed without force: the NMR-CUFF

Portable equipment for nuclear magnetic resonance (NMR) is becoming increasingly attractive for use in a variety of applications. One of the main scientific challenges in making NMR portable is the design of light-weight magnets that possess a strong and homogeneous field. Existing NMR magnets can provide such magnetic fields, but only for small samples or in small regions, or are rather heavy. Here we show a simple yet elegant concept for a Halbach-type permanent magnet ring, which can be opened and closed with minimal mechanical force. An analytical solution for an ideal Halbach magnet shows that the magnetic forces cancel if the structure is opened at an angle of 35.3° relative to its poles. A first prototype weighed only 3.1 kg, and provided a flux density of 0.57 T with a homogeneity better than 200 ppm over a spherical volume of 5mm in diameter without shimming. The force needed to close it was found to be about 20 N. As a demonstration, intact plants were imaged and water (xylem) flow measured. Magnets of this type (NMR-CUFF = Cut-open, Uniform, Force Free) are ideal for portable use and are eminently suited to investigate small or slender objects that are part of a larger or immobile whole, such as branches on a tree, growing fruit on a plant, or non-metallic tubing in industrial installations. This new concept in permanent-magnet design enables the construction of openable, yet strong and homogeneous magnets, which aside from use in NMR or MRI could also be of interest for applications in accelerators, motors, or magnetic bearings.

Application of optimal control to CPMG refocusing pulse design

We apply optimal control theory (OCT) to the design of refocusing pulses suitable for the CPMG sequence that are robust over a wide range of B(0) and B(1) offsets. We also introduce a model, based on recent progress in the analysis of unitary dynamics in the field of quantum information processing (QIP), that describes the multiple refocusing dynamics of the CPMG sequence as a dephasing Pauli channel. This model provides a compact characterization of the consequences and severity of residual pulse errors. We illustrate the methods by considering a specific example of designing and analyzing broadband OCT refocusing pulses of length 10t(180) that are constrained by the maximum instantaneous pulse power. We show that with this refocusing pulse, the CPMG sequence can refocus over 98% of magnetization for resonance offsets up to 3.2 times the maximum RF amplitude, even in the presence of ±10% RF inhomogeneity.

Permanent magnet assembly producing a strong tilted homogeneous magnetic field: towards magic angle field spinning NMR and MRI

We introduce a cylindrical permanent magnet design that generates a homogeneous and strong magnetic field having an arbitrary inclination with respect to the axis of the cylinder. The analytical theory of 3 D magnetostatics has been applied to this problem, and a hybrid magnet structure has been designed. This structure contains two magnets producing a longitudinal and transverse component for the magnetic field, whose amplitudes and homogeneities can be fully controlled by design. A simple prototype has been constructed using inexpensive small cube magnets, and its magnetic field has been mapped using Hall and NMR probe sensors. This magnet can, in principle, be used for magic angle field spinning NMR and MRI experiments allowing for metabolic chemical shift profiling in small living animals.