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.     

Versatile coil design and positioning of transverse-field RF surface coils for clinical 1.5-T MRI applications

Clinical MRI/MRS applications require radio frequency (RF) surface coils positioned at an arbitrary angle with respect to B₀. In these experimental conditions the standard circular loop (CL) coil, producing an axial RF field, shows a large signal loss in the central region of interest (ROI). We demonstrate that transverse-field figure-of-eight (FO8) RF surface coils design are not subject to the same amount of signal loss in the central ROI as loop coils when their orientations are changed. The 1.5-T CL and FO8 prototypes (diameter = 10 cm) were built on Plexiglas using copper strips (width = 4 mm, thickness = 100 m). The two linear elements of the FO8 coil were 1 cm apart. Axial spoiled gradient echo (SPGR) images of a phantom containing doped water were acquired with the coil plane at =0°, 45°, and 90°. As increases, the CL images show, in the central ROI, a signal that decreases from a maximum value to zero. Whereas the FO8 images show, in the same ROI, a signal that varies little from the maximum value (20%). Optimized FO8 coils can be oriented with the coil plane positioned along any direction with respect to B₀ without significant signal loss. Transverse RF coil design should be useful for clinical MRS studies and also for parallel imaging techniques where versatile RF coils disposed along arbitrary directions are required.     

基于双层优化的微电网系统规划设计方法

规划设计是微电网系统核心技术体系之一。从分布式电源的综合优化(组合优化、容量优化)和分布式电源间的调度优化两个方面对其展开研究。根据分布式电源特性,提出了适用于并网型微电网系统和独立型微电网系统的双层优化规划设计模型。上层优化采用基于NSGA-II的多目标遗传算法计算系统最优配置;下层优化采用混合整数线性规划算法(MILP)计算系统最优运行方案。运用所建立模型,分别针对并网型和独立型微电网系统作了案例计算,验证了所提方法的正确性。     

Applications of specialized coils for high-resolution MRI on a whole-body scanner

To investigate the application of a mini-coil surface system for high-resolution MRI, 60 volunteers were examined in a 1.5-T whole-body scanner. Two replaceable probe heads were available: a circular 2.5-cm coil and a quadratic 5-cm coil, both of which were placed directly on the skin. The skin layers, Achilles tendon and finger joints were examined with the 2.5-cm coil and a FOV of 25 × 25 mm². A matrix of 256 × 256 pixels resulted in a pixel size of 0.098 × 0.098 mm². For imaging of the carpal tunnel, the 5-cm coil was used in transverse orientation. The FOV was 50 × 50 mm² so that a matrix of 256 × 256 pixels led to a pixel size of 0.195 × 0.195 mm². The resulting spatial resolution permitted visualization of the epidermis, dermis and subcutis, resulting in clear definition of anatomical detail of the musculoskeletal system. MRI measurement of skin-layer thickness did not correlate with histometric data (p<0.05). This discrepancy was due in part to shrinkage of the tumor specimen on histologic preparation. Other causes include the motion artifacts and the limited accuracy of determining thickness on the MRI display unit.     

基于VHDL语言的可编程逻辑系统的设计方法

提出一种基于VHDL语言的可编程逻辑系统的设计方法。阐述了可编程逻辑器件和VHDL语言的特点,说明了采用VHDL语言和可编程逻辑器件进行数字系统设计的有效性,并给出设计步骤。VHDL语言和可编程逻辑器件的结合作为一种强有力的设计方式,将为设计者的产品上市带来创纪录的速度。     

A method of waveform design based on mutual information

A novel method called the general water-filling, which is suitable when clutter is not negligible, is proposed to solve the waveform design problem of broadband radar for the recognition of multiple extended targets. The uncertainty of the target’s radar signatures is decreased via maximizing the mutual information between a random extended target and the received signal. Then, the general water-filling method is employed to the waveform design problem for multiple extended targets identification to increase the separability of multiple targets. Experimental results evaluated the efficiency of the proposed method. Compared to chirp signal and water-filling signal, our method improves the classification rates and even performs better at low signal-to-interference-plus-noise ratio (SINR). __________ Translated from Journal of Xidian University, 2008, 35(4): 678–684 [译自: 西安电子科技大学学报(自然科学版)]     

基于一发多收线圈阵列的频域电磁法未爆弹探测技术

为了实现未爆弹目标的高效探测及定位,开发了一种小回线激励-阵列式线圈接收的主动电磁探测系统。采用偏心结构的收发线圈配置,实现了一次场的对消。通过接收线圈阵列排布,有效地提高了系统探测效率。提出了一种基于阵列线圈响应差分比较的未爆弹目标水平位置定位方法,并实验验证了可行性。研究结果表明,探测系统对于典型环状金属柱体的探测深度达到1 m,定位方法的计算值与真值误差在3.76 cm,实现了对未爆弹目标的准确探测和定位。     

Determination of appropriate RF blocking impedance for MRI surface coils and arrays

Surface and phased array receiving coils in MRI typically require that RF excitation be accomplished using the body coil. This process requires that the receiving coils contain blocking circuitry to increase the overall circuit impedance during RF excitation and withstand the electromotive force induced by the applied electromagnetic field. The aim of this study was to determine the optimal impedance range required during RF excitation based on an assessment of image quality. The experimental results are fit by an exponential model and establish criteria that can be applied for general receiver coil design.     

融合超高频及脉冲电流法的局部放电监测系统

基于脉冲电流法和超高频法的基本原理和特点,研制出融合超高频法及脉冲电流法的局部放电监测系统.系统采用简易的超高频信号波形转换装置,将超高频信号转换成方波信号作为脉冲电流传感器的触发信号,避开了超高频方法需要高速采集和海量存储的缺点.与传统的脉冲电流法比较,系统有效地去除了局部放电信号中所包含的脉冲型干扰.实验结果表明该系统可以准确地识别局部放电及干扰信号,有针对性地找出干扰源并有效地去除干扰信号.     

基于MATLAB的FIR数字滤波器的方法设计

数字滤波器是一种具有选择并区分不同频率功能的电路,是在时域内过滤离散信号的数字系统。采用的是窗函数法。研究的重点目的是依据FIR滤波器的特性,分析了FIR滤波器的窗函数设计方法的介绍和选择使用,并利用MATLAB信号处理工具箱来实现验证FIRDF程序的设计。给出了详细的设计理论和具体步骤,并将设计的滤波器应用到一个混和正弦波信号,以验证滤波器的性能,实验证明该滤波器达到了预期的滤波功能。     

基于FPGA的脉冲信号发生/测试仪一体化设计

脉冲信号由于具有抗干扰能力强、无噪声积累、便于测试传输等诸多优点,已经成为现代测控领域的一类主要信号(源).基于FPGA(现场可编程门阵列)设计并实现了的集双通道脉冲信号发生和参数综合测试于一体的脉冲信号发生/测试仪.以Altera Cyclone FPGA系列的EP1C3T144器件为处理器,通过对PLL(锁相环)产生的100 MHz时基脉冲计数的方法,实现了对双通道脉冲信号频率、占空比、相位差等参数的综合测量,同时可根据上位机设置的参数输出数目可控的脉冲波形.脉冲测试和发生精度可达到10 ns级,适用于以脉冲信号为对象的测试或控制系统的调试或检验.     

脉冲电流选线技术对故障电弧的影响

脉冲电流选线技术需要短时改变中性点接地方式,接地方式的改变会给故障电弧熄灭带来负面的影响。针对故障电弧自行熄灭的条件,研究了电弧熄灭后故障点电压的恢复情况,结果表明,接地方式的改变可能加快故障点电压的恢复速度,增加电弧熄灭后复燃的几率。为了避免脉冲电流选线技术对电弧熄灭的不利影响,提出了改变接地方式的合理时间范围并给出了相应的计算方法;另外,指出了添加限流电阻可以减小对电弧熄灭的不利影响。     

Analysis of the diffracted current basis functions used in the hybrid MoM-PO method

The combined moment method (MoM)-physical optics (PO) approach proposed by Bilow fails in some cases. Based on the theory of diffraction and the fundamental theory of electromagnetism, Bilow’s diffracted current basis function was modified both within and outside the transition regions. The improved MoM-PO technique is validated by comparison with exact solutions for a right-angled perfectly conducting wedge at normal incidence. Translated from Chinese Journal of Radio Science, 2006, 21(4):514–517 [译自: 电波科学学报]     

A simple method for mapping the B1 field distribution of linear RF coils

Inhomogeneity of the radio frequency (RF) field B₁ leads to intensity variations in MR images and to spatial dependence of spectral line amplitudes. In this paper, a simple method of measuring the B₁ field components of an unsegmented linear coil is described. The method is designed for the coils operating up to 20 MHz. The B₁ field distribution is replaced by the static magnetic field caused by DC current flowing through the coil. The technique involves rotating the coil 90° so that measured B₁ component is aligned with B₀ and measuring the shift of resonance frequency using a spectroscopic imaging sequence. Experimental results were in good agreement with the theoretical computations.     

一种基于空间矢量的APF直流侧电容电压设计和优化方法

并联型有源电力滤波器(SAPF)的直流侧电容电压直接影响谐波补偿性能。合理的电容电压设定值既可以保证补偿效果,又可以降低直流侧电容的耐压值选取要求。针对三相并联型有源电力滤波器直流侧电容电压优化设计这一问题,通过对典型的负载条件下谐波电流进行分析,推导出在完全补偿谐波电流的情况下变流器的输出电压矢量值。基于空间矢量脉冲宽度调制方式(SVPWM),分析了直流侧电容电压选取方法。仿真结果验证了该方法的有效性。     

Adaptive control design for VSC-HVDC systems based on backstepping method

An adaptive control design is proposed to improve dynamic performances of voltage source converter high voltage direct current (VSC-HVDC) systems. The adaptive controller design for nonlinear characteristics of VSC-HVDC systems, which is based on backstepping method, considers parameters uncertainties. For an original high-order system, the final control laws can be derived step by step through suitable Lyapunov functions. Thus, the design process is not complex. The effectiveness of the proposed adaptive controllers is demonstrated through digital simulation studies on a VSC-HVDC power system, using the PSCAD/EMTDC software package. The simulation results show that the controllers contribute significantly toward improving the dynamic behavior of the VSC-HVDC system under a wide range of operating conditions.     

A method of new filter design based on the co‐occurrence histogram

We have proposed that the co‐occurrence frequency image (CFI) based on the co‐occurrence frequency histogram of the gray value of an image can be used in a new scheme for image feature extraction. This paper proposes new enhancement filters to achieve sharpening and smoothing of images. These filters are very similar in result but quite different in process from those which have been used previously. Thus, we show the possibility of a new paradigm for basic image enhancement filters making use of the CFI. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(1): 36–42, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20699     

一种基于NSGA-II的低频谐振滤波器优化设计方法

舰船电力系统需要为船上各种用电设备提供安全稳定的电能,因此采用了各种电力电子装置完成电能变换,这些装置不可避免的会带来各种谐波干扰。由于受到体积重量的限制,在大功率电力系统中,若采用简单的EMI滤波器滤除低次谐波,由于所需的电容电感量较大,电感需要承担所有的工作电流,工程实现较为困难。在已知谐波频率的情况下,本文采用谐振滤波原理可在保证滤波效果的情况下有效减小滤波器的体积,并基于NSGA-II算法提出了一种谐振滤波器的优化设计方法。首先阐述谐振滤波的基本原理及设计方案,然后将谐振滤波器的初期投资、滤波后直流侧谐波含量,滤波设备的体积作为三个目标,利用NSGA-II算法求解谐振滤波器的参数。最终通过实验进行验证,设计的谐振滤波器具备良好的综合性能。     

基于复合形法的风力机桨叶优化设计

采用Glauert模型设计1.3 MW的风力机桨叶,采用修正的片条理论为气动计算模型,以风力机年能量输出最大为优化目标,提出以风力机年平均输出功率和叶轮实度为优化目标,采用复合形法搜索最优的桨叶气动外形.对比Glauert模型的初始设计,利用复合形法进行优化大大降低了桨叶弦长和扭角.