Biplanar gradient coil design by simulated annealing

Simulated annealing has been applied to the design of biplanar gradient coils for use in NMR microscopy. This method allows a variety of coil parameters to be considered, such as homogeneity over a specified region of interest (ROI), power dissipation, and efficiency. Chosen parameters are represented in an overall figure of merit which is then minimized by the simulated annealing approach. Unlike most analytical techniques which rely on the use of the stream function, with this technique the coil properties are calculated directly from the wire positions, so there is no stage of approximation between current density and the actual coil design. Using this technique, we have designed biplanar x and z gradient coils for use in an 11.7-T NMR microscope. In each case, the coils were composed of straight wire units. The plane spacing was set at 10 mm and the ROI was a central cube of side 6 mm. Starting from a design generated using the target field approach, simulated annealing was applied with the aim of minimizing the ratio of power dissipation per unit current to the square root of coil efficiency while maintaining adequate gradient homogeneity. The efficiency and power dissipation per unit current of the resulting x and z coils were 117 mT m^–1 A^–1 and 0.1 W A^–2, and 99 mT m^–1 A^–1 and 0.4 W A^–2, respectively.     

开放式磁共振系统中梯度磁场的快速计算方法

梯度线圈是磁共振系统的核心部件之一,其产生的梯度磁场是成像空间定位的物理基础。在开放式磁共振系统中,大量铁磁介质会导致实际梯度磁场偏离设计值,因此需要对实际梯度磁场进行校验。本文针对开放式磁共振的结构特点,提出了一种基于扇环形状单元的积分方程法,可以计算二维/三维模型中的梯度磁场。针对两种常见的开放式磁共振结构,通过将本文方法的计算结果与其他三种方法(直接法、镜像法和有限元法)进行对比,证明了本文方法在保证计算精度的同时,可以大幅减少计算时间,在重复计算和迭代优化中具有显著优势。实验结果证明了该算法具有良好的可靠性。     

均匀梯度磁场的研究

从分析单个圆线圈磁场特点出发，探讨利用圆线圈产生均匀梯度磁场的原理和方法，给出一种圆线圈装置的设计，并对磁场梯度均匀性进行数值估计．数值计算表明这种设计得到的磁场梯度均匀性相对误差高于位置偏移的二次方以上．     

Controlled<Emphasis Type="Bold"> E</Emphasis>-field gradient coils

Peripheral neural stimulation is a major problem in current gradient coil designs. Induced current problems in patients relate directly to gradient strength and modulation frequency. Current designs of gradient coil tend to limit ultra-high-speed imaging methods such as echo-planar imaging through the effect of induced currents which produce tingling sensations and involuntary muscle twitch. Neural stimulation could also trigger epileptic fits and/or cardiac fibrillation. For reduction of induced currents, an important aspect is the coil geometry. It is desirable to design the gradient coil in such a way as to prevent closed loop circulating currents within the body. Preliminary results using a four-sector gradient coil with rectangular geometry, operating in a low mutual coupling mode, indicate significant reduction in the E-field within the subject volume of the coil. Reduction in induced currents in the patient allows safer operation at higher magnetic field strengths together with faster scans currently prohibited through neural stimulation effects in standard coil geometries.     

Quiet MRI with novel acoustic noise reduction

Fast scan techniques, which are used to reduce scanning times, have raised scanning noise levels in magnetic resonance imaging (MRI) systems, resulting in greater patient discomfort and stress. It is well known that this noise is caused by vibration of the gradient coil due to the Lorentz forces generated by the current in the gradient coil, which is placed in a static magnetic field. We have confirmed that MRI noise can be substantially reduced by sealing the gradient coil in a vacuum chamber to block airborne vibration propagation, by supporting the gradient coil independently to block solid vibration propagation and by decreasing the eddy currents induced in RF coils, the RF shield and the static-field-magnet cryostat. Based on these findings, we have developed a silent MRI system in which scanning noise is markedly reduced under a wide range of scanning conditions.     

Quiet MRI with novel acoustic noise reduction.

Fast scan techniques, which are used to reduce scanning times, have raised scanning noise levels in magnetic resonance imaging (MRI) systems, resulting in greater patient discomfort and stress. It is well known that this noise is caused by vibration of the gradient coil due to the Lorentz forces generated by the current in the gradient coil, which is placed in a static magnetic field. We have confirmed that MRI noise can be substantially reduced by sealing the gradient coil in a vacuum chamber to block airborne vibration propagation, by supporting the gradient coil independently to block solid vibration propagation and by decreasing the eddy currents induced in RF coils, the RF shield and the static-field-magnet cryostat. Based on these findings, we have developed a silent MRI system in which scanning noise is markedly reduced under a wide range of scanning conditions.     

The modular (twin) gradient coil—high resolution, high contrast, diffusion weighted EPI at 1.0 Tesla

The modular (twin) gradient coil is a novel and effective approach to obtaining flexible and high power gradient performance without (i) peripheral nerve stimulation and (ii) the need for resonant or expensive high voltage gradient power supply units (PSU). This whole-body gradient system contains, on the same former, two independent sets of gradient coils, theconventional coil set and theshort-body coil set. Each gradient axis is able to operate, independently, in any one of three modes. The third,combined, mode is realized by connecting theconventional coils andshort-body coils in series. Through careful design of the shape and size of the linear volume of each mode of operation, the modular gradient coil is able to utilize the power, from a single gradient PSU, more efficiently and more appropriately, as determined by the application. In theshort-body andcombined modes the gradient fields are linear over a volume suitable for whole head/neck, liver and cardiovascular applications. In theconventional mode, a reduced performance is possible but over a much larger (conventional) imaging volume. Utilizing a semi-conductor switching arrangement it is possible to switch between modes in as little as 1 ms. By mixing different modes of operation on different gradient axes it is possible to utilize more efficiently, and safely, the properties of gradient performance best suited to the sequence requirements. Diffusion weighted EPI (DW-EPI) is a particular technique that demands the extremes of gradient system performance in terms of both amplitude and slew rate. DW-EPI has been implemented, using the modular gradient system, on a 1.0 Tesla whole-body MRI system. The preliminary results presented here serve to illustrate the advantages of the modular gradient coil in of itself as well as the direct benefits it provides for DW imaging at 1.0 Tesla.     

Principles and applications of FLASH NMR imaging

Routine clinical NMR scanners apply low-flip-angle gradient-echo sequences as fast-imaging modalities. Fast low-angle shot (FLASH) NMR imaging is the first version of a large family of fast gradient-echo methods. It is based on the application of reduced flip angles for NMR excitation, the acquisition of magnetic field gradient echoes, and considerably shortened repetition times. Under these conditions, transverse magnetization survives. This magnetization can be destroyed in spoiled FLASH or used for imaging in refocused FLASH. The measuring time of FLASH NMR images is dependent on gradient hardware and is under optimal technical conditions user selectable between less than 100 ms and 1 s. Short imaging times give the possibility to apply magnetization preparation before imaging. This technique allows the acquisition of image contrast with respect to any selected parameter, e.g.T ₁ T ₂, or diffusion constant. This FLASH version has been called snapshot-, turbo-, or magnetization-prepared RAGE.     

Design and fabriacation of a three-axis multilayer gradient coil for magnetic resonance microscopy of mice

There is great interest in the non-destructive capabilities of magnetic resonance microscopy for studying murine models of both disease and normal function; however, these studies place extreme demands on the MR hardware, most notably the gradient field system. We designed, using constrained current minimum inductance methods. and fabricated a complete, unshielded three-axis gradient coil set that utilizes interleaved, multilayer axes to achieve maximum gradient strengths of over 2000 mT m⁻¹ in rise times of less than 50 μs with an inner coil diameter of 5 cm. The coil was wire-wound using a rectangular wire that minimizes the deposited power for a given gradient efficiency. Water cooling was also incorporated into the coil to assist in thermal management. The duty cycle for the most extreme cases of single shot echo planar imaging (EPI) is limited by the thermal response and expressions for maximum rates of image collection are given for burst and continuous modes of operation. The final coil is capable of the collection of single shot EPI images with 6 mm field of view and 94 μm isotropic voxels at imaging rates exceeding 50 s⁻¹.     

基于主动磁检测技术的车辆检测新方法

为了给智能交通管理系统提供交通信息,提出了一种车辆检测的新方法.主动磁检测器包括一个发射线圈和一个接收线圈,发射线圈持续地辐射交流磁场,当车辆从检测器上方通过时,车辆的金属外壳中就会产生涡电流,从而产生涡流二次场,那么接收线圈周围的磁场就会发生改变.通过测量接收线圈周围的磁场是否改变来达到检测车辆的目的.检测概率为96...     

A method for the design of MRI radiofrequency coils based on triangular and pulse basis functions

This paper presents a numerical technique for the design of an RF coil for asymmetric magnetic resonance imaging (MRI) systems. The formulation is based on an inverse approach where the cylindrical surface currents are expressed in terms of a combination of sub-domain basis functions: triangular and pulse functions. With the homogeneous transverse magnetic field specified in a spherical region, a functional method is applied to obtain the unknown current coefficients. The current distribution is then transformed to a conductor pattern by use of a stream function technique. Preliminary MR images acquired using a prototype RF coil are presented and validate the design method.     

氧气浓度差磁致通风数值模拟

利用超导磁体提供的强梯度磁场，可以驱动具有不同氧气浓度的空气之间产生流动而形成通风。为研究氧气浓度差磁致通风的本质和作用机理，文中对该通风方式进行了理论分析，并就具体的超导磁体系统作用下的圆管内磁致通风进行了数值模拟。结果表明：需通风区与环境空气之间的氧气浓度差越大，超导磁体线圈电流密度越高，氧气浓度差磁致通风的流速就越高，通风效果越好。利用超导磁体驱动具有不同氧气浓度的空气之间产生通风是一种新型通风方式。     

A method for the design of MRI radiofrequency coils based on triangular and pulse basis functions.

This paper presents a numerical technique for the design of an RF coil for asymmetric magnetic resonance imaging (MRI) systems. The formulation is based on an inverse approach where the cylindrical surface currents are expressed in terms of a combination of sub-domain basis functions: triangular and pulse functions. With the homogeneous transverse magnetic field specified in a spherical region, a functional method is applied to obtain the unknown current coefficients. The current distribution is then transformed to a conductor pattern by use of a stream function technique. Preliminary MR images acquired using a prototype RF coil are presented and validate the design method.     

空心线圈电流互感器性能分析

空心线圈电流互感器的原理、结构及输出信号等与传统的电磁式电流互感器有很大不同，其性能易受外界磁场及环境温度等因素的影响。该文对影响空心线圈电流互感器性能的磁场及温度等干扰因素进行详细分析，在理论分析的基础上提出相应的改进措施，并研制了一台220kVGIS用空心线圈电流互感器，给出了实验结果。实验表明，改进后的空心线圈电流互感器受环境温度及外界磁场的影响很小，且具有较好的频率特性和暂态特性，互感器满足0．2级精度要求。     

Magnetic field gradient system for nuclear magnetic resonance microimaging

In this study we present an orthogonal magnetic field gradient system for nuclear magnetic resonance (NMR) microimaging applications. The construction details are given for a prototype assembly for proton microscopy inside a 50-mm vertical bore magnet, which is designed to fit into a commercial 300-MHz NMR probe. This system has been used to acquire images of the human spinal cordin vitro. Its performance has been evaluated and compared to that predicted by computer simulation.     

Magnetic instability of ac multifilamentary superconductors in spatially distributed magnetic field

Alternating current (ac) superconducting machines such as superconducting generators, transformers, or resistive current limiters need large‐current‐capacity conductors. These conductors are generally fabricated as multistrand cables stacked with multifilamentary NbTi superconductors whose current capacity is a few tens of amperes. ac quench current degradation has often been observed in ac use of such multistrand cables. Several reasons for this degradation have been pointed out: nonuniformity of each strand current, mechanical disturbances, thermomagnetic instability, and ac losses. However, it has not as yet been overcome. Since the angle between the strand axis and the cable axis changes along the cable axis in multistrand cables, the strands are exposed to a spatially (axially) distributed magnetic field, which has longitudinal and transverse components changing periodically due to multiple cabling. This paper mainly discusses the thermomagnetic instability due to the distributed transverse magnetic field, which was compared with the self‐field instability and the longitudinal field instability experimentally and theoretically. It was confirmed that the ac quench current degradation due to the distributed transverse field could be induced by the following: nonlinear E–j characteristic, strong dependence of E–j characteristic on magnetic field in low field region, uniformity of the axial current profile inside the superconductor with the high resistive matrix, and the poor thermal diffusion of CuNi/NbTi composites. © 2000 Scripta Technica, Electr Eng Jpn, 131(1): 45–55, 2000     

电力线路短路故障时通信线路暂态横向电压的计算

电力线路发生短路故障时 ,在通信线路上由于电磁感应产生横向电压 ,此电压的计算对于通信线路设计和保护配置具有重要意义。运用传输线理论 ,建立了考虑双线耦合作用时通信线路的等效电路模型。利用该模型 ,可分析计算不同频率条件下通信线上的横向电压。结合快速傅里叶变换 (FFT)技术 ,可获得通信线上各点的时域暂态横向电压。     

晶闸管投切电容式消弧线圈的设计与应用研究

分析了自动调谐消弧线圈的发展现状及存在的问题,提出了晶闸管投切电容（TSC）式消弧线圈,阐明了其工作原理和设计方法。理论分析和现场试验结果表明,TSC式消弧线圈具有响应速度快、调节范围宽、伏安特性线性度好等特点,由此构成的自动调谐消弧线圈接地装置电容电流测量准确,补偿效果好,具有良好的应用前景。     

Comparison of three commercially available radio frequency coils for human brain imaging at 3 Tesla

Objective To evaluate a transverse electromagnetic (TEM), a circularly polarized (CP) (birdcage), and a 12-channel phased array head coil at the clinical field strength of B ₀ = 3T in terms of signal-to-noise ratio (SNR), signal homogeneity, and maps of the effective flip angle α. Materials and methods SNR measurements were performed on low flip angle gradient echo images. In addition, flip angle maps were generated for α_nominal = 30° using the double angle method. These evaluation steps were performed on phantom and human brain data acquired with each coil. Moreover, the signal intensity variation was computed for phantom data using five different regions of interest. Results In terms of SNR, the TEM coil performs slightly better than the CP coil, but is second to the smaller 12-channel coil for human data. As expected, both the TEM and the CP coils show superior image intensity homogeneity than the 12-channel coil, and achieve larger mean effective flip angles than the combination of body and 12-channel coil with reduced radio frequency power deposition. Conclusion At 3T the benefits of TEM coil design over conventional lumped element(s) coil design start to emerge, though the phased array coil retains an advantage with respect to SNR performance.     

轴对称线圈静磁场的计算与应用

本文研究了包括圆环线圈与螺管线圈在内的轴对称线圈以及圆弧段电流单元所产生的静磁场中矢量磁位与磁通密度的计算，着重讨论了计算中完全椭圆积分与奇点的处理。在此基础上，对０．１５Ｔｅｓｌａ磁共振成像电阻式磁体进行了分析，设计了用于永磁磁共振成像的梯度线圈。