Recent advances in image reconstruction, coil sensitivity calibration, and coil array design for SMASH and generalized parallel MRI

Parallel magnetic resonance imaging (MRI) techniques use spatial information from arrays of radiofrequency (RF) detector coils to accelerate imaging. A number of parallel MRI techniques have been described in recent years, and numerous clinical applications are currently being explored. The advent of practical parallel imaging presents various challenges for image reconstruction and RF system design. Recent advances in tailored SiMultaneous Acquisition of Spatial Harmonics (SMASH) image reconstructions are summarized. These advances enable robust SMASH imaging in arbitrary image planes with a wide range of coil array geometries. A generalized formalism is described which may be used to understand the relations between SMASH and SENSE, to derive typical implementations of each as special cases, and to form hybrid techniques combining some of the advantages of both. Accurate knowledge of coil sensitivities is crucial for parallel MRI, and errors in calibration represent one of the most common and the most pernicious sources of error in parallel image reconstructions. As one example, motion of the patient and or the coil array between the sensitivity reference scan and the accelerated acquisition can lead to calibration errors and reconstruction artifacts. Self-calibrating parallel MRI approaches that address this problem by eliminating the need for external sensitivity references are reviewed. The ultimate achievable signal-to-noise ratio (SNR) for parallel MRI studies is closely tied to the geometry and sensitivity patterns of the coil arrays used for spatial encoding. Several parallel imaging array designs that depart from the traditional model of overlapped adjacent loop elements are described. Summary of material presented at the 2001 ISMRM workshop on MRI hardware, Cleveland, OH, USA.     

Fat suppression by echo interference

Two spectral components of a binary system, like water and fat in proton magnetic resonance imaging (MRI) can be separated by adding two signals acquired with a different delay (the Dixon method). The same effect can be obtained in a single acquisition by interference of two shifted echoes. The standard multiecho imaging technique based on the Carr-Purcell-Meiboom-Gill (CPMG) sequence has been modified by adding a time shift of /2 after excitation. As a result, in the second and further periods of the sequence, the signal is a sum of two echoes which have opposite phases for this spectral component, which is off-resonance by 1/2. A magnitude image based on the first period of such nonsymmetric CPMG shows both components, whereas the second period gives an image of the on-resonance component only. The flip angle of the refocusing pulses must be adjusted to give equal amplitudes of the interfering echoes. A similar effect is obtained with the RARE sequence based on nonsymmetric CPMG with low-angle refocusing pulses.     

Basic considerations on the impact of the coil array on the performance of Transmit SENSE

Transmit SENSE adapts the idea of parallel imaging to RF transmission. Using multiple independent transmit coils, the duration of a spatially selective RF pulse can be reduced. It is known from parallel imaging that a suboptimal coil-array geometry might lead to an ill-conditioned sensitivity matrix and, thus, to a non-homogenous noise amplification in the resulting image. The current paper investigates the consequences of suboptimal coil arrays for Transmit SENSE. Two possible consequences of a suboptimal coil array are studied in the framework of numerical simulations: the incorrect excitation of the desired spatial pattern and the increase of the specific energy absorption rate (SAR), i.e. the RF power required to excite the desired pattern. Incorrect pattern excitation occurs only in pathologic coil-array scenarios. The increase of the SAR is very moderate for a large range of coil-array geometries. Using spiral excitation k-space trajectories leads to superior results compared to Cartesian trajectories. The problem of an ill-conditioned matrix inversion does not seem to play a major role in Transmit SENSE. Consequently, the freedom in designing coil arrays seems to be much larger in Transmit SENSE than in SENSE in the receive mode.     

Post-processing water-fat imaging technique for fat suppression in a low-field MR imaging system,evaluation in patients with rheumatoid arthritis

Purpose: To evaluate the feasibility of the phase difference-based post-processing water-fat imaging method for fat suppression at low-field in imaging of arthritic joints.Materials and methods: Thirty joints (wrist, 10; elbow, 10; knee, 10) in 30 patients with rheumatoid arthritis were imaged using a 0.23TMRI unit. Contrast-enhanced Tl-weighted (Tlw) three-dimensional (3D) gradient-echo (GRE) images with and without fat suppression along with short inversion time inversion-recovery (STIR) images were evaluated by two radiologists. Contrast-enhanced Tlw 3D GRE images and corresponding post-processed fat-suppressed images were scored for conspicuity and delineation of enhancing synovial hypertrophy. The uniformity of fat suppression was evaluated between Tlw 3D GRE fat-suppressed images and STIR images, and general image quality was estimated for all of the three techniques by consensus. For a quantitative analysis, the enhancing synovial hypertrophy-to-fat contrast-to-noise (CNR) values for the T1W 3D GRE images with and without fat suppression were measured. For comparison, synovial bright signal-to-fat CNR values for the STIR images were measured.Results: The post-processing water-fat imaging technique for fat suppression was successfully applied in all examinations. Conspicuity and delineation of enhancing tissue were superior in fat-suppressed Tlw 3D GRE images compared to non-fat-suppressed images (P < 0.0001). As expected, the enhancing synovial hypertrophy tissue-to-fat CNRs were significantly higher in fat-suppressed Tlw 3D GRE images compared to non-fat-suppressed images (P < 0.0001). General image quality was assessed to be best in non-fat-suppressed images, and the difference was significant compared to fat-suppressed images (P < 0.05) and STIR images (P < 0.05).Conclusion: The phase difference-based post-processing water-fat imaging technique for fat suppression can be successfully used at low-field, and it provides high-quality fat suppression images in imaging of arthritic joints.     

Residual vibration suppression using initial value compensation for repetitive positioning

This paper presents a novel residual vibration suppression methodology for repetitive fast‐response and high‐precision positioning in machine tool drives. In sequential positioning motions, as the interval period of position references becomes shorter, the residual vibration in response due to undesired initial values deteriorates the positioning accuracy, since the positioning controller is generally designed on the condition that initial state variables are zero. In this research, an Initial Value Compensation (IVC) approach is proposed under the theoretical study on effects of the initial state values on the position transient response. The IVC can appropriately assign poles and zeros of the transfer characteristic of position output for the initial values by applying an additional input corresponding to the initial state variables, enabling the response to be residual vibration free. The desired positioning performance, as a result, can be achieved in repetitive motions with arbitrary interval period. The effectiveness of the proposed compensation has been verified by numerical simulations and experiments using a positioning device of industrial machine tools. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 157(3): 63–72, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20276 Copyright © 2006 Wiley Periodicals, Inc.     

高次谐波产生的原因,危害及其抑制措施

阐述了高次谐波产生的原因、危害 ,介绍了高次谐波的抑制方法并对这些方法进行了比较 ,认为有源电力滤波器是谐波抑制装置的发展方向。     

FACTS装置对次同步振荡的抑制机理研究

交流灵活输电系统（Flexible AC Transmission System,FACTS）在电力系统中得到了越来越广泛的应用。由于FACTS装置的快速灵活调节性能,研究其抑制次同步振荡的机理具有重大的现实意义和工程应用价值。介绍了次同步振荡的基本原理,并详细阐述了常见FACTS装置对次同步振荡的抑制机理。     

变电站铁磁谐振仿真分析及抑制措施研究

以某110kV变电站为原型,在考虑了母线零、正序参数和进出线路几何参数情况下,利用ATP-EMTP电磁暂态仿真软件,对该变电站35kV侧由于出线单相接地引起的母线电压互感器的谐振情况及电压互感器一次侧中性点经非线性电阻接地,电压互感器开口三角短时投入阻尼电阻等措施的消谐情况进行了仿真计算。对母线上装设中性点接地的三相电容器组,采用电容式电压互感器等消谐方式的有效性进行了探讨。对谐振产生的原因和各种消谐措施的原理和消谐效果进行了分析比较。结合仿真结果和系统参数分析得出:单相接地故障消失时刻对于能否激发系统谐振起着关键性作用;电压互感器开口三角短时接入阻尼电阻和电压互感器一次侧中性点经非线性电阻接地2种措施消谐效果较为理想;H.A.Peterson的试验结论不能作为判断系统是否产生谐振的确切参数标准。     

Improvement of sensorless control performance for IPMSM by suppression of harmonic current using Fourier transform and repetitive control

In the case of the sensorless control for the PMSM that estimates position and velocity from EMF using a model of synchronous motor with the voltage and current, it is desirable for voltage and current to be perfect sine waves. However, in reality, voltage and current contain various harmonic components, which negatively affect the estimation ability of the sensorless control. In this paper, we propose a method of improving the sensorless control performance for the IPMSM by suppression of harmonic component in current control loop of the vector control using Fourier transform and repetitive control. To suppress harmonic components, compensation signals are acquired using Fourier transform and repetitive control, and feedforward control is implemented with the acquired compensation signals. To evaluate the proposed system, the sensorless control system for the IPMSM is considered as an example. The system is constructed on the basis of the extended EMF disturbance observer, and the improvement potential of the sensorless control performance is demonstrated. The effectiveness of the proposed method is confirmed by experimental results. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(4): 76–86, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20095     

基于磁通控制消弧线圈的注入式选线新方法

提出了一种谐振注入式接地故障选线新方法,可用于谐振接地系统单相接地故障选线。该选线方法充分利用了基于磁通控制消弧线圈的运行特点以及谐振频率电流能够突出故障信息的特点,可注入较大幅值的谐振频率电流以增强故障信息。其原理是在接地故障发生前后分别通过消弧线圈二次侧的逆变器注入相同的谐振频率电流,对电力系统零序参数形成2次快照,采用比值法及增补判据对2次快照的电流值、电压值进行分析,判定不满足相关判据的回路为故障线路。理论研究及仿真证实,该方法可以在消弧线圈补偿的同时进行选线,可以有效地检测高、低阻接地故障。     

采用改进陷波器的声反馈抑制

声反馈现象普遍存在于各种扩声系统中。声反馈会严重影响声音的清晰度,甚至会产生自激啸叫,而较严重的啸叫可能会使系统中的放大器、扬声器中的高音单元烧毁。所以,声反馈抑制是扩声系统中的一个重要问题。采用多频带FFT算法估计自激频率,估计精度高,鲁棒性强;同时对传统陷波器带宽不变的缺陷进行了改进,使之更适合声反馈的抑制,减少对声音的损伤。仿真结果表明,声反馈得到有效抑制,原始信号被较好的保留。     

消弧线圈自动调谐装置应用分析

介绍了66kV以下系统中性点消弧线圈自动调谐装置的原理,指出传统消弧线圈存在的问题。通过实例论证了消弧线圈自动调谐装置应用的必要性和优越性。     

基于DSP的新型消弧线圈接地系统控制器

介绍配电网新型快速可调节消弧线圈接地系统中采用TMS320F2812数字信号处理器DSP(Digital Signal Processor)构成的上位机控制器的硬件和软件结构以及实现的功能,利用DSP的高速计算能力,实时完成多路电压电流信号的快速傅里叶变换FFT(Fast Fourier Transform)运算等,提高实时处理配电系统各种信号的能力。     

三绕组谐波屏蔽变压器的研究

针对独立电力系统中抑制谐波的实际需要,本文对采用三绕组变压器进行谐波抑制的方案进行了研究.该方案的特点是,在中压绕组上连接各次谐波滤波器.根据三绕组变压器的结构特点,通过合理设计中压绕组的结构,可以使中压绕组的等值阻抗接近于零.同时合理配置中压绕组外接各谐波滤波器的参数,使其对某些特定次数谐波阻抗为最小,从而抑制这些谐波分量.本文阐述了这种方案的工作原理,并进行了仿真分析,最后通过实验验证了这种方案是有效可行的.     

某220kV变电站谐波治理的研究

针对因电铁牵引负荷造成谐波严重超标的220 kV变电站提出了谐波治理的措施,即合理选择电容器串抗支路的设计参数,达到既能补偿无功又能滤除谐波的目的.通过计算分析,该方案能够满足无功出力的要求且具有良好的谐波效果.     

基于有源逆变分相注入的电压消弧与位移电压抑制方法

针对配电网发生单相接地故障时极易产生电弧,以及在非故障情况下由于三相对地参数不相同而导致的三相电压不对称问题,提出一种基于有源逆变分相注入的电压消弧与位移电压抑制方法.在三相线路首端分别挂接有源逆变,通过脉宽调制信号控制有源逆变电路的输出电流.当配电网出现单相接地故障时,有源逆变向系统分相注入零序电流,通过零序回路调节...     

Robust design of vibration suppression control system for crane using sway angle observer considering friction disturbance

It is desirable for a container crane to operate smoothly and quickly. For this purpose, the control system of a container crane should be capable of antisway control for suppressing vibrations. A vision sensor system is often used to detect the sway angle. However, since a control system with a vision sensor has a delay time when determining the angle, it sometimes leads to deterioration of control performance owing to the delay time. In order to overcome this problem, this paper proposes a new antisway crane control system based on a dual‐state observer with sensor‐delay correction. However, because of nonlinear friction in the crane, the estimation accuracy achieved by using the observer is poor. To overcome this problem, this paper proposes a disturbance observer considering friction disturbance. The control performance and e?ectiveness of the proposed robust control system based on the estimated information are shown to be satisfactory by experimental results. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(3): 36–46, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22412     

瞬态脉冲骚扰及抑制方法

随着电子技术的发展 ,量度继电器、继电保护及自动化装置 (以下简称继电器及装置 )已实现微机化及数字化。电力系统恶劣的电磁环境中经常出现电磁骚扰 ,严重影响量度继电器及装置的正常工作 ,其中影响较大的是瞬态脉冲骚扰。从分析瞬态脉冲骚扰的产生原因着手 ,总结出各种瞬态脉冲骚扰的特征进而提出抑制的方法     

具有寄生通带抑制的微带带通滤波器的设计

寄生通带的抑制程度是微带滤波器的关键性能参数之一.介绍了1种能够在较宽频带内有效抑制寄生通带的设计方法,通过将带通滤波器和低通滤波器级联、联合仿真优化,最终得到在通带的二倍频与三倍频区域都具有足够抑制的微带带通滤波器原型.给出了具体的设计原理,以及使用ADS和Ansoft Designer的设计步骤与流程.最后的实测结...     

Auto-SENSE view-sharing cine cardiac imaging

Cardiac MR cine imaging during breath hold is a compromise between spatial and temporal resolution and duration of breath hold. Especially for sick patients who have problems holding their breath, a short acquisition time is mandatory for all sequences. A combination of Auto-SENSE parallel imaging and view-sharing was implemented for fast cine imaging of the human heart and applied to healthy volunteers. Compared to conventional Fourier imaging, data acquisition could be accelerated by a factor of 3.6. Neither a pre-scan nor additional lines in k-space are required to generate the sensitivity maps in Auto-SENSE.