SQUID-Detected Magnetic Resonance Imaging in Microtesla Magnetic Fields

We describe studies of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) of liquid samples at room temperature in microtesla magnetic fields. The nuclear spins are prepolarized in a strong transient field. The magnetic signals generated by the precessing spins, which range in frequency from tens of Hz to several kHz, are detected by a low-transition temperature dc SQUID (Superconducting QUantum Interference Device) coupled to an untuned, superconducting flux transformer configured as an axial gradiometer. The combination of prepolarization and frequency-independent detector sensitivity results in a high signal-to-noise ratio and high spectral resolution (～1 Hz) even in grossly inhomogeneous magnetic fields. In the NMR experiments, the high spectral resolution enables us to detect the 10-Hz splitting of the spectrum of protons due to their scalar coupling to a ³¹P nucleus. Furthermore, the broadband detection scheme combined with a non-resonant field-reversal spin echo allows the simultaneous observation of signals from protons and ³¹P nuclei, even though their NMR resonance frequencies differ by a factor of 2.5. We extend our methodology to MRI in microtesla fields, where the high spectral resolution translates into high spatial resolution. We demonstrate two-dimensional images of a mineral oil phantom and slices of peppers, with a spatial resolution of about 1 mm. We also image an intact pepper using slice selection, again with 1-mm resolution. In further experiments we demonstrate T₁-contrast imaging of a water phantom, some parts of which were doped with a paramagnetic salt to reduce the longitudinal relaxation time T₁. Possible applications of this MRI technique include screening for tumors and integration with existing multichannel SQUID systems for brain imaging.     

Strong, Stray Static Magnetic Fields

This paper considers the generation of a strong stray magnetic field using rare-earth permanent magnets. Generating a strong field in a region displaced from a magnet is considered here in the context of magnetic resonance hardware design; however, optimizing magnetic field strength is of broad utility in many areas. The local magnetization direction in a permanent magnet is related to the field strength at a position outside of the magnet. This local direction can be optimized to obtain the strongest field. The paper derives, for a magnet of fixed volume, expressions for the dimensions yielding the strongest field. It discusses practical magnet arrangements that approximate the ideal magnetization pattern and demonstrates that a field greater that 80% of the optimal value can be obtained from a relatively simple arrangement of magnetized blocks.     

静磁场在材料生产过程中的应用研究评述

在材料电磁过程研究中，静磁场尤其是强磁场材料科学是当今世界的研究热点。本文从静磁场作用下生成的洛仑兹力和磁化力两个角度系统地归纳总结了静磁场技术在材料生产领域的应用原理和实践。对静磁场下的洛仑兹力，主要介绍了流体流动、波动和对流控制、电磁振动及电磁超声波等方面的研究现状；对强磁场下的磁化力，主要介绍了其在相变、结晶配向、磁悬浮、磁对流等方面的研究进展。最后对强磁场材料科学的研究趋势和发展前景做了展望。     

Cyclotron Resonance in ZnO Heterostructures at High Magnetic Fields

The carrier-density dependence of the cyclotron resonance was studied in ZnMgO/ZnO heterostructures between n=5.4×10¹² cm^?2 and n=7.5×10¹² cm^?2. The effective mass was obtained as m ^?=0.32m ₀, and the significant carrier-density dependence of the effective mass was not observed in the present samples. This is partially due to the suppression of the resonant polaron effect in the dense carrier system. Oscillatory behavior appears in the cyclotron resonance spectra at the higher magnetic field side of the resonant field. The oscillatory period is perfectly coincident with the one of the Shubnikov-de Hass oscillation.     

Passive Shim Design and a Shimming Approach for Biplanar Permanent Magnetic Resonance Imaging Magnets

This paper presents a new passive shim design method and a novel shimming procedure to correct the magnetic field inhomogeneities generated by C-shape permanent biplanar magnetic resonance imaging magnets. The method expresses the shim distribution as a sum of orthogonal functions multiplied by unknown amplitudes. The oscillating modes of the shim magnetization-thickness function are normalized within a finite disk. By minimizing the shim set weight and constraining the magnetization-thickness function, the method produces a continuous map of the required shim contribution. The map defines the shim shape and a discrete process then determines the regions where no shim contributions are needed. With this methodology, passive shims capable of generating magnetic field harmonics with minimal impurities and ferro-shim pieces can be generated. The paper reports a study of magnetic coupling among the iron pieces and its influence over the magnetic field harmonics of linear and nonlinear iron, and demonstrates that the exclusion of the magnetic coupling in the shimming process produces an unacceptable error in the final shimmed field homogeneity. The proper selection and arrangement of individual shim sizes produces a better conditioned field source matrix and hence improves the design. A number of examples show that the new method can effectively cancel target impurity harmonics while controlling high-order harmonics.     

基于矢量合成的静止式旋转磁场试验装置

论述了基于矢量合成的静止式旋转磁场试验装置的结构和原理,对其磁场进行了数值分析及实测,并给出其用于电能表质量检测的实例.通过控制正交线圈的电流实现对磁场方向、强度、频率、相位的控制,克服了传统单轴旋转线圈装置磁场方向不能为任意,因惯性约束无法迅速定位不便于自动控制,以及不满足标准要求的相位条件3点不足.提出了描述磁场方向一致性的特征量"平均偏角".数据表明,该装置的磁场均匀性不亚于传统装置.装置由计算机控制,能够自校准.     

A Testing Approach for MOS Circuit Using Single-Photon Detectors Under High Magnetic Fields

The MOS circuits must be tested thoroughly for insuring the reliability. A new testing approach for MOS circuits is presented in this paper, which makes use of single-photon detectors and high magnetic fields. If there are faults in the circuit under test, the photon emission from the circuit components is detected by a single-photon detector, the faults are located by the amount of the emitted photons. The following two techniques are proposed in this paper. First, the high magnetic field is applied to the circuit under test, i.e., the circuit is put in high magnetic field environment. To some extent this technique can solve the problem that some faults have poor strengths of emitted photons under general environment. Second, the special circuit input vectors are designed by using binary decision diagrams. The input vectors can make the positions of circuit components to produce signal transitions or switching behaviors, therefore the photon emission strengths of circuit components are enhanced. A lot of experimental results show that the faults in MOS circuits can be tested accurately by the approach proposed in this paper.     

New Approach to High-Pressure Nuclear Magnetic Resonance with Anvil Cells

A novel approach that uses radio-frequency microcoils in the high-pressure region of anvil cells with Nuclear Magnetic Resonance (NMR) experiments is described. High-sensitivity Al NMR data at 70 kbar for Al metal are presented for the first time. An expected decrease in the Al Knight shift at 70 kbar is observed, as well as an unexpected change in the local charge symmetry at the Al nucleus. The latter is not predicted by chemical structure analysis under high pressure.     

Smoothing of Magnetic Fields for Superconducting Bearings by Multilayers

Losses in magnetic bearings, whether conventional or superconducting, are caused by any magnetic inhomogeneities in the direction of motion. These can be smoothed by inserting a permeable sheet between the magnet and superconductor. However, this will also have the effect of reducing the field of the main magnet, which supplies the levitating force. Drawing on the parallel with shielding, where shields with air-gaps are more effective than solid shields of the same thickness (Rucker in Philos. Mag. 37:95, 1894), the same principle can be applied to smoothing layers. Analytic solutions are easy to obtain by using an anisotropic permeability in the smoothing layer. An air gap makes the normal permeability about unity while maintaining the parallel smoothing permeability which shorts out small wavelength inhomogeneities. For inhomogeneities which are mainly in the direction of motion we can striate the layer so that the in plane permeability is also inhomogeneous. The results are applied to a magnet track made of permanent magnets joined together with an effective gap of 0.1 mm. A suitable ferromagnetic layer can reduce the dip in field by a factor of 53 while only reducing the main field by 3 %.     

Equivalent Circuit Approach for Evaluating Low-Frequency Magnetic Fields in the Presence of Non-Ferromagnetic Plates

An equivalent circuit approach was presented in this paper to evaluate low-frequency magnetic fields in the presence of non-ferromagnetic plates. Thin plates and conductors, as well as others, were modeled as interconnected and coupled circuit components. The eddy currents on the plates were then solved with a circuit analysis technique. The resultant magnetic field was calculated with the Biot-Savart law. The proposed approach was tested on two plate models, and the results were compared with those obtained from the boundary-element method and the laboratory experiment. A good agreement among these results was found. The proposed method is effective in handling multi-plate problems at low frequency.      

Calculation of Charge Carrier Concentration Profile in a Wide Potential Well with Electric and Magnetic Fields

We investigated an equilibrium state of Fermi electrons in modulation doped structures with a wide quantum well in a strong parallel magnetic field. We studied the charge carrier system with a sufficiently high density, such that the de Broglie wavelength of electrons is smaller than the potential well width. We have formulated hydrodynamic equations for this carrier system both in the absence of magnetic field and in a parallel magnetic field. We have obtained analytical solutions for the charge carrier concentration as a function of coordinates in the potential well. In a quantum area near the interface, we carried out quantum mechanical calculations taking into account the effect of electric and magnetic fields. The concentration profile is presented for modulation doped Si/SiGe/Si heterostructures. We discuss large positive magnetoresistance in a strong parallel magnetic field in these structures.     

Electric and Magnetic Fields Generated by SHS

Fast-moving reaction zones (combustion fronts) may generate transient electric potentials (up to 1.5 V) and weak magnetic fields (up to 20 nT) due to ionization processes and ion movement within and behind the reaction front. These time-varying electric and magnetic fields are most probably generated by an electric double layer within individual particles and a temporal excess of ion carriers within the sample. The shape and magnitude of the induced temporal signals depend on the reaction zone propagation mode, reaction mechanism, and reactant properties.     

Theory of Cyclotron Resonance and Magneto-Optics in n- and p-Type InMnAs in Ultra-high Magnetic Fields

We present a theory for the electronic and optical properties of n- and p-type In_1–x Mn _x As in ultra-high magnetic fields. An eight-band effective mass model based on the Pidgeon–Brown model and including the wavevector dependence of the electronic states as well as the s–d and p–d exchange interactions with Mn d-electrons is used to determine the electronic states. The optical properties such as cyclotron resonance are computed using Fermi's golden rule. Comparison of the theory with ultra-high magnetic field (>50 T) cyclotron resonance experiments shows that the electron cyclotron resonance peak shifts with Mn doping and that the shift allows one to extract the Mn-electron/hole exchange parameters, and . The hole cyclotron resonance shows multiple resonance peaks, which we attribute a heavy to heavy and light to light hole transitions.     

T-matrix Approach for Calculating the Electromagnetic Fields Diffracted by a Corrugated,Anisotropic Grating

Abstract

The diffraction of electromagnetic plane waves incident on a corrugated grating made of a uniaxial dielectric material is studied using a fully vectorial treatment based on a T-matrix approach and the Rayleigh hypothesis. The optic axis is considered parallel to the mean surface of the periodic interface. The theory is exemplified numerically for the case of sinusoidal gratings made of sodium nitrate.     

Density and Thermal Conductivity Measurements for Silicon Melt by Electromagnetic Levitation under a Static Magnetic Field

The density and thermal conductivity of a high-purity silicon melt were measured over a wide temperature range including the undercooled regime by non-contact techniques accompanied with electromagnetic levitation (EML) under a homogeneous and static magnetic field. The maximum undercooling of 320 K for silicon was controlled by the residual impurity in the specimen, not by the melt motion or by contamination of the material. The temperature dependence of the measured density showed a linear relation for temperature as: ρ(T) = 2.51 × 10³−0.271(T−T _m) kg · m⁻³ for 1367 K < T < 1767 K, where T _m is the melting point of silicon. A periodic heating method with a CO₂ laser was adopted for the thermal conductivity measurement of the silicon melt. The measured thermal conductivity of the melt agreed roughly with values estimated by a Wiedemann–Franz law.     

High-Precision Temperature Measurement System for Magnetic Resonance Imaging

Measurement Techniques - To determine the compliance of an implantable medical device with the safety requirements in magnetic resonance imaging, an experimental assessment of the heating of this...     

Structure–Relaxivity Relationships of Magnetic Nanoparticles for Magnetic Resonance Imaging

Magnetic nanoparticles (MNPs) have been extensively explored as magnetic resonance imaging (MRI) contrast agents. With the increasing complexity in the structure of modern MNPs, the classical Solomon–Bloembergen–Morgan and the outer‐sphere quantum mechanical theories established on simplistic models have encountered limitations for defining the emergent phenomena of relaxation enhancement in MRI. Recent progress in probing MRI relaxivity of MNPs based on structural features at the molecular and atomic scales is reviewed, namely, the structure–relaxivity relationships, including size, shape, crystal structure, surface modification, and assembled structure. A special emphasis is placed on bridging the gaps between classical simplistic models and modern MNPs with elegant structural complexity. In the pursuit of novel MRI contrast agents, it is hoped that this review will spur the critical thinking for design and engineering of novel MNPs for MRI applications across a broad spectrum of research fields.