Elucidation of accuracy in calibration of MR signal intensity based on transmission amplitude method

To calibrate magnetic resonance (MR) signal intensity that depends on radio frequency (RF) coil loading, the transmission amplitude (TRA) for the excitation in the transmit-receive RF coil has been used as a good index in the so-called TRA method. As this TRA method needs neither an internal reference nor an additional external reference for the calibration, its accuracy is free from reference measurements. This study elucidated the calibration accuracy of MR signal intensities based on the TRA method. A cylindrical gel phantom was used for accuracy measurements with a 1.5-T MRI unit with conventional T1 imaging as a simple pulse sequence for various loading conditions. The brain parenchyma of eight healthy volunteers also showed calibrated MR signal deviations. The error of the phantom calibration measurements was 2.18% (S.D.%). The background noise intensity of images was theoretically derived to correlate with the impedance mismatching of the RF coil, which is inevitable for fixed tuning, even for automatic tuning that is not always exact. Taking into account this noise intensity, the calibration method was modified to reduce its error to 1.50%. The standard deviations of the calibrated values in the thalamus and frontal white matter were 2.9 and 3.8%, respectively. We suggest that the modified TRA method is a practical and reliable technique to obtain clinical numeric evidence.     

基于瞬态强度的射频指纹识别方法

射频指纹识别是指无线通信中的发射机识别,用于加强无线安全,其难点在于有效的特征提取,特征提取方法主要分为瞬态和稳态分析,考虑到瞬态波形的差异性,将平均功率与幅度峰值之比定义为瞬态强度,并将其作为指纹特征来识别发射机.实验仿真中,采集了不同类型笔记本无线网卡和同一类型不同系列的波形进行分类性能估计,结果表明,相较于时频域的希尔伯特黄变换和短时傅里叶变换幅值特征,瞬态强度的指纹特征,使射频指纹识别有更高的识别准确率和更短的分类时间.     

基于有限元方法500kV交流输电线路表面电场强度的研究

本文基于有限元方法详细讨论了500 kV交流输电线路导线表面电场的分布规律,并定量分析了线路参数及导线表面特性对导线表面最大电场强度的影响.结果表明,线路采用多分裂、大截面、倒三角布置方式,可以降低导线表面电场强度;不光滑导线将导致导线表面电场增大.     

基于蓝牙技术的电能表无线检定技术研究

传统的电能表检定通常采用人工接线方式,该方式效率低下,操作不当会导致器件损坏,存在安全隐患。为实现高效、可靠地批量电能表误差检定,文中利用低功耗蓝牙技术实现电能表的无线检定。摒弃蓝牙普通模式,采用蓝牙特殊模式,将蓝牙模块从协议层转发信号,改为从底层寄存器直接转发,极大提高了信号转发的稳定性,经多次测试,稳定性可以达到1μs以下,可以满足现在所有准确度等级电能表的检定要求。在检测时,无需将检表台体上的光电头与待检电能表的光脉冲接口和电脉冲接口相连接,减少人工参与,提高了批量电能表的误差检验效率。文中设计的基于蓝牙技术的无线光电脉冲校测系统适用于具有无线发送模块的待检电能表检测。     

基于量子技术的交流电压幅值测量准确度研究

在基于量子技术的交流信号测量中,采样是最常使用的方法,而在此方法中,差分电压的测量精度是决定交流电压幅值测量准确度的主要因素。为此,分析了交流量子电压台阶数、采样率和窗函数对测量准确度的影响。在交流量子电压不存在过渡过程的情况下,当采样率为常数时,通过改变交流量子电压的台阶个数,发现补偿后的相对误差随着台阶个数增加而减小;当交流量子电压的台阶个数为常数时,通过改变采样率,发现补偿后的相对误差随着采样率的提高而减小。在过渡过程和噪声干扰均存在的情况下,采用增加窗函数处理后,有效地提高了测量准确度,可使测量准确度至少提高两个数量级。     

基于振动信号的气体绝缘金属封闭输电线路局部放电诊断方法

为对气体绝缘金属封闭输电线路(GIL)局部放电缺陷进行识别与诊断,本文提出一种基于1.5维能量谱与粒子群优化极限学习机(PSO-ELM)的局部放电缺陷诊断方法.首先,通过计算局部放电引起的GIL异常振动信号的1.5维能量谱,得到不同类型放电情况的能量波动特征;其次,构建PSO-ELM模型,以1.5维能量谱作为特征量对GIL局部放电故障进行识别与诊断;最后,通过不同方法的对比,验证该方法的优越性,为GIL在输配电系统中的安全、稳定运行提供依据.     

An adaptive improved unsaturated bistable stochastic resonance method based on weak signal detection

Stochastic resonance can detect weak periodic signals from strong background noise without loss of signal energy. However, the classical bistable stochastic resonance has the inherent output saturation defect, which limits the detection performance of system. And it is more difficult to detect signal with strong background noise. In this article, we constructed improved unsaturated bistable stochastic resonance to overcome this shortcoming. The improved bistable potential function makes the output signal more easily oscillate in two potential wells. To improve the stability and the accuracy of the method, we further propose an adaptive improved unsaturated bistable stochastic resonance (AIUBSR) by constructing a synthetic index (SI). The SI combines zero-crossing ratio and structural correlation coefficient, which can measure the periodicity of output signal and the accuracy of detective frequency at the same time. Theoretical analysis and numerical simulations show that the proposed AIUBSR can have good weak signal detection capability in strong background noise.     

Suppression of MR gradient artefacts on electrophysiological signals based on an adaptive real-time filter with LMS coefficient updates

Electrocardiogram (ECG) acquisition is still a challenge as gradient artefacts superimposed on the electrophysiological signal can only be partially removed. The signal shape of theses artefacts can be similar to the QRS-complex, causing possible misinterpretation during patient monitoring and false triggering/gating of the MRI. For their real-time suppression, an adaptive filter is proposed. The adaptive filter is based on the noise-canceller configuration with LMS coefficient updates. The references of the noise canceller are the three gradient signals that are acquired simultaneously with the noisy ECG. Tests were done on patients, on volunteers and using an MR-safe ECG simulator. The noise cancellers performance was measured offline, simulating real-time processing by point-by-point operations. To create worst-case scenarios, clinical sequences with strong- and fast-switching gradients have been chosen. The noise-cancelling filter reduces the gradient artefacts peak amplitudes by 80–99% after adaptation, without changing the desired ECG signal shape. The estimated reduction of total average power of the MR gradient artefacts is 62–98%. The proposed filter is capable of reducing artefacts due to strong- and fast-switching gradients in real-time applications and worst-case situations. The quality of the ECG is sufficiently high that a standard one-lead QRS-detector can be used for gating/triggering the MRI. For permanent patient monitoring, further improvements are needed.     

基于ZigBee的485信号传输系统设计

设计了一个基于ZigBee技术的485信号无线传输组网系统,着重介绍了系统的无线接口硬件的设计,针对RS485网络主要应用于工业现场,采用了宽输入的双电源隔离供电,并对485信号进行了光电隔离处理,极大地提高了485电路通信的可靠性,保证了系统稳定运行。采用VB.NET语言编写了无线传输网络的管理软件,在ZigBee模块的透明操作模式下可实现对网络的管理与控制。     

Adaptive stochastic resonance method for weak signal detection based on particle swarm optimization

In order to solve the parameter adjustment problems of adaptive stochastic resonance system in the areas of weak signal detection, this article presents a new method to enhance the detection efficiency and availability in the system of two-dimensional Duffing based on particle swarm optimization. First, the influence of different parameters on the detection performance is analyzed respectively. The correlation between parameter adjustment and stochastic resonance effect is also discussed and converted to the problem of multi-parameter optimization. Second, the experiments including typical system and sea clutter data are conducted to verify the effect of the proposed method. Results show that the proposed method is highly effective to detect weak signal from chaotic background, and enhance the output SNR greatly.     

基于IAS精确估计的太阳轮齿面剥落检测方法研究

瞬时角速度(instantaneous angular speed, IAS)信号的估计精度易受编码器自身误差影响,影响其故障检测结果。针对上述问题提出了一种局部多项式微分估计方法对IAS信号进行估计。首先通过局部多项式微分抑制编码器细分误差产生的锯齿状噪声和估计误差导致的信号不连续问题,实现了IAS信号的高精度获取,然后使用精确估计的IAS信号通过局部同步拟合技术提取太阳轮故障特征。通过实验和仿真说明,本文提出的局部多项式微分估计能较好地抑制编码器误差导致的IAS信号噪声,提高IAS信号的信号比,结合局部同步拟合能够有效地提取太阳轮齿面剥落故障特征。     

基于表面电荷法的输电线路表面电场强度计算

三相输电线路导线表面电荷的分布是不均匀的,各点电场强度的大小也不同.文中采用表面电荷法计算导线表面的电荷密度,然后求出导线表面各点的最大电场强度值.计算方法能够反映出电荷密度及电场强度沿导线圆周分布的情况,根据计算结果优化高压输电线路的布置.     

MR imaging of the prostate in clinical practice

Magnetic resonance imaging (MRI) is the imaging tool of choice in the evaluation of prostate cancer. The main applications of MR imaging in the management of prostate cancer are: (1) to guide targeted biopsy when prostate cancer is clinically suspected and previous ultrasound-guided biopsy results are negative; (2) to localize and stage prostate cancer and provide a roadmap for treatment planning; and (3) to detect residual or locally recurrent cancer after treatment. Other MR techniques such as proton MR spectroscopic imaging (MRSI), diffusion-weighted imaging (DWI), and contrast-enhanced MRI (CE-MRI) complement conventional MR imaging by providing metabolic and functional information that can improve the accuracy of prostate cancer detection and characterization. In everyday clinical practice, and to account for patient comfort, MR imaging studies are limited to 1 h. To obtain consistently high-quality images, a well-designed protocol is necessary. Routine MR imaging can be supplemented by other MR techniques such as MRSI, DWI or CE-MRI depending on the expertise available and the clinical questions that need to be answered. This review summarizes the role of MR imaging in the management of prostate cancer and describes practical approaches to implementing anatomic, metabolic and functional MR imaging techniques in the clinic.     

交流输电线路交叉跨越区空中电场计算分析

为了实现利用输电线路空间电场分布特征的无人机巡检智能导航避障,研究交叉跨越区域空间电场强度分布规律,利用三维电场估算法和Ansys有限元分析软件,对交流500 k V垂直跨越220 k V线路进行三维建模计算。结果表明,在与交叉区域水平距离保持不变的情况下,随着测试点高度的变化,电场强度呈现先增大后减少的变化规律;此外,高电压等级线路对交叉区域电场强度分布起主导作用。此外,在保持测试点高度不变的情况下,随着测试点与交叉区域距离的增大,其电场强度值呈现明显的单调递减变化趋势。利用小型无人机搭载电场测试仪对交叉区域电场实测,实测值和理论值误差均在允许范围内,证明了分析结果的有效性,所研究对输电工程的空间电磁分析提供参考意义。     

基于STD标准的ATS信号组件模型研究与实现

按照ATS软件平台的通用化、模块化设计思想,本文采用信号组件形式构建ATS软件平台。首先对STD标准进行了简要介绍,设计了基于STD标准的ATS信号组件模型,并针对测试与激励对各个信号组件之间的交互关系进行了分析;然后以ResourceManager资源管理器组件为例,对该模型中的关键信号组件进行了开发;最后以信号组件为基础,编写了ATS测试程序,并以某组合电源模块测试为例进行了测试验证。结果表明该方法达到了预期测试目的,解决了测试程序的可移植性问题。     

高精度波导六端口测量系统校准的实现与优化

六端口反射计校准技术是确定六端口测量装置系统常数的方法。校准的精度决定了测量精度,因此校准技术是六端口技术的核心。本文采用基于高精度功率计的波导六端口反射计测量系统实现了R-B法校准,在此基础上利用C.A.Hoer法对R-B法的校准结果进行优化。测试结果表明,R-B法校准精度较低但已接近理论真值,采用C.A.Hoer法对R-B法进行迭代优化可以在较宽的频带内实现高精度校准。     

基于时分处理的BOC信号通道时延标定算法

卫星导航信号发射通道的设备自身时延是星地组合时延的一个部分,卫星在地面测试中必须进行通道时延标定。针对导航卫星BOC（binary offset carrier）调制导航信号发射通道绝对时延,提出了一种采用时分处理的时延标定算法。在完成导航卫星输出BOC导航信号和时间保持系统产生的秒脉冲信号双通道同步采样后,在数字域对BOC采样信号进行预处理和串并变换,将高速数据流变换为多路低速率数据流,对每路信号频率码相位捕获,从而得到BOC信号发射通道绝对时延。计算机仿真表明在采样率10 Gsa/s时提出算法的标定不确定度优于0.2 ns。     

基于高频信号的输电线路主动式保护

现行输电线路的故障类型和故障性质判别均依据线路中电气量的变化特征被动判断,对于某些线路运行状况,往往因缺乏主动性而造成保护误动或拒动。针对这个问题,借鉴纵差保护输电线载波通信技术,分析高频信号在输电线路各种故障时的传播特性,提出了一种基于高频信号的输电线路主动式保护。该保护通过向输电线路发射高频信号,主动判断输电线路的故障类型和故障性质。仿真验证结果表明,这种输电线路主动式保护可以准确判断出输电线路中各种类型的故障,并通过延时循环判断,可靠区分瞬时性故障和永久性故障。     

基于反行波与信号处理的特高压直流输电线路纵联保护方法

现阶段直流输电线路纵联保护单纯地采用叠加原理,未考虑直流输电系统的强非线性,保护在实际工程中存在适应性问题。针对此问题,提出一种纵联保护新方法。从保护方法的适应性角度出发,提出基于描述函数法的信号线性化处理技术。通过分析直流输电线路区内、外故障发生后一段时窗内反行波的传输特性,发现发生区内故障时,线路两端反行波波形间相似度较高;发生区外故障时,线路两端反行波波形间相似度较低;进而提出一种基于反行波的纵联保护方法,该方法利用Hausdorff距离算法度量线路两端反行波的相似度,构造直流输电线路故障识别判据。仿真结果表明,该保护方法能可靠区分直流线路区内、外故障,且在发生高阻接地故障时具有较高灵敏性。     

基于信号复杂度衰减的特高压直流输电线路雷电暂态识别方法

特高压直流输电线路雷击引起的暂态信号高频分量是行波保护和暂态保护误动的主要因素之一。发生线路雷击故障和普通接地短路故障时,电压信号的幅值因故障电流入地通路的存在快速衰减,信号复杂度低。雷击未故障时,不存在故障电流入地通路,信号衰减慢,复杂度高。基于小波变换分析电压信号频率分量的衰减,采用突变时刻前、后两个数据窗内小波奇异熵的比值表征信号复杂度的变化。再结合电压信号高频分量的含量分析,实现雷电暂态信号的识别,并通过能量相对熵实现故障极的选择。在PSCAD/EMTDC中搭建仿真模型,结果表明,该方案能很好地进行暂态信号的识别,受故障距离、过渡电阻等因素的影响小。