Use Of High Magnetic Fields Research Articles

Modeling and suppression of respiration‐related physiological noise in echo‐planar functional magnetic resonance imaging using global and one‐dimensional navigator echo correction

A major source of noise in functional magnetic resonance imaging (fMRI) arises from modulations in the local magnetic field in the head due to motion of the subject's chest through the respiratory cycle, and this physiologic noise can nullify the gains in statistical power expected by the use of higher magnetic fields for fMRI. In particular, fMRI data acquired using echo-planar imaging (EPI) are very sensitive to these spatially and temporally varying respiration-induced frequency offsets. In this study, accurate 3D magnetic field maps in the head were measured and used to determine the frequency offsets at the two extremes of the respiratory cycle. From these maps, spatially dependent frequency variations from about -1.0 Hz to +1.5 Hz were measured in the brain through the respiratory cycle. Simulations of a typical axial EPI fMRI experiment acquired in the presence of this measured field variation were performed, demonstrating regional image intensity variations between 1 and 5% in single pixel time series. The inadequacy of either global or 1D navigator echo corrections to measure and suppress respiratory-induced noise in fMRI time series is demonstrated. The nature of the spatial variations observed suggests that 2D approaches should be considered.

EXCITON SPIN STATES IN NANOCRYSTAL QUANTUM DOTS REVEALED BY SPIN-POLARIZED RESONANT PHOTOLUMINESCENCE AND RAMAN SPECTROSCOPY

We performed spin-polarized resonant Raman and resonant photoluminescence excitation spectroscopy (also known as "fluorescence line narrowing") on ZnS -capped CdSe nanocrystal quantum dots in high magnetic fields to 33 Tesla and temperatures down to 1.7K, which allows detailed investigation of the excitonic spin states. In these experiments, spin-polarized electrons and holes are resonantly injected by circularly polarized light into colloidal quantum dots of specific size, using a narrowband tunable dye laser and a fiber-coupled probe that is specially-designed for use in high-field magnets. In addition to the expected broad features associated with excitonic recombination and Raman-like peaks associated with quantized acoustic phonons, the photoluminescence spectra measured at magnetic fields larger than 10 Tesla develop a sharp peak, which moves roughly linearly with applied magnetic field. Further, the energy of this high-field peak varies systematically as a function of nanocrystal size. However, unlike typical electron spin flip transitions, the mode energy extrapolates to a finite value at zero magnetic field, suggesting the existence of an additional size-dependent exchange mechanism.

Advances in high-field magnetic resonance imaging.

Among advances in magnetic resonance imaging (MRI), the increase of the magnetic field strength is perhaps one of the most significant. The use of high magnetic fields for in vivo magnetic resonance is motivated by a number of considerations. Advantages are increases in signal-to-noise ratio, blood-oxygenation level-dependent contrast, and spectral resolution, while disadvantages include potential reduction of contrast in anatomic imaging owing to lengthening of T1 and effects of susceptibility of high fields. To address these challenges, technical advances have been made in various aspects of MRI, allowing high-field MRI to provide exquisite morphological and functional details in clinical and research settings. This review provides an overview of technical issues and applications of high-field MRI.

Appunti dal Congresso ASNR di Vancouver Risonanza Magnetica a 3 Tesla

The theme of this year's 40th Congress of the American Society of Neuroradiology was 3 Tesla magnetic resonance. For many years, research into the use of magnetic resonance systems with 3 T magnets mainly focused on spectroscopy and functional magnetic resonance. More recent studies also used 3 T magnets for MR diagnosis, trying to optimize sequences for both anatomic imaging and for MR angiography and diffusion on the basis of protocols currently adopted with 1.5 T magnets1–3. The use of higher magnetic fields improves the signal/noise ratio and the chemical-shift sensitivity thereby enhancing spatial resolution (supported by the higher signal/noise ratio) and increasing the reliability of spectroscopy and functional MR imaging. The major technical problems encountered with 3 T systems are the increased number of artefacts due to magnetic sensitivity and chemical shift, the increase in tissue heating potential and the longer T1 longitudinal relaxation times. The main advantages of high magnetic field resonances are the higher signal/noise ratio, greater spectra dispersion, improved image resolution, faster acquisition times and greater sensitivity to differences in magnetic susceptibility. The drawbacks include a lower signal/noise ratio in relation to artefacts caused by magnetic susceptibility, longer T1 relaxation times, shorter T2 and T2* relaxation times, convergent tissue relaxation times, increased absorption of RF energy and a more inhomogeneous B1 signal. The advantages and limitations of higher magnetic field magnets have already been encountered at each stage in the development of MR technology. Once again, the best strategies need to be devised for the use of these new systems. Briefly, the increased signal produced by high field magnets (3 T) offers different advantages. In conventional diagnosis, the spatial resolution of the image is improved or acquisition times shortened, or when contrast is poor with current techniques as in functional MR or spectroscopy. Even the increase in artefacts due to magnetic susceptibility and chemical shift can be exploited advantageously. For example, the enhanced magnetic susceptibility produces a greater loss of signal intensity in sequences with gadolinium bolus perfusion, thereby improving assessment of brain haemodynamics and tissue vitality for a rational selection of candidates for stroke therapy and surgery. Studies were presented from different stroke centres in the United States using protocols comprising sequences for conventional anatomical investigation, weighted diffusion sequences, MR angiography and bolus perfusion. Image quality was equivalent or superior to that obtained with 1.5 T systems, especially angiographic sequences which were improved and faster with easier identification of the occluded branch. It was also demonstrated that the examination can be performed while the patient is receiving an infusion of thrombolytic drugs with real time monitoring of the pharmaceutical effect on the thrombosis. A group of researchers then presented their findings studying patients with an 8T MR system23–25. Exposure to such a high magnetic field was well tolerated and the study focused on evaluation of the small cerebral vessels exploiting sensitivity differences in blood oxygenation as deoxyhaemoglobin is paramagnetic and the effected of magnetic susceptibility exaggerated by such a high field. Using inhomogeneous local fields with gradient-echo images together with the signal/noise ratio of an 8 T system, minute cerebral vessels with a diameter of around 200 micron could be visualized. What emerged from the Congress presentations was that we can no longer be satisfied with the MR systems in use today when examinations which are currently long and cumbersome can be carried out more quickly, often with real time results and much shorter processing times. The impression was that the 1.5 T MR systems most of us use will soon be as obsolete as the 0.5 T systems and neuroradiologists' work will be increasingly shifted from morphological investigation to functional analysis, entailing new expertise. My only concern is the exposure of patients to such high magnetic fields, fearing the biological effects, especially when follow-up examinations have to be repeated. I hope that patients' welfare will not be disregarded in the search for ever greater morphological detail and that the new systems will be used when their true worth is of real benefit.

The Grenoble High Magnetic Field Laboratory

The Grenoble High Magnetic Field Laboratory (GHMFL) was built to meet the requirements of scientists for experiments implying the use of high magnetic fields. It is established in Grenoble, France. It is a French–German laboratory, jointly operated by the Max Planck Institut für Festkörperforschung, in Stuttgart (MPI-FKF) and the Centre National de la Recherche Scientifique (CNRS). Scientists have access to the magnetic field facility provided they have written a proposal for experiments in high magnetic fields and have obtained the approval of the Program Committee to carry out their research. Since the foundation of the laboratory, the research effort has been mainly devoted to solid state physics (i.e. research on metals, superconductors, semiconductors, magnetism, etc.) which can be performed in a volume as small as a few cm 3. Five years ago the laboratory has undertaken the development of 20 MW magnets. Two of them are now in operation and deliver fields up to 30 T in a 50 mm room temperature bore. A 20 MW magnet is composed of two concentric 10 MW magnets. The outer 10 MW part alone achieves a field up to 10 T in a 376 mm diameter bore. It is obvious that this large diameter is convenient for MHD flow studies under high magnetic fields.

Piezoelectrically driven rotator for use in high magnetic fields at low temperatures

We report the development of a piezoelectrically driven rotator for use in high magnetic fields at low temperatures. This rotator, controllable with two lead wires instead of a driving shaft, is designed to fit into a narrow sample space with a diameter of 13 mm and is capable of making rotations with a high angular resolution of ∼0.01° in magnetic fields reaching 33 T. In addition, estimated heat dissipation of less than 100 μW accompanied by rotation allows continuous rotations in a dilution refrigerator (T<100 mK).

Microscope system for use in high magnetic fields

We describe a microscope system, with video output, that operates within the confines and the high magnetic field of the 195 mm wide bore, 20 T resistive magnet at the National High Magnetic Field Laboratory. The accessibility of this system allows the user to vary the field of view and focus in situ and to produce images with up to 10 micron resolution. It allows for temperature regulation of samples to within 1 °C and in situ sample manipulation. Images of a biological specimen, Xenopus laevis embryos, in high magnetic field are presented.

Newly Applied Two-Dimensional Solid-State NMR Correlation Techniques for the Characterization of Organically Modified Silicates

This paper highlights the use of two-dimensional (2D) solid-state NMR correlation techniques to probe the chemical homogeneity of organically modified silicate networks. Specifically, 29Si{1H} heteronuclear correlation (HETCOR) NMR experiments have revealed the spatial proximity of the two types of Si units present in a gel obtained from co-hydrolysis of methyldiethoxysilane and triethoxysilane. Similar information has also been obtained by using 2D 1H homonuclear correlation NMR spectroscopy. Such experiments were only possible by combining the use of high magnetic field (14.10 T) with fast MAS spinning rate (30 kHz).

Conductor materials for high field magnets

High field magnets require the development and fabrication of large quantities of wire which possess both high strength and high electrical conductivity (the latter is usually expressed as a percentage of the IACS, international annealed copper standard, where 100%IACS = 1.7241 μ cm). This combination of properties can be obtained by the co-deformation of a variety of materials either in macroscopic form or in the form of as-solidified composites. A number of these materials have strength levels close to the theoretical strength and thus require detailed consideration of the mechanisms of strengthening which operate in materials with ultra fine scale microstructures. In addition, consideration must be given to the development of rational systems of design for the selection and fabrication of materials for use in high field magnets.

A sensitive and versatile torque magnetometer for use in high magnetic fields

We describe the design and construction of a sensitive dc torque magnetometer suitable for liquid as well as solid state samples. It can be used from room temperature down to very low temperatures (mK range) in magnetic fields produced either by superconducting or resistive magnets, in particular in the high fields (B≈30 T) produced by Bitter and hybrid magnets. The highest resolution that we have attained so far is about 0.3 pNm, which is mainly the result of a symmetric design. In typical laboratory conditions (at B=10 T) this corresponds to a magnetization resolution of about 3×10−11 emu. Other features include modularity, linearity, feedback and forward bias capabilities, and an in situ calibration of the signal.

Solvent Magnetization Artifacts in High-Field NMR Studies of Macromolecular Hydration

With the use of high magnetic fields and improved quality factor ratings of the probeheads in modern NMR spectrometers, radiation damping becomes more and more important. In addition, the demagnetizing field effect from protonated solvents gains significance with the increase of the magnetic field strength. During a typical NMR pulse sequence the magnetic fields caused by these effects become time-dependent, which makes the system nonlinear and may, for example, measurably influence the precession frequencies of all nuclei in the sample. Since radiation damping can affect signals that are several kilohertz away from the solvent resonance, the amplitude, phase, and frequency of the desired signals can be disturbed so as to give rise to spectral artifacts. In particular when difference methods are used to obtain the final spectrum, the data sets may be severely deteriorated by such artifacts. This paper investigates effects from the demagnetizing field and from radiation damping with a selection of pulse sequences in use for studies of macromolecular hydration, and strategies are described for the detection and elimination of the ensuing artifacts.

Electronic Structure of Manganese(III) Compounds from High‐Frequency EPR Spectra

The use of high magnetic field and high frequency in an unconventional spectrometer has provided very informative EPR spectra of a manganese(III) octahedral complex for the first time. The parameters of the spin Hamiltonian operator are in fair agreement with those calculated with ligand‐field theory. High‐frequency EPR is thus a powerful tool for the structural investigation of complexes that contain metal ions with integer spins.

Development and applications of in vivo clinical magnetic resonance spectroscopy.

4.1 CURRENT STATUS. While an extensive clinical literature of MRS of muscle, brain, heart and liver has been achieved, the MRS technique is not considered essential for routine diagnosis because it is inherently insensitive and metabolic changes tend to be small. However, MRS techniques have proven to be of considerable value for prognosis in some circumstances, notably for predicting outcome following hypoxic-ischaemic injury in the newborn and also in predicting graft viability following organ transplantation. The chemical specificity of MRS has been illustrated, and exploiting the non-invasive nature of the technique, metabolic fingerprinting of pathophysiological processes throughout the natural history of a wide variety of diseases is now being accomplished. Particularly exciting are the applications of 13C MRS for measuring hepatic and muscle glycogen levels, for example in diabetics, and the use of hepatic 31P MRS for assessing liver function in cirrhosis. Other areas of excitement are the applications of 1H MRS in assessing neuronal function in epilepsy and stroke, and for measuring the evolution of lactate in stroke and hypoxic-ischaemic encephalopathy. Emphasis on technique development continues, and applications still tend to be technology-led. The availability of routine clinical MRI systems with spectroscopy capabilities has given MRS studies wider applicability. The recent improvements in spatial resolution have been impressive and the technique is slowly becoming more quantitative. 4.2. FUTURE PERSPECTIVES. Given the flexibility of clinical magnetic resonance techniques, particularly magnetic resonance imaging, it is likely that MRI will be the diagnostic tool of choice in a wider range of diseases, such as multiple sclerosis, stroke, neurodegenerative conditions, sports injuries and in staging malignancies. Since proton magnetic resonance spectroscopy packages have become a routine addition to many MRI systems, it is feasible to select the MRI sequences of most value in highlighting anatomical and pathological abnormalities and to incorporate specifically selected MRS sequences to emphasize biochemical differences. Improvements in technical methodologies are central to further developments. For example, use of internal coils, such as implantable or endoscopic coils, will enable small regions of tissue to be studied in considerable detail, which may otherwise be inaccessible to measurement. Chemical MRS studies have benefited from the use of higher magnetic fields, and the same may be expected for clinical MRS studies. Whole-body magnets up to 4 T have been used in a few centres, and certainly 3 T systems are becoming more widely available with the recent tremendous interest in functional imaging. Certainly, better control of artefacts can be expected; for example, improved definition of spectral changes due to voluntary or involuntary movements. Wider use of proton decoupling methods will improve the specificity of the spectra, by allowing definitive assignments of overlapping resonances, as well as the sensitivity. Comparing PET and MRS studies, it is becoming increasingly obvious that both will be required in parallel to explore parameters of brain metabolism and function. The ability to measure 13C MR signals in the brain has been demonstrated, which allows measurements of glutamate and glucose turnover. MRS measurements have the advantage of not requiring a radioactive isotope, as well as being insensitive to activity-related changes in regional cerebral blood flow. Also the study of cerebral glucose metabolism by MRS is very promising, allowing a resolution and sensitivity comparable to PET. A combination of MRS and PET studies will allow the pathogenesis of neuropsychiatric disorders to be better understood. (ABSTRACT TRUNCATED)

A low temperature scanning tunneling microscope for use in high magnetic fields

We describe the design and performance of a low temperature (1.3–4.2 K) scanning tunneling microscope (STM) setup that can be used in magnetic fields up to 9.5 T. Due to the compact, rigid, and concentric design, the STM is very insensitive to external vibrations. The influence of external magnetic fields on the performance of the STM has been minimized by making a version completely out of titanium (instead of stainless steel). The STM routinely shows atomic resolution on various samples over the whole magnetic field range. We demonstrate the spectroscopic capability by local spectroscopy of the Abrikosov flux lattice on NbSe2.

Low temperature near field spectroscopy and microscopy

A low temperature near field scanning optical microscope has been constructed. Images of InGaAs quantum wires have been obtained at 4.2 K with a resolution < 100 nm (λ/8). Low temperature spectroscopy and images of 270 nm GaAs quantum dots show vertical dependence in both intensity and spectral shape in collection mode operation. The microscope is designed for use in high magnetic fields and top loads into a 1.2 inch cryostat. A pulled optical fiber [1] tip is mounted in a simple and novel lens collection scheme for reflection mode NSOM. Additional side mounted fibers with focusing GRIN lenses are used for detecting the fiber tip resonance, providing independent shear force microscopy.

Advances in dilution refrigeration in high magnetic fields

3He/ 4He dilution refrigerators have become a standard laboratory tool used to cool experiments to the milli-kelvin temperature range. Many of these applications involve the use of high magnetic fields. One system includes a 20.7 T superconducting magnet (the highest field ever achieved using multi filamentary superconducting wire in compact magnet form) and it can cool samples to 20 mK. Other systems have been designed for use with rapidly sweeping superconducting magnets, and even for pulsed liquid nitrogen cooled magnets, with maximum fields up to 60 T.

The use of high magnetic fields for the study of magnetism and superconductivity in intermetallic compounds

Magnetic fields have a large impact on the magnetic and superconducting properties of solids. High magnetic fields are required to reach magnetic saturation along a hard magnetic direction in a variety of rare-earth intermetallics, to break the ferrimagnetic moment configuration in specific 3d-4f intermetallics, to quench the strongly correlated electron states in heavy-fermion compounds, to reach the upper critical fields in several classes of superconductors, to study flux-pinning phenomena in the high-T/sub c/ superconductors, etc. In the present review, the attention is focused on the field interval 20-50 tesla. Experiments in this field range are the privilege of specialized high magnetic field laboratories. There is a lively activity in this area of research with the number of participating institutes continuously growing.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

4993414 Moving material projection imaging system using nuclear magnetic resonance

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.

Magnetic field production for particle physics

Particle physics studies require larger machines involving the use of higher magnetic fields. The dimensions of the magnetic parts of the accelerators, the intensity, homogeneity of the required fields and the particular geometry of some components impose the wide use of superconductivity. The main superconducting components of the accelerators are the R.F. cavities, beam magnets and detectors. In the present paper the superconducting components used in particle physics accelerators are revised and the involved today available materials are considered.

Platinum NMR thermometry in high magnetic fields

A platinum nuclear magnetic resonance (NMR) thermometer has been developed for use in high magnetic fields (10 T) at low dilution refrigerator temperatures (10 mK). The design consists of fine platinum wires cooled by a heat exchanger of pressed silver powder. Nuclear magnetic heat capacity, thermal and magnetic relaxation times and inhomogeneous line broadening are quantitatively addressed. A version of the thermometer that fits within a 1 mm diameter tube has been used to measure temperatures down to 7.9 mK in a dilute 3He– 4He solution in an 8 T field.