The low-field nuclear magnetic resonance (LF NMR) system allows rapid, on-site, and nondestructive detection of molecular structures using portable equipment. However, LF NMR suffers from limited resolution, low sensitivity, and poor stability, resulting in peak overlap and limitations in practical applications. In this work, an accelerated spectroscopic method that combines nonuniform sampling (NUS) with a fast and accurate low-rank reconstruction method is proposed to rapidly acquire and faithfully reconstruct high-resolution 2-D spectra. Benefiting from the NUS and the group sparsity constraint, this method reduces the acquisition time and reconstruction time to overcome the effects of field drift while preserving low-intensity broad peaks. Through theoretical analyses, both high-field and low-field nuclear magnetic resonance (NMR) spectra reconstruction experiments, and in situ reaction monitoring, we show that this accelerated method is effective and efficient in terms of preserving sensitivity, improving the spectral resolution, and monitoring the molecular structures of reactants in real time. Thus, this proposed accelerated method may expand the applicability of low-cost LF NMR systems in fast detection.
During the past several decades, inexpensive compact nuclear magnetic resonance (NMR) instruments have been widely used for on-site detections of chemical identity and sample analyses in industrial applications. In general, via shim coils and control strategies, automatic search shimming methods are capable of improving the magnetic field uniformity so that optimized signals can be acquired and particularly suited for rapid detections in compact NMR instruments. However, because these methods inherently endure the inefficiency in multidimensional shims and the sensitivity of starting points, it is time-consuming to obtain acceptable results in a field infected with distortions and strays. Here, two remedies that improve the shimming speed for the search shimming method are proposed. First, analyzing and compensating for magnetic susceptibility effects, we relocate starting points closely to optima and reduce the complexity of global exploration. Second, modifying the simplex movement with adaptive scale factors and gradient-like centroid, we have improved the convergence of the simplex shimming algorithm in a multidimensional search space. Via theoretical analyses and lab experiments, we find that the proposed shimming method outperforms regular shimming methods in terms of handling complex magnetic fields and reducing shimming times and hope that our study can help further improve NMR shimming methods in the future.
A better fit to the detection area has always been one of the magnetic resonance imaging (MRI) coil development goals. For many application scenarios and organism detection sites, conformal birdcage coil (CBC) has a better attachment and higher filling factor than conventional circular birdcage coil, improving the MRI image signal-to-noise ratio effectively. However, the structural asymmetry of CBC causes the original resonant mode of the circular birdcage coil to split, with a bigger frequency difference as the deformation increases, affecting the strength and uniformity of the corresponding radio frequency fields significantly in an indistinct way. In this paper, we present an extended expression form of non-circular birdcage coils, which is more general than the existing research results. Based on the extended expression, we propose a more conformal, runway-shaped CBC structure that can achieve higher sensitivity. We also introduce a new design method for 1 H/ 19 F double-tuned CBC, which utilizes mode splitting generated by deformation without inserting additional tuning networks. This method employs numerical analysis and simulation to design the coil structure and tuning, allowing both 1 H and 19 F resonant modes to achieve optimized RF field performance simultaneously. To verify the method, a runway-shaped 1 H/ 19 F double-tuned CBC with specific parameters applicable to a 7 T small-bore MRI system is developed. The effectiveness and applicability of our proposed theories, structures, and methodologies have been fully validated through homogeneous phantom and subject MRI experiments conducted with this coil. Compared to commercial probes with coils of similar physical dimensions, CBC probes exhibit good usability, conformability, higher sensitivity, and better uniformity for flat tissue structures.
We present the design, fabrication, characterization, and optimization of a TPM (twin parallel microstrip)-based nuclear magnetic resonance (NMR) probe, produced by using a low-loss Teflon PTFE F4B high frequency circuit board. We use finite element analysis to optimize the radio frequency (RF) homogeneity and sensitivity of the TPM probe jointly for various sample volumes. The RF homogeneity of this TPM planar probe is superior to that of only a single microstrip probe. The optimized TPM probe properties such as RF homogeneity and field strength are characterized experimentally and discussed in detail. By combining this TPM based NMR probe with microfluidic technology, the sample amount required for kinetic study using NMR spectroscopy was minimized. This is important for studying costly samples. The TPM NMR probes provide high sensitivity to analysis of 5 µl samples with 2 mM concentrations within 10 min. The miniaturized microfluidic NMR probe plays an important role in realizing down to seconds timescale for kinetic monitoring.
The homogeneous magnetic field is highly desired for achieving high-resolution nuclear magnetic resonance (NMR) spectra. Inevitably, however, nonzero magnetic susceptibility (non-ZMS) structures near the detection region, such as radio-frequency (RF) coils and brackets among others, accentuate the magnetic field inhomogeneity ( $\beta $ ) and broaden NMR spectral lines. In this study, we discover that when cylindrical sensors are perpendicular to the applied magnetostatic field, inlaying the RF coil into a tubular bracket that coaxially overlays the tube can reduce $\beta $ within the detection region of this RF coil. This inlay depth can be optimized by enforcing magnetic dipole fields induced by the RF coil to equal those of the materials replaced by this RF coil. Based on this mechanism, we have developed an inlaid-coil sensor for a lab-built compact NMR spectrometer, which is merely additionally installed with a tubular Teflon bracket. Simulated results demonstrate that, as this depth approaches the optimal theoretical value, $\beta $ within the detection region gradually decreases, supporting our identification of such a mechanism. Furthermore, experimental results reveal that the inlaid-coil sensor can acquire higher-resolution NMR spectra than the bracketless sensor can, validating the compensation mode based on this mechanism. This approach can achieve magnetostatic-field compatibility among different materials inside cylindrical sensors perpendicular to the imposed magnetostatic field. Hopefully, our study can help provide a guide for the magnetostatic-field design of complex cylindrical sensors in emerging NMR technologies such as microsystems, hyperpolarization, and in situ detection.
Abstract Magnetic resonance (MR) technology has been widely employed in scientific research, clinical diagnosis and geological survey. However, the fabrication of MR radio frequency probeheads still face difficulties in integration, customization and miniaturization. Here, we utilized 3D printing and liquid metal filling techniques to fabricate integrative radio frequency probeheads for MR experiments. The 3D-printed probehead with micrometer precision generally consists of liquid metal coils, customized sample chambers and radio frequency circuit interfaces. We screened different 3D printing materials and optimized the liquid metals by incorporating metal microparticles. The 3D-printed probeheads are capable of performing both routine and nonconventional MR experiments, including in situ electrochemical analysis, in situ reaction monitoring with continues-flow paramagnetic particles and ions separation, and small-sample MR imaging. Due to the flexibility and accuracy of 3D printing techniques, we can accurately obtain complicated coil geometries at the micrometer scale, shortening the fabrication timescale and extending the application scenarios.
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