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。     

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     

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。     

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.     

某220 kV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.     

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     

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...     

EMI滤波器与浪涌抑制技术的结合

介绍了电磁干扰(EM I)滤波器的基本类型以及由基本类型组成的EM I电源滤波器,同时也介绍了2种常用的防雷限压器件———压敏电阻(MOV)和瞬态抑制二极管(TVS);主要讨论了将滤波和抑制浪涌电压2种功能结合在一起的复合功能EM I滤波器(具有限压功能的LC滤波器和复合功能的EM I电源滤波器);给出了LC滤波器在雷电浪涌的差模和共模冲击下,随电容值变化的压敏电阻电压波形和电容电压波形(输出电压波形);最后对电路结构进行了简单扩展。     

消弧线圈限制过电压探讨

简述了中性点不接地系统在配电网规模增大、传输容量扩大、传输距离延长后,系统对地电容电流增大,故障时故障点形成的电孤不能自行熄灭,弧光过电压长时间加在系统的绝缘上危急设备安全,采用消弧线圈限制过电压的措施。     

Chirp子脉冲频率步进信号的距离副瓣抑制

分析了Chirp子脉冲频率步进信号的高分辨一维距离像距离副瓣产生的原因及传统加权处理带来的不良影响,提出一种易于工程实现的Chirp子脉冲非线性频率步进信号。通过对同步进载频的子脉冲相干积累改变频谱的能量分布,实现不同窗函数的模拟加权并还原成线性频率步进信号,在抑制距离副瓣的同时提高输出高分辨一维距离像信噪比。仿真结果表明,该方法抗噪性能好,与传统加权处理相比,在主副比基本相同的情况下,输出一维距离像信噪比有所提高。     

Robust gain-scheduled control for vibration suppression

Limited control authority is a key issue in the field of structural control and is a major research area since most of the practical control problems are dominated by constraints on the control signal. The paper presents a simple and practical gain-scheduled controller design procedure for active vibration suppression of a three-storey flexible structure. First, system identification experiments are performed and the plants uncertainty is derived. Next, robust controller design with constraint on the control signal is presented. For a better trade-off between control performance and control constraint a gain-scheduling approach is investigated. Stability analysis of the gain-scheduled controller is analysed using a parameter-independent Lyapunov function (quadratic stability) as well as a parameter-dependent Lyapunov function (biquadratic stability). Finally, the gain-scheduled controller is tested experimentally when the flexible structure is excited with a scaled historical earthquake record (1940 El Centro record). Successful experimental results show that the proposed robust gain-scheduled control approach offers good performance in the case of control authority limitation.Nomenclature M mass matrix - C damping matrix - K stiffness matrix - q relative displacement vector to base - m_a active mass - active mass acceleration - ground (base) acceleration - K_gap transfer gain of the displacement sensor - K_acc transfer gain of the acceleration sensor - K_AMD transfer gain of the active mass damper - _id frequency range of system identification experiments - r control reference - u control signal - d external disturbance - y control output - e control error - x state vector - p(t) time-varying parameter - N_p parameter boxs dimension - V(x) Lyapunov function - V(x,p) parameter-dependent Lyapunov function - largest parameter box where quadratic stability holds - S(s),T(s) sensitivity and complementary sensitivity transfer functions - G_n(s),G(s) transfer function of nominal and real plant - K_rate rate feedback gain - P_n(s) transfer function of nominal plant modified by rate feedback - P(s) transfer function of real plant modified by rate feedback - G_red transfer function of the reduced-order plant - _m(s) multiplicative uncertainty - W_S(s),W_T(s) performance and robustness weighting functions - G_c(s) controllers transfer function - G_c1(s),G_c2(s) transfer function of robust controller for vertex 1 and vertex 2 - G_cs(p,s) transfer function of the gain-scheduled controller - u_A amplitude of the control signal - K_min,K_max minimum and maximum controller gain - K(p) scheduled controller gain - J₁,J₂,J₃,J₄,J₅ performance evaluation parameters     

电动汽车充电机的谐波分析与治理

电动汽车充电机是非线性很强的负载,充电时会给电网带来较大的谐波污染,分析充电机谐波并进行谐波抑制对今后大规模充电站的建设具有重要意义。利用MATLAB/Simulink软件建立6脉波不控整流充电机模型,分析单台和多台充电机接入电网产生的谐波。最后,使用无源滤波器来抑制谐波,达到了预期效果,为今后大规模充电站建设提供一种理论参考。     

继电保护装置干扰防护若干措施

高压变电站、发电厂是具有高强度电磁场环境的特殊区域,容易对继电保护装置造成干扰,使保护误动或拒动,危及系统安全稳定运行。为此,介绍了现阶段广东省所采取的若干抗干扰措施,并对这些措施的原理及实现方法进行了简单的阐述。