Linear Field Gradient Research Articles

A large beam high efficiency radio frequency neutron spin flipper.

A design for a radio frequency (RF) neutron spin flipper obtained from magneto-static and neutron spin transport simulations is presented. The RF flipper constructed from this design provides a flipping probability of 0.999 or better for a beam size 6cm wide and 15cm high and a wavelength band between 0.4 and 0.6nm. Three permanent magnet guide field sections with air gaps provide a linear field gradient along the beam propagation direction over a large cross-sectional area. An RF oscillator based on coupling the resonant coil of a Hartley oscillator to the excitation coil was developed, which provides a higher current and, thereby, a larger RF amplitude, as compared to a conventional RF power amplifier. Two opaque He3 neutron spin filters were employed to measure the flipping probability of the flipper with very high precision. A spatially uniform flipping probability of 0.9995(2) or higher was measured over the large cross-sectional area neutron guide. This RF neutron spin flipper will be employed in a polychromatic beam reflectometer at the National Institute of Standards and Technology Center for Neutron Research. This design can be applied to other polarized neutron instruments or applications requiring a very high continuous flipping probability of the neutron spin for a large cross-sectional area beam.

Internal Force Analysis of Parabolic Arch Considering Shear Effect under Gradient Temperature

This paper theoretical analysis the internal force of the fixed parabolic arches under radient temperature gradient field incorporating shear deformations. The effective centroid of the arch-section under linear temperature gradient is derived. Based on force method and energy method, the analytical solutions of the internal force of fixed parabolic arches at pre-buckling under linear temperature gradient field are derived. A parameter study was carried out to study the influence of linear temperature gradient on the internal force of the fixed parabolic arches with different rise-span ratio and varying slenderness ratio. It is found that the temperature gradient and the rise-span ratio has a significant influence on the internal force of the parabolic arches, the influence of shear deformation causes the bending moment increase while the axial force decreases, and the axial force of parabolic arches decreases as the rise-span ratio increases.

Development and validation of 3D MP-SSFP to enable MRI in inhomogeneous magnetic fields.

We demonstrate the feasibility of MRI with missing-pulse steady-state free precession (MP-SSFP) in a 4T magnet with artificially degraded homogeneity. T1 , T2 , and diffusion contrast of MP-SSFP was simulated with constant and alternate radiofrequency (RF) phase using an extended phase graph. To validate MP-SSFP performance in human brain imaging, MP-SSFP was tested with two types of artificially introduced inhomogeneous magnetic fields: (1) a pure linear gradient field, and (2) a pseudo-linear gradient field introduced by mounting a head-gradient set at 36 cm from the magnet isocenter. Image distortion induced by the nonlinear inhomogeneous field was corrected using B0 mapping measured with MP-SSFP. The maximum flip angle in MP-SSFP was limited to ≤10° because of the large range of resonance frequencies in the inhomogeneous magnetic fields tested in this study. Under this flip-angle limitation, MP-SSFP with constant RF phase provided advantages of higher signal-to-noise ratio and insensitivity to B1+ field inhomogeneity as compared with an alternate RF phase. In diffusion simulation, the steady-state magnetization in constant RF phase MP-SSFP increased with an increase of static field gradient up to 8 to 21 mT/m depending on simulation parameters. Experimental results at 4T validated these findings. In human brain imaging, MP-SSFP preserved sufficient signal intensities, but images showed severe image distortion from the pseudo-linear inhomogeneous field. However, following distortion correction, good-quality brain images were achieved. MP-SSFP appears to be a feasible MRI technique for brain imaging in an inhomogeneous magnetic field.

Highly Shielded Gradient Coil Design for a Superconducting Planar MRI System.

In a planar, superconducting magnetic resonance imaging (MRI) system, the gradient assembly is placed into the groove of the SC magnet. Conventional gradient coil design method considers the shielding of pole face only, but neglects the surrounding metal structures at the coil side, thus leading to large stray field leakage and resulting in serious eddy current artefact. A novel coil shielding method was proposed in this work by a full consideration of the stray fields on the pole face and also the coil ends. The gradient coil design exemplification of a 0.7T planar superconducting MRI system was presented. In the new design, the maximum stray field at the surface of the ambient structure was reduced more than six times for the transverse coils and around four times for the longitudinal coil. The highly shielded gradient coil also produced linear gradient fields (<5%) over the imaging volume.

In-Plane Instability of Fixed Arches under Linear Temperature Gradient Field and Uniformly Distributed Radial Load

This paper focuses on an in-plane instability analysis of fixed arches under a linear temperature gradient field and a uniformly distributed radial load, which has not been reported in the literature. Combining a linear temperature gradient field and uniformly distributed radial load leads to the changes in axial expansion and curvature of arches, producing the complex in-plane nonuniform bending moment and axial force. Therefore, it is necessary to explore the in-plane thermoelastic mechanism behavior of fixed arches under a linear temperature gradient field and a uniformly distributed radial load in the in-plane instability analysis. Based on the energy method and the exact solutions of internal force before instability, the analytical solutions of the critical uniformly distributed radial load considering the linear temperature gradient field associated with in-plane thermoelastic instability of arches are derived. Comparisons show that agreements of analytical solutions against FE (finite element) results are excellent. Influences of various factors on in-plane instability are also studied. It is found that the change of the linear temperature gradient field has significant influences on the in-plane instability load. The in-plane instability load decreases as the temperature differential of the linear temperature gradient field increases.

Portable Low-Cost MRI System Based on Permanent Magnets/Magnet Arrays

Portable low-cost MRI systems have the potential to enable point-of-care and timely MRI diagnosis, and to make this imaging modality available to routine scans and to underdeveloped areas. With simplicity, no maintenance, no power consumption, and low cost, permanent magnets or arrays are attractive to use as a source of magnetic field to realize portability and low cost for a scanners. However, when taking the Fourier imaging approach and using linear gradient fields, homogeneous fields are required, thus either a bulky magnet is needed, or the imaging volume is too small to image an organ if the magnet is scaled down. Recently, with the progress on image reconstruction based on non-linear gradient field, field patterns without spatial-linearity can be used as spatial encoding magnetic fields to encode MRI signals for imaging. As a result, the requirements for the homogeneity of the field can be relaxed, which allows permanent magnets(arrays) with reduced sizes, reduced weight to image bigger volumes covering organs such as a head. It offers chances to construct a truly portable low-cost MRI scanner. For this exciting potential application, permanent magnets(arrays) have attracted increased attention. A magnet(array) is strongly associated with the imaging volume, reconstruction methods, and RF excitation and coils, etc. through field patterns and homogeneity. This paper offers a review on permanent magnets(arrays) of different kinds, especially those can be used for spatial encoding towards the development of portable and low-cost MRI systems. It is aimed to familiarize the readers with relevant knowledge, literature, and the latest updates of the development on permanent magnets for MRI. Perspectives on and challenges of using permanent magnets to supply a patterned magnetic field, without spatial-linearity nor high homogeneity, for reconstruction in a portable setup are discussed.

Dynamic per slice shimming for simultaneous brain and spinal cord fMRI.

Simultaneous brain and spinal cord functional MRI is emerging as a new tool to study the central nervous system but is challenging. Poor B0 homogeneity and small size of the spinal cord are principal obstacles to this nascent technology. Here we extend a dynamic shimming approach, first posed by Finsterbusch, by shimming per slice for both the brain and spinal cord. We shim dynamically by a simple and fast optimization of linear field gradients and frequency offset separately for each slice in order to minimize off-resonance for both the brain and spinal cord. Simultaneous acquisition of brain and spinal cord fMRI is achieved with high spatial resolution in the spinal cord by means of an echo-planar RF pulse for reduced FOV. Brain slice acquisition is full FOV. T2*-weighted images of brain and spinal cord are acquired with high clarity and minimal observable image artifacts. Fist-clenching fMRI experiments reveal task-consistent activation in motor cortices, cerebellum, and C6-T1 spinal segments. High quality functional results are obtained for a sensory-motor task. Consistent activation in both the brain and spinal cord is observed at individual levels, not only at group level. Because reduced FOV excitation is applicable to any spinal cord section, future continuation of these methods holds great potential.

Phantom-based field maps for gradient nonlinearity correction in diffusion imaging.

Gradient coils in magnetic resonance imaging do not produce perfectly linear gradient fields. For diffusion imaging, the field nonlinearities cause the amplitude and direction of the applied diffusion gradients to vary over the field of view. This leads to site- and scan-specific systematic errors in estimated diffusion parameters such as diffusivity and anisotropy, reducing reliability especially in studies that take place over multiple sites. These errors can be substantially reduced if the actual scanner-specific gradient coil magnetic fields are known. The nonlinearity of the coil fields is measured by scanner manufacturers and used internally for geometric corrections, but obtaining and using the information for a specific scanner may be impractical for many sites that operate without special-purpose local engineering and research support. We have implemented an empirical field-mapping procedure using a large phantom combined with a solid harmonic approximation to the coil fields that is simple to perform and apply. Here we describe the accuracy and precision of the approach in reproducing manufacturer gold standard field maps and in reducing spatially varying errors in quantitative diffusion imaging for a specific scanner. Before correction, median B value error ranged from 33 - 41 relative to manufacturer specification at 100 mm from isocenter; correction reduced this to 0 - 4. On-axis spatial variation in the estimated mean diffusivity of an isotropic phantom was 2.2% - 4.1% within 60 mm of isocenter before correction, 0.5% - 1.6% after. Expected fractional anisotropy in the phantom was 0; highest estimated fractional anisotropy within 60 mm of isocenter was reduced from 0.024 to 0.012 in the phase encoding direction (48% reduction) and from 0.020 to 0.006 in the frequency encoding direction (72% reduction).

连续线性温度梯度场对啁啾脉冲放大系统中啁啾光纤光栅的色散调节效应

连续线性温度梯度场对啁啾脉冲放大系统中啁啾光纤光栅的色散调节效应

LINEAR AND NONLINEAR THERMOELASTIC ANALYSES OF BEAMS UNDER TEMPERATURE GRADIENT

In a structure, beams are often connected with other members such as columns, which provide considerable restraints against the rotation and extension of the beam ends. When a beam is subjected to an in-plane temperature gradient field, the temperature gradient tends to change the curvature of the beam in the transverse direction and expand the beam in the axial direction. The restrained actions will produce bending moments and compressive forces in the beam, which increase with an increase of the temperature differential and average temperature of the temperature gradient field. When these actions reach critical values, the elastically restrained beam may bifurcate from its primary in-plane equilibrium state to a lateral-torsional buckled equilibrium configuration. This paper carries out linear and nonlinear thermoelastic analyses of an elastically restrained beam of doubly symmetric open thin-walled cross-section that is subjected to a linear temperature gradient field over its cross-section. It is found that geometric nonlinearity influences the thermoelastic responses of the beam to the temperature changes significantly. The influence decreases with an increase of the stiffness of the elastic restraints.

Out-of-Plane Buckling of Beams Having In-Plane Elastic Restraints under Gradient Thermal Action

AbstractWhen a beam is subjected to an in-plane linear temperature gradient field, the thermal effects of the temperature gradient field tend to change the curvature of the beam in the transverse direction and to expand the beam in the axial direction. The rotation and expansion of the ends of the beam may be restrained elastically by its supports or by adjacent members such as columns. The restrained thermal actions will produce bending moments and compressive forces in the beam, and when these actions reach critical values the elastically restrained beam may bifurcate from its primary in-plane equilibrium configuration to an out-of-plane buckled equilibrium configuration. This paper investigates the thermoelastic out-of-plane buckling of an elastically restrained beam with a doubly symmetric cross section that is subjected to an in-plane linear temperature gradient field. The analytical solution for the critical temperature gradient for thermoelastic out-of-plane buckling of the beam is derived. It is f...

TU-EF-BRA-01: NMR and Proton Density MRI of the 1D Patient

TU-EF-BRA-01: NMR and Proton Density MRI of the 1D Patient

Visualization of Acid Treatments With 3D Nuclear-Magnetic-Resonance Imaging

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 168198, “The First Visualization of Acid Treatments on Carbonates With 3D Nuclear-Magnetic-Resonance Imaging,” by M. Krebs, SPE, Clausthal University of Technology; B. Lungwitz, SPE, A. Souza, SPE, A. Pépin, SPE, S. Montoya, P. Schlicht, and A. Boyd, SPE, Schlumberger; and E. Vidoto, R. Polli, and T. Bonagamba, University of Sao Paulo, prepared for the 2014 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, USA, 26–28 February. The paper has been peer reviewed and is scheduled to appear in SPE Journal. Core-flow tests are usually conducted to test and model stimulation treatments at laboratory scale, to predict the performance of such treatments in carbonate reservoirs. The visualization of wormholes created within core-flow tests requires novel technologies for evaluation and pathway-prediction purposes. Unfortunately, past visualization techniques were always associated with the destruction of the core sample. This paper describes the use of nuclear-magnetic-resonance imaging (NMRI) as a nondestructive visualization method in correlation with 3D rock imaging. Introduction The experiments described in the complete paper were conducted with different core samples such as Indiana limestone, Silurian dolomite, Winterset limestone, and carbonate outcrops. The core samples were selected according to their properties, which greatly differ for each type with respect to their origin. The core-flow tests or acidizing tests performed in this study serve the purpose of simulating field treatments at laboratory scale. NMRI applies a gradient static magnetic field such that the strength of the field decreases with increasing distance away from the source. The Larmor frequency becomes dependent on the position of the nuclei, which will be associated with a single value of the gradient magnetic field. Hence, it is possible to locate the nuclei in a region radially around the source. More specifically, NMRI uses mostly pulsed gradient magnetic fields that belong to a group of linear gradient fields in order to link acquired signals to a specific volume element. Therefore, it is possible to locate the nuclei more exactly in the 3D space instead of only defining a region. The gradient fields are used for spatial encoding to acquire a location-dependent signal, and they are the slice-selection gradient, phase-encoding gradient, and frequency-encoding gradient.

TU-EF-BRA-03: Free Induction Decay (without the Decay) and Spin-Echo Imaging

TU-EF-BRA-03: Free Induction Decay (without the Decay) and Spin-Echo Imaging

TU‐EF‐BRA‐04: Into 2, 3, and 4 Dimensions

TU‐EF‐BRA‐04: Into 2, 3, and 4 Dimensions

TU-EF-BRA-02: Longitudinal Proton Spin Relaxation and T1-Imaging

TU-EF-BRA-02: Longitudinal Proton Spin Relaxation and T1-Imaging

TU-EF-BRA-00: MR Basics I

TU-EF-BRA-00: MR Basics I

A compact high-performance low-field NMR apparatus for measurements on fluids at very high pressures and temperatures

We discuss an innovative new high-performance apparatus for performing low-field Nuclear Magnetic Resonance (NMR) relaxation times and diffusion measurements on fluids at very high pressures and high temperatures. The apparatus sensor design and electronics specifications allow for dual deployment either in a fluid sampling well logging tool or in a laboratory. The sensor and electronics were designed to function in both environments. This paper discusses the use of the apparatus in a laboratory environment. The operating temperature and pressure limits, and the signal-to-noise ratio (SNR) of the new system exceed by a very wide margin what is currently possible. This major breakthrough was made possible by a revolutionary new sensor design that breaks many of the rules of conventional high pressure NMR sensor design. A metallic sample holder capable of operating at high pressures and temperatures is provided to contain the fluid under study. The sample holder has been successfully tested for operation up to 36 Kpsi. A solenoid coil wound on a slotted titanium frame sits inside the metallic sample holder and serves as an antenna to transmit RF pulses and receive NMR signals. The metal sample holder is sandwiched between a pair of gradient coils which provide a linear field gradient for pulsed field gradient diffusion measurements. The assembly sits in the bore of a low-gradient permanent magnet. The system can operate over a wide frequency range without the need for tuning the antenna to the Larmor frequency. The SNR measured on a water sample at room temperature is more than 15 times greater than that of the commercial low-field system in our laboratory. Thus, the new system provides for data acquisition more than 200 times faster than was previously possible. Laboratory NMR measurements of relaxations times and diffusion coefficients performed at pressures up to 25 Kpsi and at temperatures up to 175 °C with crude oils enlivened with dissolved hydrocarbon gases (referred to as "live oils") are shown. This is the first time low-field NMR measurements have been performed at such high temperatures and pressures on live crude oil samples. We discuss the details of the apparatus design, tuning, calibration, and operation. NMR data acquired at multiple temperatures and pressures on a live oil sample are discussed.

Multi-coil magnetic field modeling

The performance of multi-coil (MC) magnetic field modeling is compared to dedicated wire patterns for the generation of spherical harmonic (SH) shapes as these are the workhorse for spatial encoding and magnetic field homogenization in MR imaging and spectroscopy. To this end, an example 48 channel MC setup is analyzed and shown to be capable of generating all first through fourth order SH shapes over small and large regions-of-interest relevant for MR investigations. The MC efficiency for the generation of linear gradient fields shares the same order of magnitude with classic and state-of-the-art SH gradient coils. MC field modeling becomes progressively more efficient with the synthesis of more complex field shapes that require the combination of multiple SH terms. The possibility of a region-specific optimization of both magnetic field shapes and generation performance with the MC approach are discussed with emphasis on the possible trade-off between the field accuracy and generation efficiency.MC shimming has been shown previously to outperform current SH shimming. Along with the efficiency gains of MC shimming shown here, the MC concept has the potential to (1) replace conventional shim systems that are based on sets of dedicated SH coils and (2) allow optimal object-specific shim solutions similar to object-specific RF coils.

Dielectrophoretic Mobility: An Approach for Measuring Electrical Properties of a Single Microbead in Aqueous Medium

An approach to measure the permittivity and conductivity of a small neutral particle in an aqueous conductive medium by utilizing dielectrophoretic mobility is presented. Particle mobility is caused by the dielectrophoretic force set off by the particle polarization in a linearly decreasing electric field. Polystyrene carboxylate microbeads of various sizes are used as particles, conductive potassium chloride (KCl) dilution is used as their carrier fluid, and a transparent hyperbolic quadrupole electrode setup is used to produce the linear field gradient. The setup and method are characterized by determining the permittivity and the conductivity of the microbeads in three different KCl concentrations of differing conductivities. The effects of carrier fluid conductivity and particle size on the acquired permittivity and conductivity values are examined. According to the results, low particle concentration combined with practical carrier conductivity (≫ 1 mS/m) provides repeatable permittivity values for the microbeads, irrespective of their size as far as the field strength is high enough to overcome the uncertainty caused by the Brownian motion. Microbead conductivities show strong dependence on the carrier fluid conductivity, thus providing a potential method for monitoring the relative conductivity variations of the carrier fluid.