NMR Radio Frequency Research Articles

Design of NMR RF Probe Constructed With High-Temperature Superconducting Elliptical–Coupled Resonators

A high-temperature superconducting (HTS) radio frequency (RF) probe has been designed for nuclear magnetic resonance spectroscopy that is constructed of two newly developed HTS elliptical-coupled resonators that are placed on either side of the sample. The probe has a higher quality factor and a more uniform magnetic field along the z-axis than one with square resonators. The measured unloaded quality factor (Qu) of a pair of elliptical-coupled resonators fabricated using YBa2Cu3Oy thin films on an Al2O3 substrate was about 30 000 at 15 K, which is about 82 times higher than the simulated Qu of a copper counterpart at room temperature. Measurement of the input power dependence of Qu showed that a Qu of over 20 000 was obtained up to 16 dBm. Measurement of the RF magnetic field profile along the z-axis showed that the RF magnetic field was more than 85% uniform around the sample region.

Structure-Correlation NMR Spectroscopy for Macromolecules Using Repeated Bidirectional Photoisomerization of Azobenzene.

Control over macromolecular structure offers bright potentials for manipulation of macromolecular functions. We here present structure-correlation NMR spectroscopy to analyze the correlation between polymorphic macromolecular structures driven by photoisomerization of azobenzene. The structural conversion of azobenzene was induced within the mixing time of a NOESY experiment using a colored light source, and the reverse structural conversion was induced during the relaxation delay using a light source of another color. The correlation spectrum between trans- and cis-azobenzene was then obtained. To maximize the efficiency of the bidirectional photoisomerization of azobenzene-containing macromolecules, we developed a novel light-irradiation NMR sample tube and method for irradiating target molecules in an NMR radio frequency (rf) coil. When this sample tube was used for photoisomerization of an azobenzene derivative at a concentration of 0.2 mM, data collection with reasonable sensitivity applicable to macromolecules was achieved. We performed isomerization of an azobenzene-cross-linked peptide within the mixing time of a NOESY experiment that produced cross-peaks between helix and random-coil forms of the peptide. Thus, these results indicate that macromolecular structure manipulation can be incorporated into an NMR pulse sequence using an azobenzene derivative and irradiation with light of two types of wavelengths, providing a new method for structural analysis of metastable states of macromolecules.

Shape-changing magnetic assemblies as high-sensitivity NMR-readable nanoprobes.

Fluorescent and plasmonic labels and sensors have revolutionized molecular biology, helping visualize in vitro cellular and biomolecular processes1–3. Increasingly, such probes are now designed to respond to wavelengths in the near infrared region, where reduced tissue autofluorescence and photon attenuation enable subsurface in vivo sensing4. But even in the near infrared, optical resolution and sensitivity decrease rapidly with increasing depth. Here we present a sensor design that obviates the need for optical addressability by operating in the NMR radio-frequency (RF) spectrum, where signal attenuation and distortion by tissue and biological media are negligible, where background interferences vanish, and where sensors can be spatially located using standard magnetic resonance imaging (MRI) equipment. The RF-addressable sensor assemblies presented here are comprised of pairs of magnetic disks spaced by swellable hydrogel material; they reversibly reconfigure in rapid response to chosen stimuli, to give geometry-dependent, dynamic NMR spectral signatures. Sensors can be made from biocompatible materials, are detectable down to low concentrations, and offer potential responsive NMR spectral shifts approaching a million times those of traditional magnetic resonance spectroscopies. Inherent adaptability should allow such shape-changing systems to measure numerous different environmental and physiological indicators, affording broadly generalizable, MRI-compatible, RF analogues to optically-based probes for use in basic chemical, biological and medical research.

Motion-adapted pulse sequences for oriented sample (OS) solid-state NMR of biopolymers

One of the main applications of solid-state NMR is to study the structure and dynamics of biopolymers, such as membrane proteins, under physiological conditions where the polypeptides undergo global motions as they do in biological membranes. The effects of NMR radiofrequency irradiations on nuclear spins are strongly influenced by these motions. For example, we previously showed that the MSHOT-Pi4 pulse sequence yields spectra with resonance line widths about half of those observed using the conventional pulse sequence when applied to membrane proteins undergoing rapid uniaxial rotational diffusion in phospholipid bilayers. In contrast, the line widths were not changed in microcrystalline samples where the molecules did not undergo global motions. Here, we demonstrate experimentally and describe analytically how some Hamiltonian terms are susceptible to sample motions, and it is their removal through the critical π/2 Z-rotational symmetry that confers the "motion adapted" property to the MSHOT-Pi4 pulse sequence. This leads to the design of separated local field pulse sequence "Motion-adapted SAMPI4" and is generalized to an approach for the design of decoupling sequences whose performance is superior in the presence of molecular motions. It works by cancelling the spin interaction by explicitly averaging the reduced Wigner matrix to zero, rather than utilizing the 2π nutation to average spin interactions. This approach is applicable to both stationary and magic angle spinning solid-state NMR experiments.

TH-D-304A-01: Development of Multi-Parametric Molecular Imager by Integrating Overhauser-Enhanced MRI (OMRI) with Prepolarized MRI (PMRI)

Purpose: To develop a multi‐parametric molecular imager to assess tumor hypoxia and redox status by integrating Overhauser‐enhanced MRI (OMRI) with prepolarized MRI (PMRI). Method and Materials: PMRI system acquires anatomic MR images comparable to conventional MRI. With a moderate hardware augmentation of the PMRI, we implemented field‐cycled OMRI and obtained electron paramagnetic resonance(EPR)images of nitroxide spin probes. Phantoms were made of 2‐cc tubes filled with pure water, <5‐mM solutions of 3‐carbamoyl‐PROXYL. A custom MRI console was used to insert 25‐W EPR radiofrequency (RF) pulses that enhanced NMR signal of the coupled water protons. A saddle coil (3.5 cm diameter, 2.5 cm length) tuned to 186 MHz was used as the EPR irradiator. To elicit the Overhauser effect, each NMR RF pulse (2.23 MHz) was preceded by the EPR RF pulse during a low field‐cycled interval. Free induction decay (FID) was observed at the EPR B0 swept from 4.2 mT to 9.5 mT. For OMRI, the low‐field cycle was fixed at 4.8 mT to exploit the low‐frequency hyperfine peak of the nitroxide. Generic gradient‐echo 0.05‐T MR images were obtained with either prepolarization or Overhauser enhancement. Results: FID acquisition of a 2.5‐mM nitroxide phantom identified three hyperfine lines of 14 N at 4.8, 6.2 and 8.1 mT. Each peak represented an enhancement factor of 13 with 300‐ms EPR RF pulse. Increasing enhancements were observed with longer durations of EPR irradiation, which indicated 0.71‐s T1 of the phantom. We observed similar intensities of water and <5‐mM nitroxide solutions from 0.15‐T PMRI. The OMRI showed characteristic EPR intensities and clearly separated the 2.5‐mM and 5‐mM nitroxide phantoms from the water phantom. Conclusion: The integrated OMRI‐PMRI system demonstrated sensitivity to both EPR and NMR. The imager can potentially acquire physiological information in small animals accurately co‐registered with high‐quality anatomic NMRimages.

Orbital domain state and finite size scaling in ferromagnetic insulating manganites.

55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca0.20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied by a strong cooling-rate dependent increase of certain FM neutron Bragg peaks. (55,139)(1/T(1)) spin-lattice and (139)(1/T(2)) spin-spin relaxation rates are strongly enhanced on approaching this temperature from below, signaling a genuine phase transition at T(tr) approximately 70 K. The disappearance of the FM metallic signal by applying a weak external magnetic field, the different NMR radio-frequency enhancement of the FM metallic and insulating states, and the observed finite size scaling of T(tr) with Ca (hole) doping, as observed in powder La(1-x)CaxMnO3 samples, are suggestive of freezing into an inhomogeneous FM insulating and orbitally ordered state embodying "metallic" hole-rich walls.

Advancing NMR sensitivity for LC-NMR-MS using a cryoflow probe: application to the analysis of acetaminophen metabolites in urine.

Cryogenic cooling of the NMR radio frequency coils and electronics to give greatly enhanced sensitivity is arguably the most significant recent advance in NMR spectroscopy. Here we report the first cryogenic probe built in flow configuration and demonstrate the application to LC-NMR-MS studies. This probe provides superior sensitivity over conventional noncryogenic flow NMR probes, allowing the use of 100 microL of untreated urine (40% less material than previous studies that required preconcentration) and yet revealing drug metabolites hitherto undetected by LC-NMR-MS at 500 MHz. Besides the known sulfate and glucuronide metabolites, previously undetected metabolites of acetaminophen were directly observable in a 15-min on-flow experiment. Simultaneous MS data also provided knowledge on the NMR-silent functional moieties. Further, stop-flow LC-NMR-MS experiments were conducted for greater signal-to-noise ratios on minor metabolites. The cryoflow probe enables the NMR analysis of lower concentrations of metabolites than was previously possible for untreated biofluids. This strategy is generally applicable for samples containing mass-limited analytes, such as those from drug metabolism studies, biomarker and toxicity profiling, impurity analysis, and natural product analysis.

NMR spectroscopy of single neurons

The first spatially localized NMR spectra of osmolytes and metabolites from single isolated neurons have been obtained using a combination of high magnetic field strengths and NMR radio frequency (RF) microcoils. The proton spectra display peaks at high concentrations (100–300 mM) assigned to betaine and choline, and other metabolite resonances including lactate at lower concentrations in the order of 10s of millimoles. The volumes examined were approximately 10 nl, over two orders of magnitude less than previously possible. In these initial experiments; the cells were unperfused and the signal intensities of the osmolytes decrease with time, a phenomenon consistent with cell swelling. This work demonstrates the technical feasibility of NMR spectroscopy of single cells, further broadening the scope of NMR spectroscopy of living tissues from application to entire living organisms (man and animal models) and isolated tissues (perfused organs and cultured assemblies of cells) and now to single cells. Magn Reson Med 44:19–22, 2000. © 2000 Wiley-Liss, Inc.

NMR spectroscopy of single neurons.

The first spatially localized NMR spectra of osmolytes and metabolites from single isolated neurons have been obtained using a combination of high magnetic field strengths and NMR radio frequency (RF) microcoils. The proton spectra display peaks at high concentrations (100-300 mM) assigned to betaine and choline, and other metabolite resonances including lactate at lower concentrations in the order of 10s of millimoles. The volumes examined were approximately 10 nl, over two orders of magnitude less than previously possible. In these initial experiments; the cells were unperfused and the signal intensities of the osmolytes decrease with time, a phenomenon consistent with cell swelling. This work demonstrates the technical feasibility of NMR spectroscopy of single cells, further broadening the scope of NMR spectroscopy of living tissues from application to entire living organisms (man and animal models) and isolated tissues (perfused organs and cultured assemblies of cells) and now to single cells. Magn Reson Med 44:19-22, 2000.

5576622 Shielded NMR radio frequency coil and method of performing an NMR experiment

An RF coil for NMR experiments having a conductive path for flowing a field-cancelling current to isolate magnetically the RF coil from its environment. NMR data, particularly MRI data from a human subject, is collected with the receiver coil magnetically shielded to avoid loss of the received signal energy to the environment.

5565780 NMR radio-frequency coil

5565780 NMR radio-frequency coil

Observation of a new coherent transient in NMR -- nutational two-pulse stimulated echo in the angular distribution of \gamma-radiation from oriented nuclei

A new coherent transient in pulsed NMR, the two-pulse nutational stimulated echo, is reported for the ferromagnetic system 60CoFe using resonant perturbations on the directional emission of anisotropic \gamma-radiation from thermally oriented nuclei. The new spin echo is a result of non-linear nuclear spin dynamics due to large Larmor inhomogeneity active during radiofrequency pulse application. It is made readily observable through the gross detuning between NMR radiofrequency excitation and gamma radiation detection, and inhomogeneity in the Rabi frequency caused by metallic skin-effect. The method of concatenation of perturbation factors in a statistical tensor formalism is quantitatively applied to successfully predict and then fit in detail the experimental time-domain data.

Magnetism, phase composition, and hyperfine fields of melt-spun Nd-Fe-B alloys with a few percent of neodymium.

It is possible that melt-spun Nd-Fe-B alloys with a few percent of neodymium can be used as a raw material for inexpensive and bonded permanent magnets. After an appropriate heat treatment, melt-spun ${\mathrm{Nd}}_{4}$${\mathrm{Fe}}_{77.5}$${\mathrm{B}}_{18.5}$ alloy has a coercive field of 3 kOe, a remanent magnetization of 12.5 kG, and an energy product of 13 MG Oe. However, at present, there is some debate on the origin of its hard magnetic properties. In the present work, the phase composition, magnetism, and hyperfine fields of melt-spun Nd-Fe-B alloys are studied. The effect of rare-earth element substitution for Nd, such as Pr, Gd, and Dy, on $^{11}\mathrm{B}$ and $^{57}\mathrm{Fe}$ magnetic hyperfine fields is also investigated. On the basis of the experimental results, the origin of the hard magnetic properties is discussed. The results indicate that (1) the melt-spun Nd-Fe-B alloys with a low Nd concentration annealed under an optimal heat treatment consist of body-centered-tetragonal ${\mathrm{Fe}}_{3}$B (bct-${\mathrm{Fe}}_{3}$B) and a few percent of \ensuremath{\alpha}-Fe and no ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{14}$B magnetically hard phase; (2) about 5 at. % Fe atoms in ${\mathrm{Fe}}_{\mathrm{III}}$(8g) site of bct-${\mathrm{Fe}}_{3}$B are replaced by Nd atoms in the samples annealed under optimal condition. NMR radio frequency (rf) enhancement effect results demonstrate that bct-${\mathrm{Fe}}_{3}$B containing Nd atoms has better permanent magnetic properties; (3) the substitution of some rare-earth elements, such as Pr, Gd, and Dy, does not influence the hyperfine field for $^{57}\mathrm{Fe}$ in \ensuremath{\alpha}-Fe; however, the hyperfine field for $^{11}\mathrm{B}$ in bct-${\mathrm{Fe}}_{3}$B increases with the addition of Gd or Dy. Thus, it can be concluded that the hard magnetic properties of melt-spun Nd-Fe-B with a few percent of neodymium result from bct-${\mathrm{Fe}}_{3}$B containing Nd atoms, and not from the presence of the ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{14}$B magnetically hard phase.

5315251 NMR radio-frequency coil

5315251 NMR radio-frequency coil

5179332 NMR radio frequency coil with disable circuit

5179332 NMR radio frequency coil with disable circuit

5196797 Method of correcting an asymmetry in an NMR radio frequency coil and an improved radio frequency coil having N-fold symmetry and reduced eddy current

In the present invention a method is disclosed to correct the asymmetry in an NMR radio frequency coil of the type that has a pair of conductive loop elements disposed in a spaced apart relation along a common longitudinal axis. The coil has at least eight conductive elements electrically interconnecting the loop elements at point spaced along the periphery of each of the loops. A pair of corrective capacitive elements are placed 45° apart from one another and in series with a pair of the conductive segments which interconnect the loops. The present invention also relates to an NMR radio frequency coil having N-fold symmetry and reduced eddy current.

A spectrometer modification to suppress spurious signals which interfere with 2D NMR spectra

Two-dimensional NMR experiments obtained on our 500 MHz NMR spectrometer have frequently been rendered useless by the presence of bands of “noise” that resemble a reflection of the desired spectrum. 2D NMR spectra often contain artifacts as a consequence of a variety of causes, such as, errors in the flip angle and phase of excitation pulses, RF inhomogeneity, imprecise or inappropriate interpulse delays, quadrature image due to a mismatch in the phase or amplitude balance of the dual detectors used for quadrature detection, general environmental instability (e.g., temperature), and artifacts arising from nonideal application of the discrete Fourier transform (1-4). The general effects of these instabilities and imperfections have commonly been discussed in terms of “ti” and “t2” correlated noise and true quadrature images resulting from incomplete suppression of N-type or P-type signals. In the case illustrated by Fig. 1, we have determined that the “noise” signals are due to interference signals originating within the spectrometer console. These types of artifacts can appear in all types of 2D ‘H NMR spectra (e.g., COSY, NOESY, TOCSY, RCT COSY) and are unrelated to the method used for quadrature detection during the incremented time domain (e.g., phase modulation (5), hypercomplex reconstruction (6), time proportional phase incrementation (7,8)). We have also observed these artifacts in experiments where no attempt was made to provide quadrature detection during the incremented time domain. Here we illustrate the source of this artifact and present a simple means by which these spurious but real signals can be eliminated. High-resolution NMR spectrometers usually employ a superheterodyne type receiver. A superheterodyne receiver converts the NMR radiofrequency signal to an intermediate frequency (IF), prior to final phase detection, by mixing a local oscillator (LO) frequency with the NMR signal to produce a difference frequency which falls within the bandwidth of the IF amplifier circuit. There are, however, two, RF frequencies which will combine with a given LO frequency to produce the same difference frequency (9, 10). For example, an NMR frequency of 500 MHz and a LO of 506 MHz will produce an IF frequency of 6 MHz. An RF of 5 12 MHz (the image frequency) when mixed with the 506 MHz LO would also produce a frequency at the 6 MHz IF frequency. In high sensitivity superheterodyne RF receivers one of the main limiting factors determining their performance is a process called reciprocal mixing. In this process the noise present on the receivers own LO is converted into the bandpass of the IF

A connection between the Bloch equations and the Korteweg-de Vries equation

By formulating the physical problem of NMR radiofrequency excitation as an inverse problem, we are led to a family of rf amplitude modulations that realize a particular response through Bloch temporal evolution. Moreover, an exploration of the variation of one of the parameters in the solution obtained displays remarkable stability.

NMR multiple echoes observed in solidHe3

A large number of echoes-called multiple echoes-following two isolated NMR radio-frequency pulses separated by a time $\ensuremath{\tau}$, have been observed in bcc $^{3}\mathrm{He}$. A quantitative theory of this phenomenon is given, taking into account the nonlinear effect of the nuclear demagnetizing field in the equation of motion of the magnetization. In a magnetic-field gradient $G$, just before the second pulse at time $\ensuremath{\tau}$, the nuclear-spin transverse magnetization has a helical configuration of pitch $\ensuremath{\gamma}G\ensuremath{\tau}={k}_{0}$, which is converted by the second radio-frequency pulse into a sinusoidal modulation of the magnetization along the magnetic field gradient. The corresponding sinusoidal demagnetizing field modulates spatially the NMR resonance frequency so that the transverse magnetization is a superposition of helical configurations with pitch $p{k}_{0}$ (multiple of ${k}_{0}$), yielding multiple echoes at times $p\ensuremath{\tau}$. A detailed comparison with the experimental results obtained at high temperatures ($300 \mathrm{mK}&lt;T&lt;1 \mathrm{K}$) and low temperatures ($1&lt;T&lt;20$ mK), leads to a satisfactory agreement with the experiments. The main practical application of the multiple-echo study is to provide a method of measurement of the absolute value of the nuclear-spin susceptibility without knowing the shape or the number of spins of the sample. The transverse relaxation time ${T}_{2}$ and the diffusion coefficient $D$ are also obtained with this analysis. When applied to the low-temperature measurements, the multiple-echo analysis has pointed out, for the first time, the increase of the magnetic susceptibility of solid $^{3}\mathrm{He}$ in the paramagnetic phase near the ordering temperature ${T}_{c}$, while in contrast the relaxation times ${T}_{1}$ and ${T}_{2}$ and the diffusion coefficient $D$ do not change appreciably, even just above ${T}_{c}$.