Combining prospective motion correction and distortion correction for EPI: towards a comprehensive correction of motion and susceptibility-induced artifacts

Object  

State-of-the-art MR techniques that rely on echo planar imaging (EPI), such as real-time fMRI, are limited in their applicability by both subject motion and B₀ field inhomogeneities. The goal of this work is to demonstrate that in principle it is possible to accurately predict the B₀ field inhomogeneities that occur during echo planar imaging in the presence of large scale head motion and apply this knowledge for distortion correction.     

Optimization of diffusion-weighted single-refocused spin-echo EPI by reducing eddy-current artifacts and shortening the echo time

Objective

The purpose of this work was to optimize the acquisition of diffusion-weighted (DW) single-refocused spin-echo (srSE) data without intrinsic eddy-current compensation (ECC) for an improved performance of ECC postprocessing. The rationale is that srSE sequences without ECC may yield shorter echo times (TE) and thus higher signal-to-noise ratios (SNR) than srSE or twice-refocused spin-echo (trSE) schemes with intrinsic ECC.

Materials and methods

The proposed method employs dummy scans with DW gradients to drive eddy currents into a steady state before data acquisition. Parameters of the ECC postprocessing algorithm were also optimized. Simulations were performed to obtain minimum TE values for the proposed sequence and sequences with intrinsic ECC. Experimentally, the proposed method was compared with standard DW-trSE imaging, both in vitro and in vivo.

Results

Simulations showed substantially shorter TE for the proposed method than for methods with intrinsic ECC when using shortened echo readouts. Data of the proposed method showed a marked increase in SNR. A dummy scan duration of at least 1.5 s improved performance of the ECC postprocessing algorithm.

Conclusion

Changes proposed for the DW-srSE sequence and for the parameter setting of the postprocessing ECC algorithm considerably reduced eddy-current artifacts and provided a higher SNR.

     

Motion artifacts reduction in DWI using navigator echoes: A robust and simple correction scheme

NMR signal phase variation caused by macroscopic motion of an object during application of the diffusion gradient is a well-known effect in diffusion-weighted imaging (DWI) using the standard pulsed gradient spin-echo sequence (PGSE). This phase error causes severe ghost artifacts in the output image when phase encoding techniques, such as two dimensional Fourier transform (2DFT) imaging, are used. One possible way to eliminate the motion effects is the navigator echo technique. The method is based on estimating the phase error from the navigator echo and using it for the correction of the image echo. The phase errors (zero and first order) for the phase correction of the image echo are usually evaluated from the navigator echo after Fourier transform (FT) in the readout direction, correcting for both translation and rotation. We present here a simple algorithm which wnables evaluation and correction in the time domain of phase errors induced by motion. This approach has the advantage of improved correction of motional artifacts and minimized sensitivity to noise and inaccurate setting up of the experiment.     

Wavelet-encoding gradient-echo imaging sequence using real-time de-noising method for acquisition time reduction

Object  

To reduce acquisition time and increase signal-to-noise ratio (SNR) of the wavelet-encoding gradient-echo (GE-WE) sequence used in small field of view dynamic magnetic resonance imaging.     

多端柔性直流下垂控制的功率参考值修正方法

基于电压源型换流器的多端柔性直流输电(VSC-MTDC)所采用的外环有功控制方式会对直流系统潮流分布造成影响。首先提出了电压源型换流器外环有功控制通用模型,该模型基本涵盖了常用的外环有功控制方式。基于该模型推导直流网络潮流计算方法。采用直流电压下垂控制的换流器其直流输送功率会与参考值偏离,为了提高直流系统输送能力,提出了根据期望输送功率对直流功率参考值进行修正的方法。最后通过PSCAD/EMTDC时域仿真模型与潮流计算的结果对比,对所提出潮流计算及直流功率参考值修正方法的有效性进行验证。结果表明所提出的功率参考值设定方法可以使采用下垂控制的多端直流系统按期望功率传输。     

特征K值时域递归滤波估计的PPG信号伪差校正算法

光电容积脉搏波(PPG)信号采集系统极易受到伪差干扰,为了提升PPG信号的高信噪比,提高生理参数计算准确度,提出了特征K值自适应递归滤波估计的PPG信号伪差校正算法。将采集的PPG信号去除环境干扰和工频干扰之后,计算未受伪差干扰的信号主波峰峰值和重搏波峰值,从而得到每一个信号周期的特征K值。采用自适应递归滤波算法对被干扰信号的K值进行估计,利用估计得到的结果作为被干扰信号K值的真值。最后,根据估计的K值,用高斯函数法合成新的不受干扰的信号,替代原信号中被伪差干扰的部分。采用自行研制的系统进行数据采集,证明了算法的有效性和可靠性;通过血氧饱和度实验,证明了算法在可穿戴系统中的实用性。     

Highly accelerated PSF-mapping for EPI distortion correction with improved fidelity

Objective

This study presents an improved point-spread-function (PSF) mapping-based distortion correction method and accelerated PSF acquisition for distortion correction in EPI without loss of quality or reliability compared to full encoding.

Materials and methods

To correct geometric distortions accurately, the PSF in the EPI phase-encoding coordinates (EPI-PSF) was measured and used as a kernel for distortion correction. FOV reduction was applied in the PSF mapping dimension for highly accelerated PSF acquisition. A novel approach for fold-over artifact correction in this reduced dimension is introduced. Conventional gradient-echo EPI and corresponding full PSF reference data were acquired in phantoms and in human brain at 7 T. The distortion corrected EPI data with the proposed acceleration were compared to result with full encoding. Previously published interpolation methods based on shift maps, non-uniform Fourier transformation and a b-spline interpolation were compared with the proposed method.

Results

The results demonstrate that the proposed method corrects geometric distortions in EPI with high accuracy and quality despite the high acceleration. In contrast to partial parallel imaging acceleration, no noise enhancement is introduced.

Conclusion

The proposed EPI-PSF-based distortion correction improves correction of EPI and accelerates PSF reference data acquisition and computation.     

一种基于SV采样信号的局部放电监测内同步基准修正方法

高压设备局部放电信号与设备运行的工频电压具有高度的相关性,局部放电监测装置或仪器必须基于稳定可靠的工频相位信号才能正常工作。针对现有电子互感器应用场合局部放电基准相位无法引入的难题,提出一种基于SV采样信号的局部放电监测内同步基准修正方法:首先利用装置自身定时器生成一个虚拟工频周期的"内同步"信号,利用该信号对各个传感器输入信号进行相位预标定;接收合并单元送来的电压互感器的SV信号并以其作为基准对虚拟的工频相位进行修正,从而得到精确和可靠的工频相位基准。所述方案实现简单,对于常规互感器和电子互感器均适用,具有灵活性和普及性。     

An improved PSF mapping method for EPI distortion correction in human brain at ultra high field (7T)

Object  

To further improve the quality and robustness of the point-spread function (PSF) mapping method for fully automatic and accurate correction of geometric distortions in EPI at ultra high field such as 7 Tesla with high fidelity.     

考虑运行方式优化和拓扑校正控制的参考电网优化方法

针对现有参考电网模型未涉及电网现状以及未关联电网运行策略的不足,提出了考虑运行方式优化和拓扑校正控制的参考电网优化方法,为输电网灵活规划提供重要参考。基于前瞻时间尺度内各时段负荷预测值和规划网络候选支路方案,构建以最小化发电成本和输电成本之和为目标,以系统正常运行状态以及N-1事故状态下的运行安全要求为约束条件的参考电网优化模型。对优化模型中混合整数非线性约束式进行处理,将其转换为混合整数二次规划模型,并采用混合整数二次规划法对优化模型进行求解,得到最终的优化方案。最后,通过算例分析说明了所提方法的有效性。     

一种采用记忆神经网络和曲线形状修正的负荷预测方法

针对分布式电源和新型负荷的容量累积造成负荷影响因素多元化和不确定性特性增强的问题，提出一种采用记忆神经网络和曲线形状修正的负荷预测方法。在负荷峰值预测中，采用最大信息系数计算负荷峰值与影响因素的非线性相关性，实现对输入特征的筛选；综合考虑负荷峰值序列的长短期自相关性和输入特征与负荷峰值的不同程度相关性，结合Attention机制和双向长短时记忆神经网络（Bidirectional long short-term memory, BiLSTM）建立负荷峰值预测模型。在负荷标幺曲线预测中，通过误差倒数法组合相似日和相邻日建立负荷标幺曲线预测模型；针对预测偏差的非平稳特征，利用自适应噪声的完全集成经验模态分解和BiLSTM网络建立误差预测模型对曲线形状进行修正。应用中国北方某城市的区域电网负荷数据为算例，验证了所提模型的有效性。     

Optimized EPI for fMRI using a slice-dependent template-based gradient compensation method to recover local susceptibility-induced signal loss

Object  

Most functional magnetic resonance imaging (fMRI) experiments use gradient-echo echo planar imaging (GE EPI) to detect the blood oxygenation level-dependent (BOLD) effect. This technique may fail in the presence of anatomy-related susceptibility-induced field gradients in the human head. In this work, we present a novel 3D compensation method in combination with a template-based correction that can be optimized over particular regions of interest to recover susceptibility-induced signal loss without acquisition time penalty.     

Next day load curve forecasting using hybrid correction method

This work presents an approach for short-term load forecast problem, based on hybrid correction method. Conventional artificial neural network based short-term load forecasting techniques have limitations especially when weather changes are seasonal. Hence, we propose a load correction method by using a fuzzy logic approach in which a fuzzy logic, based on similar days, corrects the neural network output to obtain the next day forecasted load. An Euclidean norm with weighted factors is used for the selection of similar days. The load correction method for the generation of new similar days is also proposed. The neural network has an advantage of dealing with the nonlinear parts of the forecasted load curves, whereas, the fuzzy rules are constructed based on the expert knowledge. Therefore, by combining these two methods, the test results show that the proposed forecasting method could provide a considerable improvement of the forecasting accuracy especially as it shows how to reduce neural network forecast error over the test period by 23% through the application of a fuzzy logic correction. The suitability of the proposed approach is illustrated through an application to actual load data of the Okinawa Electric Power Company in Japan.     

一种新的有源滤波器参考信号提取方法

有源滤波器是改善电力系统电能质量的一种有效工具,参考信号的提取对其补偿性能有重要影响.提出了一种新的有源滤波器参考信号提取方法,采用交叉迭代数值逼近方法,提取单相畸变信号的参考信号,获得三个基本参数的估计:基波幅值、角频率和初相位;在此基础上,按照补偿功率最小的最优原则,推导出三相对称参考信号的表达式.算例结果显示了方法的有效性和准确性.     

一种新的有源滤波器参考信号提取方法

有源滤波器是改善电力系统电能质量的一种有效工具,参考信号的提取对其补偿性能有重要影响。提出了一种新的有源滤波器参考信号提取方法,采用交叉迭代数值逼近方法,提取单相畸变信号的参考信号,获得三个基本参数的估计:基波幅值、角频率和初相位;在此基础上,按照补偿功率最小的最优原则,推导出三相对称参考信号的表达式。算例结果显示了方法的有效性和准确性。     

A novel method for locating the source of sustained oscillation in power system using synchrophasors data

Large interconnected power systems are usually subjected to natural oscillation (NO) and forced oscillation (FO). NO occurs due to system transient response and is characterized by several oscillation modes, while FO occurs due to external perturbations driving generation sources. Compared to NO, FO is considered a more severe threat to the safe and reliable operation of power systems. Therefore, it is important to locate the source of FO so corrective actions can be taken to ensure stable power system operation. In this paper, a novel approach based on two-step signal processing is proposed to characterize FO in terms of its frequency components, duration, nature, and the location of the source. Data recorded by the Phasor Measurement Units (PMUs) in a Wide Area Monitoring System (WAMS) is utilized for analysis. As PMU data usually contains white noise and appears as multi-frequency oscillatory signal, the first step is to de-noise the raw PMU data by decomposing it into a series of intrinsic mode functions (IMF) using Improved Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (ICEEMDAN) technique. The most appropriate IMF containing the vital information is selected using the correlation technique. The second step involves various signal processing and statistical analysis tools such as segmented Power Spectrum Density (PSD), excess kurtosis, cross PSD etc. to achieve the desired objectives. The analysis performed on the simulated two-area four-machine system, reduced WECC-179 bus 29 machine system, and the real-time power system PMU data set from ISO New England, demonstrates the accuracy of the proposed method. The proposed approach is independent of complex network topologies and their characteristics, and is also robust against measurement noise usually contained in PMU data.     

基于负荷预测与关联规则修正的不良数据辨识方法

随着电力系统的快速发展,使得电网需要对海量、异构和多态的数据进行分析与辨识。传统的不良数据辨识方法辨识效率较低,且不能够高效率利用已知的全部数据信息。为解决此问题,提出了一种基于负荷预测与关联规则修正的不良数据辨识方法。根据数据量之间的内在联系,给出了一种三维矩阵的数据存储方法。建立基于回归分析法的预测模型与基于灰色关联的相关性分析模型,分析节点注入功率与温度之间的变化关系,并采用关联规则与特殊断面修正法对预测值进行修正,进而完成对注入功率的辨识。在此基础上,再通过基尔霍夫定律与残差辨识法完成对支路潮流数据的辨识工作。最后应用实际系统的仿真算例证明了该方法能够在克服残差污染和残差淹没现象的前提下准确辨识出全部的不良数据。     

Image artifacts from MR-based attenuation correction in clinical, whole-body PET/MRI

Purpose

Integrated whole-body PET/MRI tomographs have become available. PET/MR imaging has the potential to supplement, or even replace combined PET/CT imaging in selected clinical indications. However, this is true only if methodological pitfalls and image artifacts arising from novel MR-based attenuation correction (MR-AC) are fully understood.

Results

Here we present PET/MR image artifacts following routine MR-AC, as most frequently observed in clinical operations of an integrated whole-body PET/MRI system.

Conclusion

A clinical adoption of integrated PET/MRI should entail the joint image display and interpretation of MR data, MR-based attenuation maps and uncorrected plus attenuation-corrected PET images in order to recognize potential pitfalls from MR-AC and to ensure clinically accurate image interpretation.     

基于参考向量和纠错编码的测试数据压缩算法

随着系统级芯片SoC的集成度越来越高,所需的测试数据量呈指数倍增长,针对测试数据量大这一关键问题,提出了一种有效的基于参考向量和纠错码编码的压缩方案,该方案对测试数据进行三步处理:对测试向量进行分块处理;按照特定的团划分方式选出参考向量;利用纠错码和测试数据相容性的特点对测试数据进行编码.提出的编码具有更短的码字,解压...     

三相PFC电路的新型控制算法研究

在大功率三相整流领域,由于电路功率大、非线性高,传统的控制技术不能有效地达到控制效果.在总结单相有源功率因数校正研究成果的基础上,提出新的三相功率校正的控制方法-模糊自适应PID控制方法.并在Simulink建立仿真模型,验证了该方法的实用性和先进性,达到了理想的结果,在大功率整流设备中具有一定的应用价值.