Radiofrequency microcoils for magnetic resonance imaging and spectroscopy

A roadmap to high-resolution standard microcoil MAS NMR spectroscopy for metabolomics

Current microcoil probe technology has emerged as a significant advancement in NMR applications to biofluids research. It has continued to excel as a hyphenated tool with other prominent microdevices, opening many new possibilities in multiple omics fields. However, this does not hold for biological samples such as intact tissue or organisms, due to the considerable challenges of incorporating the microcoil in a magic-angle spinning (MAS) probe without relinquishing the high-resolution spectral data. Not until 2012 did a microcoil MAS probe show promise in profiling the metabolome in a submilligram tissue biopsy with spectral resolution on par with conventional high-resolution MAS (HR-MAS) NMR. This result subsequently triggered a great interest in the possibility of NMR analysis with microgram tissues and striving toward the probe development of "high-resolution" capable microcoil MAS NMR spectroscopy. This review gives an overview of the issues and challenges in the probe development and summarizes the advancements toward metabolomics.

NMR spectroscopy of a single mammalian early stage embryo

The resolving power, chemical sensitivity and non-invasive nature of NMR have made it an established technique for in vivo studies of large organisms both for research and clinical applications. NMR would clearly be beneficial for analysis of entities at the microscopic scale of about 1 nL (the nanoliter scale), typical of early development of mammalian embryos, microtissues and organoids: the scale where the building blocks of complex organisms could be observed. However, the handling of such small samples (about 100 µm) and sensitivity issues have prevented a widespread adoption of NMR. In this article we show how these limitations can be overcome to obtain NMR spectra of a mammalian embryo in its early stage. To achieve this we employ ultra-compact micro-chip technologies in combination with 3D-printed micro-structures. Such device is packaged for use as plug & play sensor and it shows sufficient sensitivity to resolve NMR signals from individual bovine pre-implantation embryos. The embryos in this study are obtained through In Vitro Fertilization (IVF) techniques, transported cryopreserved to the NMR laboratory, and measured shortly after thawing. In less than 1 h these spherical samples of just 130-190 µm produce distinct spectral peaks, largely originating from lipids contained inside them. We further observe how the spectra vary from one sample to another despite their optical and morphological similarities, suggesting that the method can further develop into a non-invasive embryo assay for selection prior to embryo transfer.

Proton-phosphorous connectivities revealed by high-resolution proton-detected solid-state NMR

Proton-detected solid-state NMR enables atomic-level insight in solid-state reactions, for instance in heterogeneous catalysis, which is fundamental for deciphering chemical reaction mechanisms. We herein introduce a phosphorus-31 radiofrequency channel in...

The normalized limit of detection in NMR spectroscopy

We derive the normalized limit of detection for frequency space (nLOD_f) as a parameter to measure the sensitivity of an NMR spectroscopy setup. nLOD_f is independent of measurement settings such as bandwidth or number of measurement points, and allows to compare performances of different setups. We demonstrate the usefulness of the new nLOD_f by comparing the sensitivity of NMR setups from various publications, which all use microcoils. Finally, we want to propose a standard measurement and report format for the sensitivity of new NMR setups.

Proton MRS on sub-microliter volume in rat brain using implantable NMR microcoils

The use of miniaturized NMR receiver coils is an effective approach for improving detection sensitivity in studies using MRS and MRI. By optimizing the filling factor (the fraction of the coil occupied by the sample), and by increasing the RF magnetic field produced per unit current, the sensitivity gain offered by NMR microcoils is particularly interesting when small volumes or regions of interest are investigated. For in vivo studies, millimetric or sub-millimetric microcoils can be deployed in tissues to access regions of interest located at a certain depth. In this study, the implementation and application of a tissue-implantable NMR microcoil with a detection volume of 850 nL is described. The RF magnetic field generated by the microcoil was evaluated using a finite element method simulation and experimentally determined by high spatial resolution MRI acquisitions. The performance of the microcoil in terms of spectral resolution and limit of detection was measured at 7 T in vitro and in vivo in rodent brains. These performances were compared with those of a conventional external detection coil. Proton MR spectra were acquired in the cortex of rat brain. The concentrations of main metabolites were quantified and compared with reference values from the literature. In vitro and in vivo results obtained with the implantable microcoil showed a gain in sensitivity greater than 50 compared with detection using an external coil. In vivo proton spectra of diagnostic value were obtained from brain regions of a few hundred nanoliters. The similarities between spectra obtained with the implanted microcoil and those obtained with the external NMR coil highlight the minimally invasive nature of the coil implantation procedure. These implantable microcoils represent new tools for probing tissue metabolism in very small healthy or diseased regions using MRS.

Untuned broadband spiral micro-coils achieve sensitive multi-nuclear NMR TX/RX from microfluidic samples

The low frequency plateau in the frequency response of an untuned micro-resonator permits broadband radio-frequency reception, albeit at the expense of optimal signal-to-noise ratio for a particular nucleus. In this contribution we determine useful figures of merit for broadband micro-coils, and thereby explore the parametric design space towards acceptable simultaneous excitation and reception of a microfluidic sample over a wide frequency band ranging from ¹³C to ¹H, i.e., 125–500 MHz in an 11.74 T magnet. The detector achieves 37% of the performance of a comparably sized, tuned and matched resonator, and a linewidth of 17 ppb using standard magnet shims. The use of broadband detectors circumvents numerous difficulties introduced by multi-resonant RF detector circuits, including sample loading effects on matching, channel isolation, and field distortion.

Implantable NMR Microcoils in Rats: A New Tool for Exploring Tumor Metabolism at Sub-Microliter Scale?

The aim of this study was to evaluate the potential of a miniaturized implantable nuclear magnetic resonance (NMR) coil to acquire in vivo proton NMR spectra in sub-microliter regions of interest and to obtain metabolic information using magnetic resonance spectroscopy (MRS) in these small volumes. For this purpose, the NMR microcoils were implanted in the right cortex of healthy rats and in C6 glioma-bearing rats. The dimensions of the microcoil were 450 micrometers wide and 3 mm long. The MRS acquisitions were performed at 7 Tesla using volume coil for RF excitation and microcoil for signal reception. The detection volume of the microcoil was measured equal to 450 nL. A gain in sensitivity equal to 76 was found in favor of implanted microcoil as compared to external surface coil. Nine resonances from metabolites were assigned in the spectra acquired in healthy rats (n = 5) and in glioma-bearing rat (n = 1). The differences in relative amplitude of choline, lactate and creatine resonances observed in glioma-bearing animal were in agreement with published findings on this tumor model. In conclusion, the designed implantable microcoil is suitable for in vivo MRS and can be used for probing the metabolism in localized and very small regions of interest in a tumor.

Magnetic resonance microscopy of samples with translational symmetry with FOVs smaller than sample size

In MRI, usually the Field of View (FOV) has to cover the entire object. If this condition is not fulfilled, an infolding image artifact is observed, which suppresses visualization. In this study it is shown that for samples with translational symmetry, i.e., those consisting of identical objects in periodic unit cells, the FOV can be reduced to match the unit cell which enables imaging of an average object, of which the signal is originated from all unit cells of the sample, with no punishment by a loss in signal-to-noise ratio (SNR). This theoretical prediction was confirmed by experiments on a test sample with a 7 × 7 mm² unit cell arranged in a 3 × 3 matrix which was scanned by the spin-echo and by single point imaging methods. Effects of experimental imperfections in size and orientation mismatch between FOV and unit cell were studied as well. Finally, this method was demonstrated on a 3D periodic sample of tablets, which yielded well-resolved images of moisture distribution in an average tablet, while single tablet imaging provided no results. The method can be applied for SNR increase in imaging of any objects with inherently low signals provided they can be arranged in a periodic structure.

Broadband radio-frequency transmitter for fast nuclear spin control

The active manipulation of nuclear spins with radio-frequency (RF) coils is at the heart of nuclear magnetic resonance (NMR) spectroscopy and spin-based quantum devices. Here, we present a miniature RF transmitter designed to generate strong RF pulses over a broad bandwidth, allowing for fast spin rotations on arbitrary nuclear species. Our design incorporates (i) a planar multilayer geometry that generates a large field of 4.35 mT per unit current, (ii) a 50 Ω transmission circuit with a broad excitation bandwidth of ∼20 MHz, and (iii) an optimized thermal management leading to minimal heating at the sample location. Using individual ¹³C nuclear spins in the vicinity of a diamond nitrogen-vacancy center as a test system, we demonstrate Rabi frequencies exceeding 70 kHz and nuclear π/2 rotations within 3.4 μs. The extrapolated values for ¹H spins are about 240 kHz and 1 μs, respectively. Beyond enabling fast nuclear spin manipulations, our transmitter system is ideally suited for the incorporation of advanced pulse sequences into micro- and nanoscale NMR detectors operating at a low (<1 T) magnetic field.

Multi-parameter Analytical Method for B1 and SNR Analysis (MAMBA): An open source RF coil design tool

In Magnetic Resonance Imaging (MRI), radio frequency (RF) coils of different forms and shapes are used to maximize signal-to-noise ratio (SNR). RF coils are designed for clinical applications and have dimensions comparable with the target body part to be imaged, and they perform best when loaded by human tissue majority of which have conductivity values higher than 0.5 S/m. However, they are not properly tuned and matched for samples having low conductivity such as solid samples with low water content. Moreover, for samples with low filling factor and low conductivity, the noise in MRI is dominated by RF coil losses. In this case, RF coil design can be optimized to improve image SNR. Here, a new software tool (Multi-parameter Analytical Method for B1 and SNR Analysis) MAMBA is presented to design and compare volume coils of birdcage, solenoid, and loop-gap design for these samples. The input parameters of the tool are the sample properties, the coil design and the hardware properties, of which a relative SNR is determined. For that, a figure of merit is calculated from the coil sensitivity, applied resonant frequency and the resistive losses of sample, coil and capacitive components. The tool was tested in an ancient Egyptian mummy head which represents an extreme case of MRI with short T2*. Two optimized birdcage coils were designed using MAMBA, constructed and compared to a commercial transmit receive head coil. Calculated relative SNR values are in good agreement with the measurements.

An NMR-compatible microfluidic platform enabling in situ electrochemistry

Combining microfluidic devices with nuclear magnetic resonance (NMR) has the potential of unlocking their vast sample handling and processing operation space for use with the powerful analytics provided by NMR. One particularly challenging class of integrated functional elements from the perspective of NMR are conductive structures. Metallic electrodes could be used for electrochemical sample interaction for example, yet they can cause severe NMR spectral and SNR degradation. These issues are more entangled at the micro-scale since the distorted volume occupies a higher ratio of the sample volume. In this study, a combination of simulation and experimental validation was used to identify an electrode geometry that, in terms of NMR spectral parameters, performs as well as for the case when no electrodes are present. By placing the metal tracks in the side-walls of a microfluidic channel, we found that NMR RF excitation performance was actually enhanced, without compromising B0 homogeneity. Monitoring in situ deposition of chitosan in the microfluidic platform is presented as a proof-of-concept demonstration of NMR characterisation of an electrochemical process.

Broadband and multi-resonant sensors for NMR

It has always been of considerable interest to study the nuclear magnetic resonance response of multiple nuclei simultaneously, whether these signals arise from internuclear couplings within the same molecule, or from uncoupled nuclei within sample mixtures. The literature contains numerous uncorrelated reports on techniques employed to achieve multi-nuclear NMR detection. This paper consolidates the subset of techniques in which single coil detectors are utilized, and highlights the strengths and weaknesses of each approach, at the same time pointing the way towards future developments in the field of multi-nuclear NMR. We compare the different multi-nuclear NMR techniques in terms of performance, and present a guide to NMR probe designers towards application-based optimum design. We also review the applicability of micro-coils in the context of multi-nuclear methods. Micro-coils benefit from compact geometries and exhibit lower impedance, which provide new opportunities and challenges for the NMR probe designer.

Scalable dissolution-dynamic nuclear polarization with rapid transfer of a polarized solid

In dissolution-dynamic nuclear polarization, nuclear spins are hyperpolarized at cryogenic temperatures using radicals and microwave irradiation. The hyperpolarized solid is dissolved with hot solvent and the solution is transferred to a secondary magnet where strongly enhanced magnetic resonance signals are observed. Here we present a method for transferring the hyperpolarized solid. A bullet containing the frozen, hyperpolarized sample is ejected using pressurized helium gas, and shot into a receiving structure in the secondary magnet, where the bullet is retained and the polarized solid is dissolved rapidly. The transfer takes approximately 70 ms. A solenoid, wound along the entire transfer path ensures adiabatic transfer and limits radical-induced low-field relaxation. The method is fast and scalable towards small volumes suitable for high-resolution nuclear magnetic resonance spectroscopy while maintaining high concentrations of the target molecule. Polarization levels of approximately 30% have been observed for 1-¹³C-labelled pyruvic acid in solution.

Dissolution-dynamic nuclear polarization is able to enhance nuclear magnetic resonance signals, but requires complex procedures to generate hyperpolarized nuclear spins. Here the authors establish a fast and facile method to transfer hyperpolarized samples into the liquid solution where the measurement is performed.

RF coils: A practical guide for nonphysicists

Radiofrequency (RF) coils are an essential MRI hardware component. They directly impact the spatial and temporal resolution, sensitivity, and uniformity in MRI. Advances in RF hardware have resulted in a variety of designs optimized for specific clinical applications. RF coils are the "antennas" of the MRI system and have two functions: first, to excite the magnetization by broadcasting the RF power (Tx-Coil) and second to receive the signal from the excited spins (Rx-Coil). Transmit RF Coils emit magnetic field pulses ( B1+) to rotate the net magnetization away from its alignment with the main magnetic field (B0 ), resulting in a transverse precessing magnetization. Due to the precession around the static main magnetic field, the magnetic flux in the receive RF Coil ( B1-) changes, which generates a current I. This signal is "picked-up" by an antenna and preamplified, usually mixed down to a lower frequency, digitized, and processed by a computer to finally reconstruct an image or a spectrum. Transmit and receive functionality can be combined in one RF Coil (Tx/Rx Coils). This review looks at the fundamental principles of an MRI RF coil from the perspective of clinicians and MR technicians and summarizes the current advances and developments in technology.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 6.

High-resolution microstrip NMR detectors for subnanoliter samples

We present the numerical optimization and experimental characterization of two microstrip-based nuclear magnetic resonance (NMR) detectors. The first detector, introduced in our previous work, was a flat wire detector with a strip resting on a substrate, and the second detector was created by adding a ground plane on top of the strip conductor, separated by a sample-carrying capillary and a thin layer of insulator. The dimensional parameters of the detectors were optimized using numerical simulations with regards to radio frequency (RF) sensitivity and homogeneity, with particular attention given to the effect of the ground plane. The influence of copper surface finish and substrate surface on the spectral resolution was investigated, and a resolution of 0.8-1.5 Hz was obtained on 1 nL deionized water depending on sample positioning. For 0.13 nmol sucrose (0.2 M in 0.63 nL H₂O) encapsulated between two Fluorinert plugs, high RF homogeneity (A_810°/A_90° = 70-80%) and high sensitivity (expressed in the limit of detection nLOD_m = 0.73-1.21 nmol s^1/2) were achieved, allowing for high-performance 2D NMR spectroscopy of subnanoliter samples.

Pushing nuclear magnetic resonance sensitivity limits with microfluidics and photo-chemically induced dynamic nuclear polarization

Among the methods to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy, small-diameter NMR coils (microcoils) are promising tools to tackle the study of mass-limited samples. Alternatively, hyperpolarization schemes based on dynamic nuclear polarization techniques provide strong signal enhancements of the NMR target samples. Here we present a method to effortlessly perform photo-chemically induced dynamic nuclear polarization in microcoil setups to boost NMR signal detection down to sub-picomole detection limits in a 9.4T system (400 MHz ¹H Larmor frequency). This setup is unaffected by current major drawbacks such as the use of high-power light sources to attempt uniform irradiation of the sample, and accumulation of degraded photosensitizer in the detection region. The latter is overcome with flow conditions, which in turn open avenues for complex applications requiring rapid and efficient mixing that are not easily achievable on an NMR tube without resorting to complex hardware.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique with an inherently low sensitivity. Here, the authors present a combination of microcoils with photo-chemically induced dynamic nuclear polarization to boost NMR sensitivity down to sub-picomole detection limits.

The Molecular Basis of Noncanonical Bacterial Morphology

Bacteria come in a wide variety of shapes and sizes. The true picture of bacterial morphological diversity is likely skewed due to an experimental focus on pathogens and industrially relevant organisms. Indeed, most of the work elucidating the genes and molecular processes involved in maintaining bacterial morphology has been limited to rod- or coccal-shaped model systems. The mechanisms of shape evolution, the molecular processes underlying diverse shapes and growth modes, and how individual cells can dynamically modulate their shape are just beginning to be revealed. Here we discuss recent work aimed at advancing our knowledge of shape diversity and uncovering the molecular basis for shape generation in noncanonical and morphologically complex bacteria.

Synthesis and characterization of monophosphinic acid DOTA derivative: A smart tool with functionalities for multimodal imaging

A new facile synthetic strategy was developed to prepare bifunctional monophosphinic acid Ln-DOTA derivatives, Gd-DO2AGAP^NBn and Gd- DO2AGAP^ABn. The relaxivities of the Gd-complexes are enhanced compared to Gd-DOTA. Monophosphinic acid arm of these Gd-complexes affords enhancement of inner sphere water exchange rate due to its steric bulkiness. The different functionalities of DO2AGAP^NBn were appended in trans positions and are designed to conjugate identical or different vectors according to the potential applications. The conjugation of Gd-DO2AGAP^ABn with E3 peptide known to target apoptosis was successfully performed and in vivo MRI allowed cell death detection in a mouse model.

A low-power high-sensitivity single-chip receiver for NMR microscopy

In this paper, we present a fully-integrated receiver for NMR microscopy applications manufactured in a 0.13μm CMOS technology. The design co-integrates a 10-turn planar detection coil together with a complete quadrature, low-IF downconversion receiver on a single chip, which operates from a single 1.5V supply with a total power dissipation of 18mW. The detector's measured time-domain spin sensitivity is 3×10(13)(1)Hspins/Hz at 7T. Additionally, the paper discusses two important aspects of NMR microscopy using planar detection coils: the link between the detection coil's spin sensitivity and the achievable image SNR and the correction of image artifacts induced by the inhomogeneous sensitivity profile of planar detection coils. More specifically, we derive analytical expressions for both the theoretical image SNR as a function of the coil's spin sensitivity and the sensitivity correction for a known coil sensitivity profile in CTI MR imaging experiments. Both expressions are validated using measured data in the imaging section of the paper. Thanks to the improved spin sensitivity of the utilized integrated receiver chip compared to a previously presented design, we were able to obtain sensitivity corrected images in a 7T spectroscopy magnet with isotropic resolutions of 9.6μm and 5μm with single-shot SNRs of 37 and 15 in relatively short imaging times of 4.4h and 24h, respectively.

High-level integration of three-dimensional microcoils array in fused silica

Rapid and facile creation of three-dimensional (3D) microcoils array in a "lab-on-a-chip" platform is a big challenge in micromachining. Here we report a method based on an improved femtosecond-laser wet-etch (FLWE) technology and metal-microsolidifying process for the fabrication of 3D microcoils array inside fused silica. Based on this approach, we fabricated microcoil arrays such as 3×3 O-shaped microcoils array and 4×4 liner microcoils array. By injecting high-melting-point alloy, the electrocircuit of microcoils array can hardly be disconnected. The microcoils array also exhibits good uniformity and a high integration level. It shows promise as a real application device.

Thermostatted micro-reactor NMR probe head for monitoring fast reactions

A novel nuclear magnetic resonance (NMR) probe head for monitoring fast chemical reactions is described. It combines micro-reaction technology with capillary flow NMR spectroscopy. Two reactants are fed separately into the probe head where they are effectively mixed in a micro-mixer. The mixed reactants then pass through a capillary NMR flow cell that is equipped with a solenoidal radiofrequency coil where the NMR signal is acquired. The whole flow path of the reactants is thermostatted using the liquid FC-43 (perfluorotributylamine) so that exothermic and endothermic reactions can be studied under almost isothermal conditions. The set-up enables kinetic investigation of reactions with time constants of only a few seconds. Non-reactive mixing experiments carried out with the new probe head demonstrate that it facilitates the acquisition of constant highly resolved NMR signals suitable for quantification of different species in technical mixtures. Reaction kinetic measurements on a test system are presented that prove the applicability of the novel NMR probe head for monitoring fast reactions.

Evaluation of coils for imaging histological slides: signal-to-noise ratio and filling factor

PURPOSE

To investigate the relative gain in sensitivity of five histology coils designed in-house to accommodate tissue sections of various sizes and compare with commercial mouse head coils.

METHODS

The coil set was tailored to house tissue sections ranging from 5 to1000 µm encased in either glass slides or coverslips.

RESULTS

Our simulations and experimental measurements demonstrated that although the sensitivity of this flat structure consistently underperforms relative to a birdcage head coil based on the gain expected from their respective filling factor ratios, our results demonstrate that it can still provide a remarkable gain in sensitivity. Our study also describes preparation protocols for freshly excised sections, as well as premounted tissue slides of both mouse and human specimens. Examples of the exceptional level of tissue detail and the near-perfect magnetic resonance imaging to light microscopic image coregistration are provided.

CONCLUSION

The increase in filling factor achieved by the histology radiofrequency (RF) probe overcomes the losses associated with electric leaks inherent to this structure, leading to a 6.7-fold improvement in performance for the smallest coil implemented. Alternatively, the largest histology coil design exhibited equal sensitivity to the mouse head coil while nearly doubling the RF planar area coverage.