Use Of High Magnetic Fields Research Articles

High-sensitivity, broadband-decoupled (13) C MR spectroscopy in humans at 7T using two-dimensional heteronuclear single-quantum coherence.

Carbon-13 ((13) C) magnetic resonance spectroscopy (MRS) has an intrinsically low NMR sensitivity that often leads to large acquisition volumes or long scan times. While the use of higher magnetic fields can overcome the sensitivity limitations, high radiofrequency (RF) power deposition associated with proton-decoupling limits the achievable gain. Two-dimensional (2D) heteronuclear single quantum coherence (HSQC) MRS is a method that uses the high chemical specificity of (13) C MRS while retaining the high sensitivity of (1) H detection. Due to the 2D nature of the method, proton-decoupled (13) C MR spectra can be obtained without the use of high-powered decoupling pulses. A novel three-dimensional (3D) localized 2D HSQC method based on 3D STEAM localization is presented and implemented at 7T. The low RF power deposition of the method allows TR variation along the indirect dimension which, in combination with controlled aliasing, leads to an acceleration of 11.8 relative to a standard 2D NMR acquisition. Artifact-free, high-quality and high-sensitivity 2D HSQC spectra were obtained for all subjects in 19 min from a small (9 mL) volume placed in the leg adipose tissue. Complete proton decoupling was achieved along the indirect (13) C dimension despite the absence of broadband proton-decoupling pulses. The high chemical specificity along the indirect (13) C dimension allowed the detection of 19 unique resonances from which the lipids could be characterized in terms of saturation and omega-6/omega-3 fatty acid ratio. It has been demonstrated that high-quality 2D HSQC NMR spectra can be acquired from human adipose tissue at 7T. The HSQC method is methodologically simple and robust and is flexible regarding trade-offs between temporal and spectral resolution. 2D HSQC has a strong potential to become a default method in natural-abundance or (13) C-enriched studies of human metabolism in vivo.

Advances in high-field magnetic resonance spectroscopy in Alzheimer's disease.

Alzheimer’s disease (AD) affects several important molecules in brain metabolism. The resulting neurochemical changes can be quantified non-invasively in localized brain regions using in vivo single-voxel proton magnetic resonance spectroscopy (SV 1H MRS). Although the often heralded diagnostic potential of MRS in AD largely remains unfulfilled, more recent use of high magnetic fields has led to significantly improved signal-to-noise ratios and spectral resolutions, thereby allowing clinical applications with increased measurement reliability. The present article provides a comprehensive review of SV 1H MRS studies on AD at high magnetic fields (3.0 Tesla and above). This review suggests that patterned regional differences and longitudinal alterations in several neurometabolites are associated with clinically established AD. Changes in multiple metabolites are identifiable even at early stages of AD development. By combining information of neurochemicals in different brain regions revealing either pathological or compensatory changes, high field MRS can be evaluated in AD diagnosis and in the detection of treatment effects. To achieve this, standardization of data acquisition and analytical approaches is needed.

The two aluminum sites in the 27Al MAS NMR spectrum of kaolinite: Accurate determination of isotropic chemical shifts and quadrupolar interaction parameters

The problem of resolving the two aluminum sites in the 27Al NMR spectrum of kaolinite has been unsuccessfully addressed for 30 years. A few years ago, it was shown that the two sites cannot be spectrally separated even by the use of high magnetic fields. Nevertheless, it is still possible to determine the NMR parameters of both sites. In this article, we present an alternative approach. We show that, at low magnetic field (7 T), the individual spinning sideband lineshapes of the outer satellite transitions are sensitive enough to differentiate information coming from the two aluminum sites. Thus, the isotropic chemical shift δ, the quadrupolar constant CQ, and asymmetry parameter ηQ of each site can be obtained by accurately fitting the full 27Al MAS spectrum acquired at low magnetic field. In return, this approach requires a carefully acquired and post-treated 27Al spectrum. It is concluded that the two sets of parameters (δ = 7.5 ppm, CQ = 3.4 MHz, ηQ = 0.8) and (δ = 8.0 ppm, CQ = 3.0 MHz, ηQ = 0.9) represent the best and the unique solution overall. Moreover, the accuracy of these experimental values is independently and fully supported by first-principles calculations of the electric field gradient. The approach presented in this article can be easily applied, not only to clays or aluminosilicate materials, but to any compounds where the NMR parameters of overlapping spectral lines have to be determined. This can also be extended to sites of unequal multiplicity and to other nuclei. Moreover, this methodology can be useful in the characterization of small structural changes occurring partly at one particular site. Indeed, when the NMR parameters are barely modified, the spectral signatures due to both affected and unaffected sites may strongly overlap, making the spectral resonances broad and badly resolved. In such a case, determining the isotropic chemical shift and the quadrupolar coupling parameters may help to exceed the simple qualitative analysis of structural changes by offering the possibility of discriminating between structural models via the experimental data.

Note: Compact optical fiber coupler for diamond anvil high pressure cells

A compact optical fiber coupler has been developed to allow transmission of light through an optical fiber to and from the high pressure region of a diamond anvil high pressure cell. Despite its small size the coupler has focusing adjustments and optics, which allows the light to be focused precisely on the sample within the pressure cell. The coupler is suitable for a wide range of optical measurements and particularly for high pressure measurements at low temperatures in cryostats with no optical windows. The use of the coupler to determine the pressure in a diamond anvil cell at 1.2 K using the ruby fluorescence spectra of ruby is demonstrated. The small size of the coupler and its construction out of nonmagnetic beryllium copper makes it suitable for use in high magnetic fields and for magnetization experiments.

Transformation Heat Treatment of Rapidly Quenched $ \hbox{Nb}_{3}\hbox{Al}$ Precursor Monitored In Situ by High Energy Synchrotron Diffraction

Nb3Al superconductors are studied for use in high field magnets. Fine-grained Nb3Al with nearly stoichiometric Al content is obtained by a rapid heating quenching and transformation process. We describe a nondestructive in situ study of the transformation process step of a rapid heating quenching Nb3Al precursor wire with ramp rates of either 120 °C/h or 800 °C/h. High-energy synchrotron X-ray diffraction measurements show the transformation from a Nb(Al)SS supersaturated solid solution into Nb3Al. When heating with a ramp rate of 120 °C/h, a strong reduction of the Nb(Al)SS (110) diffraction peak component is observed when the temperature exceeds 660 °C. Additional diffraction peaks are detectable in the approximate temperature interval 610 °C -750 °C and significant Nb3Al growth is observed above 730 °C.

Advances in glycosaminoglycanomics by 15N-NMR spectroscopy

Recent developments to enhance sensitivity in solution NMR spectroscopy such as the advent and spread of the use of high magnetic fields, cryoprobe technology, isotopic labeling techniques, and new combinations of 2D experiments have pushed the limits in structural NMR analysis of glycosaminoglycans (GAGs). This review is dedicated to the less sensitive (15)N isotope of hexosamines rather than the commonly used anomeric and ring (1)H and (13)C resonances of uronic acids and hexosamines. Given that GAG types are basically classified on the basis of their composing hexosamine types together with variations of their sulfation patterns, and epimerized forms of the adjacent uronic acids, (15)N-related NMR studies on native GAGs, oligosaccharides, or the various composing amino sugars have proved to be quite useful in the retrieval of both structural and dynamic information, despite the low number of resultant peaks. This in turn reduces significantly chemical shift degeneracy and at the same time facilitates spin and structural assignments. This review covers the principal contributions made so far by solution (15)N-NMR spectroscopy to progress in the structural biology of GAGs in the current glycomics age.

Solid-State NMR Studies of Fossil Fuels using One- and Two-Dimensional Methods at High Magnetic Field

We examine the opportunities offered by advancements in solid-state NMR (SSNMR) methods, which increasingly rely on the use of high magnetic fields and fast magic angle spinning (MAS), in the studies of coals and other carbonaceous materials. The sensitivity of one- and two-dimensional experiments tested on several Argonne Premium coal samples is only slightly lower than that of traditional experiments performed at low magnetic fields in large MAS rotors, since higher receptivity per spin and the use of 1H detection of low-gamma nuclei can make up for most of the signal loss due to the small rotor size. The advantages of modern SSNMR methodology in these studies include improved resolution, simplicity of pulse sequences, and the possibility of using J-coupling during mixing.

Stroke: High-Field Magnetic Resonance Imaging

Diagnostic modalities for the diagnosis of acute stroke have increased in number and quality. Magnetic resonance imaging has increasingly become a central tool for the management of patients with stroke. New sequences, such as diffusion and perfusion, provide insight into the infarcted core and the hypoperfused brain. The use of higher magnetic fields allows us to gain in signal strength, which can be used to improve imaging speed and/or resolution. Recent additional sequences allow perfusion without contrast and susceptibility-weighted imaging can help identify early bleeding. These new techniques should provide more information about the on going ischemic process.

Inter-speaker speech variability assessment using statistical deformable models from 3.0 Tesla magnetic resonance images

The morphological and dynamic characterisation of the vocal tract during speech production has been gaining greater attention due to the motivation of the latest improvements in magnetic resonance (MR) imaging; namely, with the use of higher magnetic fields, such as 3.0 Tesla. In this work, the automatic study of the vocal tract from 3.0 Tesla MR images was assessed through the application of statistical deformable models. Therefore, the primary goal focused on the analysis of the shape of the vocal tract during the articulation of European Portuguese sounds, followed by the evaluation of the results concerning the automatic segmentation, i.e. identification of the vocal tract in new MR images. In what concerns speech production, this is the first attempt to automatically characterise and reconstruct the vocal tract shape of 3.0 Tesla MR images by using deformable models; particularly, by using active and appearance shape models. The achieved results clearly evidence the adequacy and advantage of the automatic analysis of the 3.0 Tesla MR images of these deformable models in order to extract the vocal tract shape and assess the involved articulatory movements. These achievements are mostly required, for example, for a better knowledge of speech production, mainly of patients suffering from articulatory disorders, and to build enhanced speech synthesizer models.

Metallic composites processed via extreme deformation: Toward the limits of strength in bulk materials

We review microstructures and properties of metal matrix composites produced by severe plastic deformation of multiphase alloys. Typical processings are wire drawing, ball milling, roll bonding, equal-channel angular extrusion, and high-pressure torsion of multiphase materials. Similar phenomena occur between solids in frictional contact such as in tribology, friction stir welding, and explosive joining. The resulting compounds are characterized by very high interface and dislocation density, chemical mixing, and atomic-scale structural transitions at heterointerfaces. Upon straining, the phases form into nanoscaled filaments. This leads to enormous strengthening combined with good ductility, as in damascene steels or pearlitic wires, which are among the strongest nanostructured bulk materials available today (tensile strength above 6 GPa). Similar materials are Cu-Nb and Cu-Ag composites, which also have good electrical conductivity that qualifies them for use in high-field magnets. Beyond the engineering opportunities, there are also exciting fundamental questions. They relate to the nature of the complex dislocation, amorphization, and mechanical alloying mechanisms upon straining and their relationship to the enormous strength. Studying these mechanisms is enabled by mature atomic-scale characterization and simulation methods. A better understanding of the extreme strength in these materials also provides insight into modern alloy design based on complex solid solution phenomena.

High‐Resolution ESR Imaging of N@C60 Radicals on a Surface

AbstractElectron Spin Resonance Microcopy (ESRM) is an imaging method capable of observing stable free radicals in small samples with a spatial resolution of ∼1 micron. Currently this technique is pursued mainly for biological applications at room temperature and at relatively low static magnetic fields. Future progress, which involves the use of higher magnetic fields at cryogenic temperatures, could significantly improve sensitivity and resolution and thus make this method attractive to many solid‐state and physical science applications. Here we consider a possible application of ESRM to the field of quantum computing employing Endohedral Nitrogen fullerene (N@C60) molecules. In order to fully address the challenges of quantum computing, the resolution should be significantly improved to ∼10 nm and the sensitivity should approach the single spin level. In the present work we show some preliminary results conducted with diluted N@C60 samples. A 2D image of N@C60 dispersed on a surface was obtained at a field of ∼0.6 T and at room temperature. Under these conditions the current sensitivity of the ESRM system is around 107−108 spins, and due to the low enrichment factor of the N@C60 sample (∼10−6), we were able to reach image resolution of only ∼50 m̈m. Future steps that are taken in order to significantly improve resolution and sensitivity of the system are discussed. These include the use of highly concentrated samples to be measured at ∼4 K and at a field of more than 1 T, which would lead to a resolution of ∼100 nm and a sensitivity of ∼100 spins.

Quench induced degradation inBi2Sr2CaCu2O8+x tape conductors at 4.2 K

With the growing interest in commercial Ag-alloy sheathedBi2Sr2CaCu2O8+x powder-in-tube conductors (Bi2212) for use in high-field magnets, it is importantto understand the quench behavior and limiting criteria of a quench, includingconditions that will result in a decrease in critical current. Even thoughthe quench characteristics of low-temperature superconductors NbTi andNb3Sn are well understood, there is still a lack of data and understanding of what conditionsduring quenches in high-temperature superconducting (HTS) materials cause degradationof the conductor. In this investigation, quenches are induced in short samples of Bi2212tape conductors with local heat disturbances using a resistive heater. The voltage andtemperature evolution during quenching were recorded and analyzed to determine thenormal zone propagation velocity. Furthermore, the quench conditions where varied toidentify the threshold quench conditions which result in conductor damage. Theseconditions are quantified in terms of three parameters: the maximum temperature(Tmax), the maximum rate of temperature increase(dT/dt|max) and the maximumtemperature gradient (dT/dx|max).It is found that the normal zone propagation velocity for Bi2212 tape conductor is20–30 mm s−1. The samples showed high sensitivity during quench, exhibiting large losses in criticalcurrent under certain conditions. In all cases the sections of the tape closest to the quenchinitiation exhibited the highest peak temperatures and loss in critical current.It was found that conductor damage is avoided under the following conditions:dT/dt|max<250 K s−1,dT/dx|max<100 K cm−1 and Tmax<250 K.

Polarization microscopy probe for applications in a high magnetic field

A microscopy probe designed for use in high field magnets is presented. It was developed to observe and continuously measure the position and shape of a magnetically driven grain boundary in a magnetically anisotropic material. The approach utilizes the anisotropy of reflectivity of visible light for orientation contrast to determine the boundary location. The major components of the system are the polarizing microscope with video camera and the sample chamber with a resistive heater for annealing at elevated temperatures up to 500 degrees C in an inert gas atmosphere. The results of test measurements are presented.

Magnetic-Field-Induced Synthesis of Magnetic γ-Fe2O3 Nanotubes

We report a novel method for synthesizing γ-Fe2O3 nanotubes based on the use of high magnetic field. The characterization data confirm the formation of well crystalline γ-Fe2O3 nanotubes, ranging from ∼300 nm in diameter and up to ∼30 µm in length. They exhibit room-temperature typical ferromagnetic behavior.

Hybrid two-dimensional navigator correction: A new technique to suppress respiratory-induced physiological noise in multi-shot echo-planar functional MRI

A troublesome source of physiological noise in functional magnetic resonance imaging (fMRI) is due to the spatio-temporal modulation of the magnetic field in the brain caused by normal subject respiration. fMRI data acquired using echo-planar imaging are very sensitive to these respiratory-induced frequency offsets, which cause significant geometric distortions in images. Because these effects increase with main magnetic field, they can nullify the gains in statistical power expected by the use of higher magnetic fields. As a study of existing navigator correction techniques for echo-planar fMRI has shown that further improvements can be made in the suppression of respiratory-induced physiological noise, a new hybrid two-dimensional (2D) navigator is proposed. Using a priori knowledge of the slow spatial variations of these induced frequency offsets, 2D field maps are constructed for each shot using spatial frequencies between ±0.5 cm−1 in k-space. For multi-shot fMRI experiments, we estimate that the improvement of hybrid 2D navigator correction over the best performance of one-dimensional navigator echo correction translates into a 15% increase in the volume of activation, 6% and 10% increases in the maximum and average t-statistics, respectively, for regions with high t-statistics, and 71% and 56% increases in the maximum and average t-statistics, respectively, in regions with low t-statistics due to contamination by residual physiological noise.

Is 4.0 T MRI advantageous for current source localization of weak magnetic fields?

Abstract. Recently, several methods have been proposed for measuring the magnetic fields produced by weak electric currents using magnetic resonance imaging (MRI), including a theoretically described technique for source localization of current dipoles using signal phase. We have investigated whether these results can be replicated experimentally using 4.0 Tesla (T) MRI with a dipole phantom. While the use of high magnetic fields is advantageous in conventional functional MRI, it is unclear, a priori, whether MR current source localization also benefits. This study presents the technical challenges in MRI current source imaging and quantifies sensitivity using a phantom containing a current dipole.

Mobility Characterization by Magnetoresistance in Ultra Thin SOI Ring-FETs

We report a novel use of magneto-resistance (MR) to estimate carrier mobility in ultra-thin virgin SOI wafers. Measurements were made in pseudo-MOSFET configuration with electrical contacts in Corbino geometry and substrate biasing. The use of high magnetic fields enabled accurate extraction of electron mobility as a function of gate voltage (effective field) and film thickness. The MR mobility values are large and correlate well with the field-effect mobility determined in pseudo- MOSFETs at zero magnetic field.

Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

The use of high magnetic fields increases the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS) of brain metabolites. Practical limitations arise, however, from susceptibility-induced field distortions, which are increased at higher magnetic field strengths. Solutions to this problem include optimized shimming, provided that active, i.e., electronic, shimming can operate over a sufficient range. To meet our shim requirements, which were an order of magnitude greater than the active shim capacity of our 7 T MR system, we developed a combined passive and active shim approach. Simple geometries of ferromagnetic shim elements were derived and numerically optimized to generate a complete set of second-order spherical harmonic shim functions in a modular manner. The major goals of the shim design were maximization of shim field accuracy and ease of practical implementation. The theoretically optimized ferro-shim geometries were mounted on a cylindrical surface and placed inside the magnet bore, surrounding the subject’s head and the RF coil. Passive shimming generated very strong shim fields and eliminated the worst of the field distortions, after which the field was further optimized by flexible and highly accurate active shimming. Here, the passive-shimming procedure was first evaluated theoretically, then applied in phantom studies and subsequently validated for in vivo 1H MRS in the macaque visual cortex. No artifacts due to the passive shim setup were observed; adjustments were reproducible between sessions. The modularity and the reduction to two pieces per shim term in this study is an important simplification that makes the method applicable also for passive shimming within single sessions. The feasibility of very strong, flexible and high-quality shimming via a combined approach of passive and active shimming is of great practical relevance for MR imaging and spectroscopy at high field strengths where shim power is limited or where shimming of specific anatomical regions inherently requires strong shim fields.

Pulsed magnetization transfer imaging with body coil transmission at 3 Tesla: Feasibility and application

Pulsed magnetization transfer (MT) imaging has been applied to quantitatively assess brain pathology in several diseases, especially multiple sclerosis (MS). To date, however, because of the high power deposition associated with the use of short, rapidly repeating MT prepulses, clinical application has been limited to lower field strengths. The contrast-to-noise ratio (CNR) of MT is limited, and this method would greatly benefit from the use of higher magnetic fields and phased-array coil reception. However, power deposition is proportional to the square of the magnetic field and scales with coil size, and MT experiments are already close to the SAR limit at 1.5T even when smaller transmit coils are used instead of the body coil. Here we show that these seemingly great obstacles can be ameliorated by the increased T(1) of tissue water at higher field, which allows for longer maintenance of sufficiently high saturation levels while using a reduced duty cycle. This enables a fast (5-6 min) high-resolution (1.5 mm isotropic) whole-brain MT acquisition with excellent anatomical visualization of gray matter (GM) and white matter (WM) structures, and even substructures. The method is demonstrated in nine normal volunteers and five patients with relapsing remitting MS (RRMS), and the results show a clear delineation of heterogeneous lesions.

Development of a 20 Tesla Superconducting Magnet

A 20 T superconducting magnet was designed and developed for studying physical properties. The cryostat is an open neck Dewar vessel with a liquid nitrogen reservoir and radiation shield. A lambda plate is built into the magnet support assembly. The operating temperature can be reduced from 4.2 K to 2.2 K by pumping liquid helium through a throttle valve under the lambda plate. The central magnetic field is 20 T at 2.5 K (18 T at 4.2 K). The field homogeneity is less than +/-0.1% in a 10 mm diameter sphere volume. The inner diameter of the cold clear bore is 52 mm. The outer diameter of the coil is 349 mm. The height of the coil, excluding the magnet protection circuit, is 560.5 mm. The coil is wound with the specially developed wires for use in high magnetic fields. In the case of Nb3Sn wires, a marked increase of the critical current density is achieved with higher tin content in the bronze matrix. The wires are more resistant to external stress with a rectangular cross section. We hung the magnet into the Dewar vessel and energized it. At first the magnet was operated at liquid helium temperature 4.2 K. As a result, the magnetic field rose to over 18 T with no training quench. The maximum magnetic field was 18.5 T. Then we energized the magnet after reducing the operating temperature to 2.2 K. Finally, the magnetic field rose to over 20 T with no quench. The result of this study shows that this magnet delivers a high magnetic field stably and easily