Electromagnetic articulograph based on a nonparametric representation of the magnetic field

Electromagnetic articulograph (EMA) devices are capable of measuring movements of the articulatory organs inside and outside the vocal tract with fine spatial and temporal resolutions, thus providing useful articulatory data for investigating the speech production process. The position of the receiver coil is detected in the EMA device on the basis of a field function representing the spatial pattern of the magnetic field in relation to the relative positions of the transmitter and receiver coils. Therefore, the design and calibration of the field function are quite important and influence the accuracy of position detection. This paper presents a nonparametric method for representing the magnetic field, and also describes a method for determining the receiver position from the strength of the induced signal in the receiver coil. The field pattern in this method is expressed by using a multivariate spline as a function of the position in the device's coordinate system. Because of the piecewise property of the basis functions and the freedom in the selection of the rank and the number of the basis functions, the spline function has a superior ability to flexibly and accurately represent the field pattern, even when it suffers from fluctuations caused by the interference between the transmitting channels. The position of the receiver coil is determined by minimizing the difference between the measured strength of the received signal and the predicted one from the spline representation of the magnetic field. Experimental results show that the error in estimating the receiver position is less than 0.1 mm for a 14 x 14-cm measurement area, and this error can be further reduced by using a spline-smoothing technique.     

Induced current of high temperature superconducting loops by combination of exciting coil and thermal switch

With its commercialization, the second-generation (2G) high temperature superconducting (HTS) RE—Ba—Cu—O (REBCO, RE is rare earth) tape is extensively applied to the superconducting magnets in the high magnetic fields. However, unlike low temperature superconducting (LTS) magnets, the HTS magnet cannot operate in the persistent current mode (PCM) due to the immature superconducting soldering technique. In this paper, an exciting method for two HTS sub-loops, so-called charging and load loops, is proposed by flux pump consisting of exciting coil and controllable thermal switch. Two HTS sub-loops are made of an REBCO tape with two slits. An exciting coil with iron core is located in one sub-loop and is supplied with a triangular waveform current so that magnetic field is generated in another sub-loop. The influence of magnetic flux on induced current in load loop is presented and verified in experiment at 77 K. The relationship between the induced magnetic flux density and the current on the sub-loops having been calibrated, magnetic flux density, and induced current are obtained. The results show that the HTS sub-loops can be excited by a coil with thermal switch and the induced current increases with magnetic flux of exciting coil increasing, which is promising for persistent current operation mode of HTS magnets.     

级联型爆磁压缩发生器的等效电路计算

 动态级联型爆磁压缩发生器由多级构成,后一级俘获前一级的磁通进而将能量放大。用镜像电流法计算装置等效电感和电阻,用磁通俘获模型计算两级间磁通耦合,并假设损耗电阻正比于直流电阻。用该等效电路方法计算了一种两级动态级联型爆磁压缩发生器的静态和动态电路参数,并对其输出电流波形进行了模拟,同实际测量和实验结果进行了比较,同时对该装置通过脉冲变压器对脉冲形成线的充电过程进行了简单的模拟计算。结果表明,该计算方法对级联型爆磁压缩发生器的优化设计和应用研究具有较好的指导作用。另外两级磁通俘获模型对于间接馈电（线圈或永磁体）装置模拟计算也有一定的参考价值。     

Estimating patient dielectric losses in NMR imagers

Dielectric losses in the patient may impair radiofrequency receiver coil sensitivity, and transmitter coil efficiency, in nuclear magnetic resonance (NMR) imagers. The frequency dependence of this loss mechanism is derived. Patient losses in a solenoidal head coil used for imaging heads were simulated by a cylindrical saline phantom. The frequency dependence of the loss introduced by the phantom can indicate whether dielectric losses in the patient will be significant compared to eddy current losses. The detuning caused by the phantom is used to calculate an upper limit for the distributed stray capacitance between coil and patient. Given the approximate conductivity of the patient, an upper limit for the dielectric loss can be estimated. Some methods of reducing patient dielectric losses are suggested.     

Magnetic measurement of transport AC losses in Bi-2223/Ag tapes

Magnetic nature of the losses in superconducting wire carrying AC current implies that it should be possible to determine these losses in a contactless way. Ribbon-like samples are quite favorable for such an experiment, because a notable portion of magnetic flux related to losses ‘escapes' the sample volume and can be detected by an appropriate pick-up coil. In this case, a model describing the AC current penetration into the tape, based, e.g., on the critical state model, allows one to derive the losses from the pick-up coil signal. Because this signal is proportional to the number of coil turns, extension of the accessible range of measured voltages (and losses) can be achieved. We demonstrate the data obtained on a 1 cm long portion of a low-loss multifilamentary tape carrying AC current with frequency 35 Hz. The pick-up coil technique allowed us to reach loss level more than one order below the experimental limit for direct measurements.     

Three-dimensional electromagnetic articulography: a measurement principle

A measurement principle of the three-dimensional electromagnetic articulographic device is presented. The state of the miniature receiver coil is described by five variables representing the position in the three-dimensional coordinate system and the rotation angles relative to it. When the receiver coil is placed in the magnetic field produced from the distributed transmitter coils, its state can be optimally estimated by minimizing the difference between the measured strength of the received signal and the predicted one using the known spatial pattern of the magnetic field. Therefore, the design and calibration of the field function inherently determine the accuracy in estimating the state of the receiver coil. The field function in our method is expressed in the form of a multivariate B spline as a function of position in the three-dimensional space. Because of the piecewise property of the basis function and the freedom in the selection of the rank and the number of basis functions, the spline field function has a superior ability to flexibly and accurately represent the actual magnetic field. Given a set of calibration data, the spline function is designed to form a smooth curved surface interpolating all of these data samples. Then, an iterative procedure is employed to solve the nonlinear estimation problem of the receiver state variables. Because the spline basis function is a polynomial, it is also shown that the calculation of the Jacobian or Hessian required to obtain updated quantities for the state variables can be efficiently performed. Finally, experimental results reveal that the measurement accuracy is about 0.2 mm for a preliminary condition, indicating that the method can achieve the degree of precision required for observing articulatory movements in a three-dimensional space. It is also experimentally shown that the Marquardt method is a better nonlinear programming technique than the Gauss-Newton or Newton-Raphson method for solving the receiver state problem.     

Spatial dependence of a differential shading artifact in images from coil arrays with reactive cross-talk at 1.5 T.

Reactive cross-talk causes leakage of the reception signal between neighboring coils of a receiver array. We present here experimental and computer-simulated NMR images (based upon a simple theory) to show, for an array of two coils, that the leakage (or secondary) signal is combined phase sensitively with the primary signal in each coil, to produce (in certain geometries) a differential shading artifact, manifest as a divot of missing intensity in the image derived from one (and only one) of the two coils. The asymmetry of this effect arises from the sense of the nuclear precession, and the afflicted coil may be swapped with its mate by reversing the direction of the static magnetic field. The artifact appears most clearly in transaxial images and is shown to be forbidden in certain types of saggital images. In a simplified theory for an array of two meshes (i.e., with only two degrees of freedom) the severity of the artifact depends upon the normalized coefficient of coupling (denoted eta and related to the cross-talk in decibels, psi, by psi=-20 log eta.) While the presence of input trap circuits in a typical array doubles the degrees of freedom and complicates both the circuit theory and the circuit measurements, the cross-talk is nonetheless shown to be given by an expression of the form psi=-20 log eta', where the new primed parameter eta' embodies the impedance-matching capacitance and the resistance of the scanner's preamplifiers, as well as the mutual reactance responsible for the cross-talk. The values of cross-talk inferred from the computer simulations of the image artifact are somewhat higher (by an estimated 3 to 6 dB) than those obtained by bench top measurements; but, given that the simulations unmistakably reproduce the unique and highly characteristic visual appearance of the artifact, the proposed model for its formation is claimed to be essentially correct. Finally, it is suggested that the artifact could be corrected by means of the filtered, edge-completed, reception profile described by Wald and co-workers (Wald et al., Magn. Reson. Med. 34, 433 (1995)).     

Surface and Gradiometer Coils near a Conducting Body: The Lift-off Effect

The use of surface coils in magnetic resonance is widespread. Examples include MRI, detection of subsurface aquifers by NMR, and, more recently, landmine detection by nuclear quadrupole resonance. In many of these cases a finite-sized sample to be examined is contained within a larger medium that is a poor electrical conductor, and eddy currents induced by the RF fields provide a loss mechanism that reduces the effective quality factorQof the transmitter and receiver coils. Here the losses induced in a circular surface coil (a horizontal loop antenna) separated a distance from a dissipative medium are calculated and compared to measurements. It is shown that often the overall efficiency of the coil for magnetic resonance can be improved by displacing the coil away from the conducting medium a prescribed “lift-off” distance. The use of a gradiometer as a surface coil is also examined, and it is shown by theory and experiment that in certain circumstances such a gradiometer can be more efficient than a conventional surface coil for inspection of conducting media.     

Optimization of NMR receiver bandwidth by inductive coupling.

We show, by theory and experiment, that inductive coupling can be used in overcoupled mode to widen the bandwidth of a high-Q NMR coil with only a negligible degradation of signal-to-noise ratio over the bandwidth of interest. The receiver bandwidth depends on the coupling coefficient between the NMR coil and the coupling coil rather than on the quality factor of the NMR coil alone. The overall bandwidth can be optimized by a judicious choice of the coupling coefficient. Moreover, this method permits wireless reception without the need for retuning and rematching despite changes of NMR coils or samples. This technique has been incorporated in a 0.1-T imager for clinical routine. It achieves a typical bandwidth five times greater than that using a classic 50-omega matching method.     

The noise immunity of gradiometer coils for14N NQR land mine detection: Practical limitations

Magnetic resonance measurements in the field, such as for land mine detection using¹⁴N nuclear quadrupole resonance (NQR), must deal with large levels of environmental radio-frequency interference. One approach to minimize the level of interference which enters the NQR receiver is the use of a coil with no magnetic dipole moment. Such a coil is, ideally, sensitive only to spatial gradients of the magnetic fields and is referred to as a gradiometer. It is straightforward to estimate the amount of reduction one can expect for an ideal gradiometer. Here it is shown that for¹⁴N NQR land mine detection in practice, the ultimate level of interference received can be expected to be significantly greater than what one would expect solely due to these spatial gradients. This is due to the fact that it is quite difficult to construct an ideal gradiometer.     

Single-sided mobile NMR apparatus using the transverse flux of a single permanent magnet

This study presents a simple design for a mobile, single-sided nuclear magnetic resonance (NMR) apparatus which uses the magnetic flux parallel to the magnetization direction of a single, disc-shaped permanent magnet polarized in radial direction. The stray magnetic field above the magnet is approximately parallel to the magnetization direction of the magnet and is utilized as the B₀ magnetic field of the apparatus. The apparatus weighs 1.8 kg, has a compact structure and can be held in one's palm. The apparatus generates a B₀ field strength of about 0.279 T at the center of apparatus surface and can acquire a clear Hahn echo signal of a pencil eraser block lying on the RF coil in one shot. Moreover, a strong static magnetic field gradient exists in the direction perpendicular to the apparatus surface. The strength of the static magnetic field gradient near the center of the apparatus surface is about 10.2 T/m; one-dimensional imaging of thin objects and liquid self-diffusion coefficient measurements can be performed therein. The available spatial resolution of the one-dimensional imaging experiments using a 5×5 mm horizontal sample area is about 200 μm. Several nondestructive inspection applications of the apparatus, including distinguishing between polyethylene grains of different densities, characterizing epoxy putties of distinct set times and evaluating the fat content percentages of milk powders, are also demonstrated. Compared with many previously published designs, the proposed design bears a simple structure and generates a B₀ magnetic field parallel to the apparatus surface, simplifying apparatus construction and simultaneously rendering the selection of the radiofrequency coil relatively flexible.     

High homogeneity B(1) 30.2 MHz Nuclear Magnetic Resonance Probe for off-resonance relaxation times measurements

This paper reports on design and construction of a double coil high-homogeneity ensuring Nuclear Magnetic Resonance Probe for off-resonance relaxation time measurements. NMR off-resonance experiments pose unique technical problems. Long irradiation can overheat the sample, dephase the spins because of B(1) field inhomogeneity and degrade the signal received by requiring the receiver bandwidth to be broader than that needed for normal experiment. The probe proposed solves these problems by introducing a separate off-resonance irradiation coil which is larger than the receiver coil and is wound up on the dewar tube that separates it from the receiver coil thus also thermally protects the sample from overheating. Large size of the irradiation coil also improves the field homogeneity because as a ratio of the sample diameter to the magnet (coil) diameter increases, the field inhomogeneity also increases (Blümich et al., 2008) [1]. The small receiver coil offers maximization of the filling factor and a high signal to the noise ratio.     

A novel frequency multiplier based on magnetoelectric laminate

We report a novel frequency multiplier based on the magnetoelectric (ME) effect in a simple multiferroic laminate, made up of an amorphous FeBSiC magnetic layer bonded onto a Pb(Zr,Ti)O₃ plate wrapped with a coil. By applying an input signal with a frequency f to the coil, an output signal with 2f can be generated from the PZT plate due to the ME coupling. This ME laminate-based device can be operated in a broad frequency range and switched by a low bias magnetic field, offering potential opportunities for frequency multipliers in electrical applications.     

永磁体 N-N 相对式排列提升旋转式高温超导磁通泵开路电压的仿真研究

旋转式高温超导磁通泵可以无接触地向超导线圈中注入直流电, 在超导磁体充电方面具有独特的优势.在本文中, 我们基于 H-A 方程耦合建立了一个磁通泵二维有限元模型, 分别模拟了三块永磁体、 五块永磁体在不同的排列方式下对磁通泵开路电压的影响. 与正常的 N 极向下的排列方式相比, N-N 相对式排列改进后能够提升磁通泵的开路电压; 改进式 Halbach 排列对开路电压几乎没有影响. 在50 Hz 的旋转频率下, 永磁体 N-N 相对式排列使开路电压提升13% . 这是由于永磁体 N-N 相对式排列后, 磁通将会被挤压, 从而产生有多个峰值的磁感应强度分布, 使等效电压波形从不对称的四重峰变为多重峰. 最后, 对永磁体宽度进行了参数化扫描来分析永磁体尺寸对开路电压的影响. 通过优化磁体结构设计, 可以控制磁感应强度分布, 提升磁通泵的开路电压, 为提升实验装置的输出性能提供一种新颖的设计方向.     

Magnetic field analysis in a diamond anvil cell for Meissner effect measurement by using the diamond NV~- center

Diamond negatively charged nitrogen-vacancy(NV-) centers provide an opportunity for the measurement of the Meissner effect on extremely small samples in a diamond anvil cell(DAC) due to their high sensitivity in detecting the tiny change of magnetic field. We report on the variation of magnetic field distribution in a DAC as a sample transforms from normal to superconducting state by using finite element analysis. The results show that the magnetic flux density has the largest change on the sidewall of the sample, where NV-centers can detect the strongest signal variation of the magnetic field. In addition, we study the effect of magnetic coil placement on the magnetic field variation. It is found that the optimal position for the coil to generate the greatest change in magnetic field strength is at the place as close to the sample as possible.     

EAST极向场线圈及磁测量系统的修正

采用网格法计算空间磁场,应用最小二乘法优化极向场线圈、单匝环、磁探针位置参数及标定系数获得了更准确的位置坐标,明显地改善了磁场和磁通的计算误差,也为等离子体平衡反演算法提供更准确的格林函数。     

Development of surface-coil-type resonators consisting of an irradiation and receiver coil system for EPR measurements at 700 MHz

Surface-coil-type resonators (SCRs) consisting of irradiation and receiver coils that are highly isolated from each other were developed for electron paramagnetic resonance (EPR) measurements. Their sample space was open to free space. For these coils, a circular single-turn one-loop receiver coil and a square single-turn twin-loop irradiation coil were fabricated. The transmission lines were set to resonate at about 700 MHz. A phantom (agar, including a nitroxide radical and physiological saline solution) is located on the receiver coil, and the irradiation coil is under the receiver coil. In this condition, the isolation between the receiver and irradiation SCRs was about 40 dB at the resonant frequency. When radiowaves that were divided from the line to the irradiation SCR were applied to the line from the receiver SCR at the appropriate phase and power to cancel the direct coupling between both SCRs, the isolation increased to more than 70 dB. In the conventional SCR, the noise level increased at high incident power. Because such an increase in the noise was not observed in the irradiation-receiver SCR system, high sensitivity at high incident power was obtained.     

填充因子对核磁共振谱仪灵敏度的影响

讨论了接收线圈填充因子对核磁共振谱仪灵敏度的影响,接收线圈结构参数对灵敏度的影响,核磁共振信号极限信噪比与双T电桥检测信噪比的相互关系,对通常的接收线圈(绕在外径7毫米,内径6毫米的管子外壁)进行了改进:将接收线圈放置在该管子的内壁,实验证实,应用CH₂₀₃核磁共振谱仪(北京分析仪器厂制造),5毫米外径样品管和1%乙基苯样品,灵敏度从20:1(用通常的接收线圈)提高到30:1(用改进了的接收线圈)。     

Equivalent circuit for coil-patient interactions in magnetic resonance imaging

An equivalent circuit is presented which accurately models the performance of magnetic resonance imaging receiver coils used with conducting samples. Coil-sample interaction is determined by measuring either the complex impedance or the associated resonant frequency and quality factor when samples of different conductivity are placed in the coil. The equivalent circuit component values are obtained from these data using a global nonlinear least squares fit. This equivalent circuit contains a minimum number of components necessary for understanding and quantifying the detuning and losses caused by electric and magnetic field coupling with the sample.     

A superconducting solenoid system which cancels fluctuations in the ambient magnetic field

An extra superconducting coil added to a standard, high-field solenoid results in a self-shielding solenoid system which utilizes flux conservation to passively shield an interior volume from changes in the ambient field, such as those from elevators or subways. For this first experimental demonstration, a highly homogeneous 6 T solenoid and an added coil were arranged in one of the geometries predicted to produce effective shielding. The fluctuations in the shielded high-field region are observed to be smaller than the fluctuations in the spatially uniform ambient magnetic field by a large factor of 156, confirming the general shielding principles presented earlier. This shielding is crucial for ongoing antiproton cyclotron resonance experiments and should be useful for nuclear magnetic resonance experiments and ion cyclotron resonance experiments and for other applications where high field and high stability are required simultaneously.