Active ferromagnetic shimming of the permanent magnet for magnetic resonance imaging scanner

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging (MRI) magnet.It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for MRI applications.An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications.This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards.Simulation and experimental results verify that the method is valid and efficient.     

A design of novel type superconducting magnet for super—high field functional magnetic resonance imaging by using the harmonic analysis method of magnetic vector potentials

The approach of expanding the magnetic scalar potential in a series of Legendre polynomials is suitable for designing a conventional superconducting magnetic resonance imaging magnet of distributed solenoidal configuration. Whereas the approach of expanding the magnetic vector potential in associated Legendre harmonics is suitable for designing a single-solenoid magnet that has multiple tiers, in which each tier may have multiple layers with different winding lengths. A set of three equations to suppress some of the lowest higher-order harmonics is found. As an example, a 4T single-solenoid magnetic resonance imaging magnet with 4×6 layers of superconducting wires is designed. The degree of homogeneity in the 0.5m diameter sphere volume is better than 5.8 ppm. The same degree of homogeneity is retained after optimal integralization of turns in each correction layer. The ratio B_m/B₀ in the single-solenoid magnet is 30% lower than that in the conventional six-solenoid magnet. This tolerates higher rated superconducting current in the coil. The Lorentz force of the coil in the single-solenoid system is also much lower than in the six-solenoid system. This novel type of magnet possesses significant advantage over conventional magnets, especially when used as a super-high field functional magnetic resonance imaging magnet.     

A novel acquisition-reconstruction algorithm for surface magnetic resonance imaging

In U-shaped, hand-size magnetic resonance surface scanners, imaging is performed along only one spatial direction, with the application of just one gradient (one-dimensional imaging). Lateral spatial resolution can be obtained by magnet displacement, but, in this case, resolution is very poor (on the order of some millimeters) and cannot be useful for high-resolution imaging applications. In this article, an innovative technique for acquisition and reconstruction of images produced by U-shaped, hand-size MRI surface scanners is presented. The proposed method is based on the acquisition of overlapping strips and an analytical reconstruction technique; it is capable of arbitrarily improving spatial lateral resolution without either using a second magnetic field gradient or making any assumptions about the imaged sample extension. Numerical simulations on synthetic images are reported demonstrating the method functionalities. The presented method also makes it possible to use U-shaped, hand-size MRI surface scanners for high-resolution biomedical applications, such as the imaging of skin lesions.     

Computer-assisted design of surface coils used in magnetic resonance imaging. I. The calculation of the magnetic field

For a number of reasons, it is desirable to fabricate coils which, for a known current, shall produce predetermined values of the magnetic field intensity at a number of points within a nuclear magnetic resonance imager. The calculation of the magnetic field intensity at a set of points involves the integration of the Biot-Savart equation for all components of the segments of conductor which make up the coil. This process in itself is a rather formidable task. When this process is parameterized in terms of coil diameter, coil spacing, etc. the problem is to determine the values of these parameters to match values of magnetic field intensities which are desired. The problem thereby increases in complexity to the point where, by ordinary methods, the problem becomes intractable. This note describes an algorithm and offers a computer subroutine to calculate magnetic fields for coils of arbitrary shape and complexity for fixed currents.     

用于超导实验的背场磁体研制

在进行超导材料特性、交流损耗测量等超导实验时,需要为实验提供大小可变、均匀性好的背景磁场。文中对自行研制的背场磁体进行了测试研究,发现选择性对组合铜线圈供电能够提供合适的磁场,满足实验的要求。     

一种实验用交流超导磁体的研制

该文介绍了用于探测线圈法测量超导材料交流损耗的一种交流超导磁体的设计、加工与绕制.超导磁体设计采用成熟的螺线管磁体技术,磁体骨架为低温环氧树脂加工而成,所用超导线为NbTi/Cu复合超导绞线.为了满足磁体的稳定性,每两层超导线间留出了一条冷却通道.所绕磁体的磁场强度和均匀度都满足设计要求.     

带有双X型铁心框架的超导MRI磁体的设计

文中对带有双X型铁心框架的MR I用高温和低温超导磁体的设计进行了介绍,包括项目的市场需求、磁体的基本参数和系统热负荷的预设计结果等。     

Indigenous design and fabrication of a 6.5 T superconducting magnet and a magnetotransport measurement set-up

A low-cost apparatus for measuring Hall effect and magnetoresistance is designed and built indigenously. This includes a 6.5 T superconducting magnet and a variable temperature sample holder assembly. A superinsulated liquid helium dewar with a low liquid helium boil-off rate is chosen as the low-temperature bath for doing magnetotransport measurements. A pair of high-T _c superconducting leads for energizing the magnet reduces the liquid helium consumption further and makes it economically beneficial, especially for laboratories with limited budget. The performance of the apparatus is tested over a wide range of temperatures (4.2 to 300 K) and fields up to 6.5 T. Reproducible magnetotransport data are obtained with excellent temperature and field stability.      

FAIR收集环二极超导磁体外杜瓦泄放系统的概念设计

对FAIR(Facility for Antiproton and Ion Research)收集环-CR(Collector Ring)二极超导磁体外杜瓦的超压泄放系统进行了概念设计,给出了爆破片式的泄放系统结构,并基于ANSYS的结构分析初步确定了爆破片的结构参数;以及通过ANSYS对波纹管和外杜瓦进行了结构分析,分析结果表明波纹管和外杜瓦在超压状态下安全。文中的设计和分析研究,为外杜瓦的工程设计提供了有益的参考。     

高磁场磁共振超导磁体电源的拓扑设计与分析

针对高磁场磁共振大孔径人体成像系统中超导磁体电源在励磁时向磁体充能、退磁时把能量“逆变” 回电网的设计要求,比较了两种能量回馈型的拓扑结构。通过 Matlab 仿真分析,验证了拟采用的拓扑结构的合 理性。      

Development of a temperature-variable magnetic resonance imaging system using a 1.0T yokeless permanent magnet

A temperature variable magnetic resonance imaging (MRI) system has been developed using a 1.0 T permanent magnet. A permanent magnet, gradient coils, radiofrequency coil, and shim coil were installed in a temperature variable thermostatic bath. First, the variation in the magnetic field inhomogeneity with temperature was measured. The inhomogeneity has a specific spatial symmetry, which scales linearly with temperature, and a single-channel shim coil was designed to compensate for the inhomogeneity. The inhomogeneity was drastically reduced by shimming over a wide range of temperature from −5 °C to 45 °C. MR images of an okra pod acquired at different temperatures demonstrated the high potential of the system for visualizing thermally sensitive properties.     

Design and development of a superconducting magnet system for study of magnetoresistance in the temperature range 1.5–300 K

The design and fabrication of an all-metal cryostat, superconducting magnet and variable temperature sample holder insert for the measurement of magnetoresistance at temperatures in the range 1.5–300 K are described. A field of 50 kilogauss has been achieved at the centre of a one-inch bore superconducting solenoid and has a uniformity of 0.2% over an axial distance of 20 mm around the centre of the solenoid. Also, the design and construction of vapour cooled current leads for the magnet are discussed.     

GSI超导二极磁体的电磁分析

主要介绍超导二级磁体的结构,用有限元分析软件ANSYS对磁体在工作电流下进行了电磁分析,结果符合物理设计要求,同时为下一步的结构分析提供了电动力。     

目标场法在悬浮超导球体的球形线圈设计中的应用

文中把目标场法引入到悬浮超导球体的球形线圈设计中,根据目标场法产生均匀磁场的原理,通过离散化绕组来近似处理球形线圈在超导球体和球形线圈的间隙内产生均匀磁场的电流密度分布规律。利用有限元分析和验证,在超导球体与球形线圈之间的间隙内产生了具有很好均匀性的悬浮磁场,这会明显提高超导球体悬浮的刚度和稳定性。     

A novel radio frequency coil for veterinary magnetic resonance imaging system

In this article,a novel designed radio frequency (RF) coil is designed and built for the imaging of puppies in a V-shape permanent magnetic resonance imaging (MRI) system.Two sets of Helmholtz coil pairs with a V-shape structure are used to improve the holding of an animal in the coil.The homogeneity and the sensitivity of the RF field in the coil are analysed by theoretical calculation.The size and the shape of the new coil are optimized and validated by simulation through using the finite element method (FEM).Good magnetic resonance (MR) images are achieved on a shepherd dog.     

Analysis of current ramp for a high-temperature superconducting magnet

In this paper, an equivalent electrical circuit for response of a high-temperature superconductor (HTS) system for arbitrary time varying transport current is considered and as well as an analysis of the generated resistance losses is given. Also, for the pancake coil made with superconducting Bi-2223/AgMg tape, hysteresis behavior on the voltage-current curve is examined as a function of the current rate during ramp up and down at liquid nitrogen temperature. Behavior of voltage versus current is in good agreement with considered theory.     

A novel functional magnetic resonance imaging compatible search-coil eye-tracking system

Measuring eye movements (EMs) using the search-coil eye-tracking technique is superior to video-based infrared methods [Collewijn H, van der Mark F, Jansen TC. Precise recording of human eye movements. Vision Res 1975;15(3):447-50], which suffer from the instability of pupil size, blinking behavior and lower temporal resolution. However, no conventional functional magnetic resonance imaging (fMRI)-compatible search-coil eye tracker exists. The main problems for such a technique are the interaction between the transmitter coils and the magnetic gradients used for imaging as well as the limited amount of space in a scanner. Here we present an approach to overcome these problems and we demonstrate a method to record EMs in an MRI scanner using a search coil. The system described has a spatial resolution of 0.07 degrees (visual angle) and a high temporal resolution (22 kHz). The transmitter coils are integrated into the visual presentation system and the control/analysis unit is portable, which enables us to integrate the eye tracker with an MRI scanner. Our tests demonstrate low noise in the recorded eye traces and scanning with minimal artifact. Furthermore, the induced current in the search coil caused by the RF pulses does not lead to measurable heating. Altogether, this MR-compatible search-coil eye tracker can be used to precisely monitor EMs with high spatial and temporal resolution during fMRI. It can therefore be of great importance for studies requiring accurate fixation of a target, or measurement and study of the subject's oculomotor system.     

Computer-assisted design of surface coils used in magnetic resonance imaging. II. Rotational discrimination nonlinear regression analysis and the design of surface coils

For a number of reasons, it is desirable to fabricate coils which, for a known current, shall produce predetermined values of the magnetic field intensity at a number of points within a nuclear magnetic resonance imager.

The calculation of the magnetic field intensity at a set of points involves the integration of the Biot-Savart equation for all components of the segments of conductor which make up the coil. This process in itself is a rather formidable task. When this process is parameterized in terms of coil diameter, coil spacing, etc. the problem is to determine the values of these parameters to match values of magnetic field intensities which are desired. The problem thereby increases in complexity to the point where, by ordinary methods, the problem becomes intractable.

A generalized solution technique has been developed on a digital computer to implement the rotational discrimination nonlinear regression techniques of Faris, Law and Letcher to find the best solution to this problem. The problem is posed by integrating the Biot-Savart equation. This produces algebraic expressions for incorporation into the optimization program which is executed on a computer in a conversational mode.

This technique was employed to specify the dimensions of a rectangular surface coil for the investigation of the whole human spine.     

Effect of current sweep reversal on the magnetic field stability for a Bi-2223 superconducting solenoid

The effect of current sweep reversal on the temporal drift in magnetic field intensity for a Bi-2223 solenoid was investigated by experiment and using numerical simulation. Current sweep reversal, by as small as 1% of the peak current, was found to stabilize the drift in magnetic field intensity for a Bi-2223 tape solenoid. The field drift was due to flux creep in the Bi-2223 tape and the current sweep reversal formed a barrier for flux entrance at the upper and lower surface of the conductor, preventing flux creep. With a current reversal of several% of the peak current, the barrier formation extended over half of the solenoid and the magnetic field intensity became constant with time. The current sweep reversal technique should prove useful to stabilize an ultra-high field low/high-temperature superconducting nuclear magnetic resonance magnet operated at frequencies (field intensities) beyond 1 GHz (23.5 T).     

7T大口径NbTi超导磁体降温通电实验

介绍了用于超导导体测试用超导磁体降温通电实验,降温采用500W@4.5K制冷机系统。300—10K直接采用冷He气循环冷却超导磁体,温度低于10K直接输入LHe。该磁体在4.2K温度下可通1000A的电流,其中心磁场为7T,储能为3M J。