Modeling the static fringe field of superconducting magnets

The resonance frequency-space and the frequency gradient-space relations are evaluated analytically for the static fringe magnetic field of superconducting magnets used in the NMR diffusion measurements. The model takes into account the actual design of the high-homogeneity magnet coil system that consists of the main coil and the cryoshim coils and enables a precise calibration of the on-axis magnetic field gradient and the resonance frequency inside and outside of the superconducting coil.     

短腔、自屏蔽磁共振成像超导磁体系统的混合优化设计方法

本文提出一种用于短腔、自屏蔽磁共振成像超导磁体系统的混合优化设计方法,通过结合线性规划和非线性优化算法,设计出的磁体系统具有建造成本低、结构简单、以及线圈中最高磁场、电流安全裕度和电磁应力可控等优点.首先,通过线性规划算法在欲布置线圈空间范围内建立二维连续网格划分,搜索满足磁场约束条件的网格电流分布图;其次,将电流分布图中的非零电流簇离散成螺线管线圈,通过非线性优化算法计算出满足成像区域磁场均匀度要求、5 Gs杂散场限制、线圈中最高磁场限制、电流安全裕度以及线圈间尺寸间隔等约束条件的线圈结构参数.文中给出一个中心磁场为1.5T自屏蔽磁共振成像超导磁体系统的设计案例,在50 cm球形成像区域所产生的磁场峰峰值不均匀度为10 ppm,线圈最大长度为1.32 m.该设计方法可用于对称、非对称螺线管线圈系统以及开放式双平面线圈系统的磁共振成像磁体系统设计.     

Equivalent magnetic dipole method used to design gradient coil for unilateral magnetic resonance imaging

The conventional magnetic resonance imaging(MRI) equipment cannot measure large volume samples nondestructively in the engineering site for its heavy weight and closed structure. In order to realize the mobile MRI, this study focuses on the design of gradient coil of unilateral magnet. The unilateral MRI system is used to image the local area above the magnet. The current density distribution of the gradient coil cannot be used as a series of superconducting nuclear magnetic resonance gradient coils, because the region of interest(ROI) and the wiring area of the unilateral magnet are both cylindrical side arc surfaces. Therefore, the equivalent magnetic dipole method is used to design the gradient coil, and the algorithm is improved for the special case of the wiring area and the ROI, so the X and Y gradient coils are designed.Finally, a flexible printed circuit board(PCB) is used to fabricate the gradient coil, and the magnetic field distribution of the ROI is measured by a Gauss meter, and the measured results match with the simulation results. The gradient linearities of x and y coils are 2.82% and 3.56%, respectively, less than 5% of the commercial gradient coil requirement.     

永磁微型磁共振成像仪的梯度线圈与图像质量

通过理论分析结合计算机仿真试验剖析了磁共振成像(MRI)中梯度场线性对图像质量影响的原理,在梯度场线性有缺陷的情况下,用实例和计算机模拟直观的展示了重建图像和模拟结果. 结合研发实践发现,在永磁微小口径、低场强用于鼠类等小动物成像情况下,对梯度场线性度的要求远高于文献中提出的指标. 依靠软件改进,图像没有本质的提高,尝试补偿技术耗时费力效果也不理想. 说明了对永磁微小口径成像仪研发线性度更好的梯度线圈是提高图像质量的关键技术之一. 揭示了在理想条件下普适的理论原理,在不同的实践条件下有不同的趋近理想条件要求. 将具有独立知识产权的高线性梯度线圈安装在自行研制的仪器上得到了较好的图像.     

CCT型超导组合磁体样机结构设计

基于斜螺线管型(Canted-Cosine-Theta,简称CCT)线圈结构,研制了一台四极与六极组合的超导磁体样机,它的四极和六极梯度场分别为22 T/m和120 T/m2.该组合型磁体的好场区范围为Φ0.16 m × 1.3 m,有效长度为1.3m.其中四极与六极磁体均采用两层骨架结构,每层骨架的线槽内并排放置多层...     

MWECR CVD等离子体系统梯度磁场对沉积a-Si:H薄膜特性研究

分别研究了磁场线圈电流为115.2和137.7A以及137.7A并在加热台下加放SmCo永磁体的方法，来改变单磁场线圈分散场MWECR CVD系统等离子体室及沉积室磁场形貌.用洛伦兹拟合定量地得到了三种磁场形貌的磁场梯度.研究了磁场梯度对沉积a-Si:H薄膜性能的影响.研究表明：在衬底附近，高的磁场梯度可以获得高的沉积速率；在温度不很高时，高的磁场梯度可得到光敏性较好的a-Si:H薄膜. 关键词： 梯度磁场 洛伦兹拟合 a-Si:H薄膜 MWECR CVD系统     

Gradient-induced longitudinal relaxation of hyperpolarized noble gases in the fringe fields of superconducting magnets used for magnetic resonance

When hyperpolarized noble gases are brought into the bore of a superconducting magnet for magnetic resonance imaging (MRI) or spectroscopy studies, the gases must pass through substantial field gradients, which can cause rapid longitudinal relaxation. In this communication, we present a means of calculating this spatially dependent relaxation rate in the fringe field of typical magnets. We then compare these predictions to experimental measurements of (3)He relaxation at various positions near a medium-bore 2-T small animal MRI system. The calculated and measured relaxation rates on the central axis of the magnet agree well and show a maximum (3)He relaxation rate of 3.83×10(-3) s(-1) (T(1)=4.4 min) at a distance of 47 cm from the magnet isocenter. We also show that if this magnet were self-shielded, its minimum T(1) would drop to 1.2 min. In contrast, a typical self-shielded 1.5-T clinical MRI scanner will induce a minimum on-axis T(1) of 12 min. Additionally, we show that the cylindrically symmetric fields of these magnets enable gradient-induced relaxation to be calculated using only knowledge of the on-axis longitudinal field, which can either be measured directly or calculated from a simple field model. Thus, while most MRI magnets employ complex and proprietary current configurations, we show that their fringe fields and the resulting gradient-induced relaxation are well approximated by simple solenoid models. Finally, our modeling also demonstrates that relaxation rates can increase by nearly an order of magnitude at radial distances equivalent to the solenoid radius.     

Asymmetric MRI Magnet Design Using a Hybrid Numerical Method

This paper describes a hybrid numerical method for the design of asymmetric magnetic resonance imaging magnet systems. The problem is formulated as a field synthesis and the desired current density on the surface of a cylinder is first calculated by solving a Fredholm equation of the first kind. Nonlinear optimization methods are then invoked to fit practical magnet coils to the desired current density. The field calculations are performed using a semi-analytical method. A new type of asymmetric magnet is proposed in this work. The asymmetric MRI magnet allows the diameter spherical imaging volume to be positioned close to one end of the magnet. The main advantages of making the magnet asymmetric include the potential to reduce the perception of claustrophobia for the patient, better access to the patient by attending physicians, and the potential for reduced peripheral nerve stimulation due to the gradient coil configuration. The results highlight that the method can be used to obtain an asymmetric MRI magnet structure and a very homogeneous magnetic field over the central imaging volume in clinical systems of approximately 1.2 m in length. Unshielded designs are the focus of this work. This method is flexible and may be applied to magnets of other geometries.     

Bio-effects of high magnetic fields: a study using a simple animal model.

The desire to do clinical imaging and spectroscopy at magnetic field strengths greater than 2 Tesla (T) necessitates investigation of possible bioeffects at these high fields. A simple T-maze was utilized to evaluate the aversive effects of exposure to three levels of static magnetic field (0, 1.5, and 4 T). The right arm of the maze extended into the center of a 30-cm horizontal bore magnet, while the left arm extended into a mock magnet bore with the same dimensions. The self-shielded design of the magnet reduces the fringe field to zero within 1 m of the bore, placing the start box of the maze outside the 5-G line of the magnet. Each rat performed a total of ten trials at each level of magnetic field strength. A follow-up subset was run at 4 T with the maze reversed. At 0 T, the rats entered the magnet freely. No significant differences from the control were observed at 1.5 T. At 4 T, however, in 97% of the trials the rats would not enter the magnet. In the maze-reversed subset a majority of the rats turned toward the magnet, indicating that they had learned an aversive response from the previous trials at 4 T. However, in only 4 decisions out of 58 did the rats actually enter the magnet. Eighteen decisions to turn around were made at the edge of the magnet in a region of strong field gradients (up to 13 T/m) and a field strength up to 1.75 T.(ABSTRACT TRUNCATED AT 250 WORDS)     

Optimizing magnetization orientation of permanent magnets for maximal gradient force

The force exercised on a permanent magnet (PM) in a nonuniform field (gradient force) is dependent on the magnetization orientation of the magnet. In this paper, it is shown theoretically that the gradient force is greatest when the magnetization through the magnet, or at least at its surface, is collinear with the external field. The formulae for calculating the force between an axis-symmetric optimal magnet and a coaxial axis-symmetric coil are presented. Using the finite element method (FEM), calculations of the magnetic field distribution of an optimal cylindrical magnet and some its approximations are performed. The forces between these magnets and a pancake coil are computed and compared. For a system consisting of a magnet with a height of 1 unit and a diameter of 2 units and magnetization invariant in field and an annular pancake coil with a diameter of 2.4 units, a thickness of 0.2 units, an inner diameter of 0.4 units and a distance from the magnet of 0.2 units, the force on the optimal magnet was 1.44 times greater than the force on an axially magnetized magnet of the same size and magnetization magnitude. The optimal magnetization may be approximated by magnetization inclined at a constant angle to the axis and by a combination of axially and radially magnetized sections. With magnetization at a constant angle to the axis in the axis plane, the force was greatest when the angle was about 45°, being 1.38-fold compared to the force on an axially magnetized magnet. When the magnet was composed of an axially magnetized cylindrical core and a radially magnetized outer ring, the force was greatest when the volume of the core was approximately equal to the volume of the ring, being 1.26-fold compared to the force on an axially magnetized magnet. The optimal magnet and its approximations also provided a reduced stray field. A short review of methods of the fabrication of permanent magnets (PMs) with a continuous variation of the magnetization orientation and with radial magnetization orientation is given.The results of this study can be used to design linear electromagnetic (micro)actuators.     

Three-dimensional high-inductance birdcage coil for NQR applications

A birdcage coil capable of operating simultaneously and independently in three orthogonal dimensions has been developed. A co-rotational end-ring mode producing an RF field in the longitudinal direction was utilized in addition to the two common transverse orthogonal modes. Two conductor turns were used for each of the coil's windows, increasing its inductance by a factor of four, thereby, making the coil suitable for low-frequency applications. Two or three-frequency detection can be easily carried out with this device. Orthogonality of the coil's channels allows arbitrarily close frequency positioning of each resonant mode, potentially useful in wide-line NQR studies, in which simultaneous excitation/detection of signals from three adjacent regions of a single wide line can be performed. The coil's performance was evaluated using a three-dimensional scheme, in which a circularly polarized experiment was combined with a linearly polarized measurement at another frequency, resulting in SNR improvement by 55%.     

开放式自屏蔽全身成像高场超导MRI磁体优化设计

本文发展了开放式自屏蔽全身成像高场超导磁共振成像（MRI）磁体的优化设计方法,使设计出来的磁体仅有4 对超导线圈. 这种开放结构的超导MRI磁体优化设计方法集成了线性规划算法和遗传算法. 通过迭代线性规划算法可以在考虑成像区域（DSV）磁感应强度约束、磁场不均匀度约束、5 Gs线范围约束、线圈区域最大磁场值约束和最大环向应力约束的条件下,获得用线量最少的线圈初始形状和位置,同时可以得到每个线圈的层数和每层匝数;通过遗传算法可以提高DSV区域的磁场不均匀度,以达到高质量成像的要求. 这种集成的优化设计方法既可以灵活有效的设计开放式MRI磁体,也可以设计传统的圆柱形MRI磁体,本文通过一个1.2 T的开放式MRI磁体的设计清楚的展示了这种优化方法. 关键词： 线性规划算法 遗传算法 自屏蔽 开放式超导MRI磁体     

Probing four orders of magnitude of the diffusion time in porous silica glass with unconventional NMR techniques

The combined use of two unconventional NMR diffusometry techniques permits measurements of the self-diffusion coefficient of fluids confined in porous media in the time range from 100 microseconds to seconds. The fringe field stimulated echo technique (FFStE) exploits the strong steady gradient in the fringe field of a superconducting magnet. Using a standard 9.4 T (400 MHz) wide-bore magnet, for example, the gradient is 22 T/m at 375 MHz proton resonance and reaches 60 T/m at 200 MHz. Extremely short diffusion times can be probed on this basis. The magnetization grid rotating frame imaging technique (MAGROFI) is based on gradients of the radio frequency (RF) field. The RF gradients not necessarily need be constant since the data are acquired with spatial resolution along the RF gradient direction. MAGROFI is also well suited for unilateral NMR applications where all fields are intrinsically inhomogeneous. The RF gradients reached depend largely on the RF coil diameter and geometry. Using a conic shape, a value of at least 0.3 T/m can be reached which is suitable for long-time diffusion measurements. Both techniques do not require any special hardware and can be implemented on standard high RF power NMR spectrometers. As an application, the influence of the tortuosity increasing with the diffusion time is examined in a saturated porous silica glass.     

A novel high-gradient permanent magnet for the profiling of planar films and coatings.

The design and construction of a low-cost, permanent magnet is described. The magnet is intended for applications which require a large static gradient, such as those for which stray field imaging or fringe field diffusometry are conventionally employed. The magnet has been designed using the scalar potential method. Particular features of the magnet include a field profile such that ||B || is constant in the horizontal plane and such that B is horizontal at the midpoint between the poles. There is a vertical, and therefore orthogonal, strong gradient, G, in ||B ||. The ratio G/ ||B || is constant within a large volume and so allows measurements at a range of gradient strengths. It is this ratio which governs the shape of the pole-pieces. The constructed magnet has a typical operating field of 0.8 T, gives a gradient of 20 Tm-1, and has a useable interpole access of 20 mm. Field plot data show values consistent with the theory. In particular ||B || has a curvature of less than +/-5 microm over a 5 x 5 mm area at the target field. The magnet is most suitable for the one-dimensional profiling of thin planar samples. As an example of the magnet's use, a profile of a sandwich structure made of several polymer layers is shown. In addition, a set of one-dimensional profiles of an alkyd coating, recorded during solvent loss and cross-linking, is presented. This example demonstrates quantitative T2 measurements at a resolution of 6.5 microm across a 70-microm-thick film.     

Two-dimensional imaging with a single-sided NMR probe

A new low field unilateral NMR sensor equipped with a two-dimensional gradient coil system was built. A new NMR-MOUSE concept using a simple bar magnet instead of the classical U-shaped geometry was used to produce magnetic field profiles comparatively homogeneous in extended lateral planes defining a suitable field of view for 2D spatial localization. Slice selection along the depth direction is obtained by means of the highly constant static magnetic field gradient produced by this magnet geometry. Implementing a two-dimensional phase-encoding imaging method 2D cross sections of objects were obtained with high spatial resolution. By retuning the probe it was possible to change the depth of the selected slice obtaining a 3D imaging method. The details of the construction of the new device are presented together with imaging tests to show the quality of space encoding.     

医用重离子回旋加速器引出系统设计

紧凑型回旋加速器作为重离子医学专用装置同步加速器的注入器,其引出系统设计所用的磁场为TOSCA模型计算磁场。通过单粒子轨道计算确定引出系统的元件类型及基本参数;通过多粒子跟踪确定最终的元件参数和束流参数。为了提高引出效率,改善引出束流品质,在引出位置磁场梯度较大的位置,安放了一块C型磁铁,以改善此处的磁场梯度。同时,为了消除此C型磁铁对主磁场的影响,在此区域安放了一对线圈。计算结果表明引出系统的设计能够保证引出束流的强度和品质符合同步加速器的要求。     

Fast Optimization of a Biplanar Gradient Coil Set

This work presents an approach for fast optimization of gradient coils, using the simulated annealing method. The shielding condition derived from a target field method and the analytical evaluation of the fields produced by simple geometries were used to reduce the computing time. This method is applied to the optimization of a shielded biplanar gradient coil set. Efficiency, inductance, and homogeneity of the gradient fields produced by the optimized geometries were studied as a function of the number of wires, for the longitudinal and transverse gradient coils. A prototype of the gradient set was made to test the proposed design method. The resulting experimental values of coil efficiency, inductance, field linearity, and shielding performance exhibit good agreement between theory and experiment.     

50kA超导变压器初级绕组设计制作及低温通电实验

主要讨论了超导变压器初级绕组的设计、绕制以及低温通电实验,初级绕组主要采用2 942匝NbTi超导股线绕制,并经过真空压力浸渍而成。利用Ansys软件分析了其磁场分布。采用LHe浸泡式制冷,最大电流可达190A,中心场约为2T。     

国产MOCVD-YBCO带材高温超导线圈研制与磁场温度特性研究

超导磁体的场强与超导材料的载流能力、磁体口径和低温环境有密切关系.为了在中高温区域实现高磁场强度的超导磁体,采用国产第二代高温超导带材,成功绕制出内直径为100 mm的高温超导线圈.该超导线圈在77,65,55和46 K不同温区下进行了性能测试,其最大运行电流分别为65,147,257和338 A,对应的中心磁场强度分别为0.78,1.77,3.1和4.08 T.所研制的超导线圈的中平面上磁场基本一致.     

A distributed equivalent magnetic current based FDTD method for the calculation of E-fields induced by gradient coils

This paper evaluates a new, low-frequency finite-difference time-domain method applied to the problem of induced E-fields/eddy currents in the human body resulting from the pulsed magnetic field gradients in MRI. In this algorithm, a distributed equivalent magnetic current is proposed as the electromagnetic source and is obtained by quasistatic calculation of the empty coil's vector potential or measurements therein. This technique circumvents the discretization of complicated gradient coil geometries into a mesh of Yee cells, and thereby enables any type of gradient coil modelling or other complex low frequency sources. The proposed method has been verified against an example with an analytical solution. Results are presented showing the spatial distribution of gradient-induced electric fields in a multi-layered spherical phantom model and a complete body model.