Spiral快速成像方法的实现

介绍了在Bruker Biospec47/30超导核磁共振成像仪(4.7T)上实现Spiral快速成像方法的主要步骤,其中包括梯度场波形、k空间的采样点分布、网格重建算法和图像重建.在空间分辨率为64×64(图像8cm×8cm)的条件下,采样时间为11ms.实验表明基于自旋回波的Spiral成像方法能够有效减小偏共振效应的影响.将自旋回波Spiral方法用于柠檬果肉部分T 2 的测定,结果与常规方法得到的T 2 数值完全符合.     

融合边界条件的电阻抗图像重建仿真研究

结合成像目标的边界结构信息,提出一种新的改善电阻抗图像重建质量的方法.在生物医学电阻抗断层成像中,将传统采用圆形场域的重建模型改进为与人体胸腔边界近似的八边形场域,建立正问题数学模型,求解得到灵敏度系数矩阵,经等位线滤波反投影算法获得感兴趣区域的仿真分布图像.针对成像目标在场域中的不同分布情况,采用面积占空比对两种重建模型下的成像效果进行评价.图像及数据分析结果表明,改进方法重建图像伪影明显减少,面积占空比也更接近真实分布,提高了图像空间分辨率,达到了改善电阻抗重建图像质量的目的.     

PMC边界手征负折射平板波导模式特性及场分布

从理论上分析了以理想磁导体(perfect magnetic conductor, PMC)为边界的手征负折射平板波导的模式特性及其磁场的具体分布. 利用PMC边界条件, 导出了手征负折射波导的色散方程和磁场各分量表达式. 通过波导的色散曲线发现, 当手征参数 κ>1时, 某些区域曲线出现下凹, 左旋圆极化(left circularly polarized, LCP)波的折射率为负值, 实现了负折射特性, 截止频率也不再是传播常数β=0所对应的频率. 根据传播常数β和k₊, k₋的关系, 将色散曲线分为3个区域, 给出了各区域一阶奇模和一阶偶模磁场的具体分布, 并与一般手征媒质平板波导的磁场分布作了比较. 结果发现, 切向分量场H_y,H_z 在上下边界处为零, 满足PMC边界条件. 而由于面磁流的存在, 法向分量H_x在边界处不为零.     

用于电容层析成像技术的共轭梯度算法

针对电容层析成像技术中的“软场”效应和病定问题,基于灵敏度矩阵的奇异值分解理论,提出共轭梯度图像重建算法及其改进算法———正则化共轭梯度法.仿真实验得知:经过 200次迭代后,Landweber算法残差为0. 139 5,未加正则化的共轭梯度算法残差为 1. 357 7×10-4;完成同样操作,Landweber算法迭代耗时 9. 3s,共轭梯度法只需 6. 8s.可见,共轭梯度法是一种比其他的迭代算法收敛更快、成像效果更好的图像重建算法.     

两相流电磁成像测量正问题研究

应用有限元方法对电磁流动成像测量进行正演仿真,模拟不同测量组合下各种流型的电势分布以及对应流型下各电极的测量电压值,最后模拟6种典型测量敏感场.仿真结果表明:介质分布不同是影响电势分布差异的根本原因;核心流测量电压值曲线呈简单"U"型变化,贴井壁泡流曲线变化规律不明显,层流曲线存在拐点;敏感场分布不均匀,不对称,且存在负值.正演模拟的结果验证了电磁成像测量方法的可行性,并为下一步传感器设计和图像重构奠定基础.     

Gaussian窗函数在电容成像图像重建中的应用

为降低敏感场矩阵的小奇异值给电容成像(electri-cal capacitance tomography,ECT)问题带来的不稳定性,以提高图像重建质量,该文从正则化方法的谱滤波形式出发,提出了一种基于Gaussian窗函数的ECT图像重建正则化方法。该方法利用Gaussian窗函数的滤波特性,通过选择合适的Gaussian窗函数幂次,来达到降低敏感场矩阵的小奇异值对重建结果造成不稳定的效果,从而获得较好的图像重建结果。对5种典型介电常数分布进行了数值仿真,结果表明:该算法对于两相流不同介质分布均具有良好的适应性,可获得较好的成像结果,部分分布的重建图像轮廓较Tikhonov正则化方法更为清晰。     

基于系统矩阵优化的二维磁性粒子成像研究

磁性粒子成像(MPI)是一种新型高分辨率成像技术,利用磁性粒子在交变磁场中的非线性响应构建系统矩阵进而重建磁性纳米粒子的浓度分布,提高重建速度并降低存储空间需求和计算复杂度是实现实时成像的关键.本文将磁性粒子的非线性磁化响应特征与电磁感应定律相结合获取检测点电压信号,进一步考虑接受线圈的灵敏度可得电压信号与磁性粒子浓度的关系,利用傅里叶变换及频域矩阵展开分析了影响系统矩阵的因素,系统分析了系统矩阵频率分量的选取以及不同接收方向对重建图像的影响.结果表明,通过选取高频段信号可以优化系统矩阵分量的空间结构;通过增加频率分量可以构建线性无关方程组,使方程的解唯一化,提高重建精度和质量;通过不同接收方向系统矩阵的重组,使系统矩阵拥有更丰富的空间结构,进而提高浓度分布重建图像的质量.本研究对MPI技术进行磁性纳米粒子浓度重建起到了重要的指导作用,在新型生物医学成像领域有着广阔的应用前景.     

基于灵敏度场非线性的ECT图像重建改进算法

为了克服ECT系统重建图像精度低的问题,提出一种基于灵敏度场非线性的图像重建改进算法.首先论述了灵敏度场的原理及计算方法,然后给出了利用有限元仿真软件comsol求解灵敏度场的方法;在此基础上,在灵敏度场中提取相邻极板间的灵敏度向量,利用此向量求解图像变换系数,并将其运用到图像重建算法中;最后通过仿真实验对所提方法的有效性进行了验证.实验结果表明利用本文方法重建的图像精度更高,这也为ECT图像重建技术提供了新的途径和手段.     

ECT系统中重建高介电常数对象的算法

针对电容层析成像(ECT)系统中常用的快速图像重建算法线性反投影(LBP)算法在重建高介电常数成像对象的图像时,重建的图像质量差,提出一种高介电常数成像对象的快速图像重建算法,根据不同介电常数成像对象的成像敏感场分布图具有高度相关性的特点,采用成像敏感场灵敏度替换法提升高介电常数对象的成像敏感场灵敏度,利用正则化图像重...     

基于单电极激励模式的颅脑电阻抗图像重建方法研究

作为一种新兴的可视化技术,电阻抗层析成像(EIT)能够根据人体组织病理变化对其电导率分布进行图像重建,为疾病检测提供了一种选择.在基于EIT的脑部疾病检测中,为了改善被测区域的灵敏度分布并解决电阻抗成像中典型的不适定问题,在单电极激励数据采集模式下,提出了k阶有限差分L₁正则化目标函数,并采用增广拉格朗日和交替方向算法对目标函数进行求解,实现电导率分布的重构.研究了单电极激励模式下,外接电阻对敏感场的影响;针对脑出血和脑缺血两种病情,对比了Landweber方法、Newton-Raphson方法、Tikhonov方法、广义总变分方法(TGV)和本文方法的图像重建性能.结果表明,在脑出血和脑缺血的图像重建中,采用单电极激励模式的ALAD-LR方法可有效提高图像重建质量,并具有较强的鲁棒性.     

非均匀磁场下单个经典电子的运动

理论推导并用MATLAB数值求解了梯度磁场下单个电子的运动方程,得出了电子长时间平均位移是沿着磁场为零的线运动的结论.在此基础上,加上一个与电子平均运动方向相反的匀强电场,得出了其类似于匀减速直线运动的特性.再将情况进一步推广到随机磁场的情况,得出仅在随机磁场作用下,电子仍然沿着磁场强度等于零的线运动的结论.     

Optimizing ultrasensitive single electron magnetometer based on nitrogen-vacancy center in diamond

The measurement of the weak magnetic field in nanoscale resolution and at room temperature is always a significant topic in biological, physical, and material science. Such detection can be used to decide the characterization of the samples, such as cells, materials, and so on. Nitrogen-vacancy (NV) center in diamond has been proved to be able to detect a magnetic field with nano Tesla sensitivity and nanometer resolution at room temperature. Here we experimentally demonstrate an optimized NV center based single electron magnetometer in a commercial diamond and under a home-built optically detected magnetic resonance (ODMR) microscope. With current technology, we change the optically detected time window to get a better signal to noise ratio, and use dynamical decoupling to increase the slope of magnetic field amplitude versus fluorescence signal. By employing the 8-pulse XY-4 dynamical decoupling sequence we achieve a sensitivity of 18.9 nT (Hz)(1/2) , which is 1.7 times better than spin echo. We also propose a NV center based scanning diamond microscope for electron and nuclear spins detection as well as nanoscale magnetic resonance imaging. If it is realized, the NV center based magnetometry will have wide application in the future.     

磁性液体薄片磁控光开关效应研究

研究了在平行于激光照射方向递增的梯度磁场作用下的磁性液体薄片光透射率随时间的变化情况。结果表明,在磁场作用下,磁性液体内部形成微粒链结构,微粒链受到梯度磁场汇聚力,向薄片中心聚集,在此处形成光关闭现象。通过改变磁性液体体积分数、外磁场变化率、初始磁场大小可以达到控制调节磁性液体的响应时间和光关闭时间的效果,为磁性液体作为可控光开关提供了可行性研究。     

Influences of the shape of magnetic material on the magnetoelectric properties of the PZT/Metglas composite

In this paper, the influences of the shape of magnetic material on the magnetoelectric(ME) properties of PZT/Metglas magnetoelectric(ME) composites have been investigated. The results indicate that, with the decrease of the waist length(L_w) of the dumbbell-shaped Metglas, the magnetic flux density in the center region and ME coefficients(α_(ME)) of the composites increase, while the optimal bias magnetic field Hdc decreases on the contrary. In an AC magnetic field of 1 kHz, the maximum αME(α_(Max)) of the composite with L_w= 20 mm exhibits 1.3 times larger than that of the one with L_w=50 mm, and the optimal Hdc deceases by 15%. At the resonant frequencies of each composites, α_(Max) is enhanced by 1.3 times as L_w decreases from 50 to 20 mm. The simulation made by Comsol Multiphysics and the theoretical analysis based on an equivalent magnetic circuit confirm the experimental results.     

Nanoscale imaging magnetometry with diamond spins under ambient conditions

Magnetic resonance imaging and optical microscopy are key technologies in the life sciences. For microbiological studies, especially of the inner workings of single cells, optical microscopy is normally used because it easily achieves resolution close to the optical wavelength. But in conventional microscopy, diffraction limits the resolution to about half the wavelength. Recently, it was shown that this limit can be partly overcome by nonlinear imaging techniques, but there is still a barrier to reaching the molecular scale. In contrast, in magnetic resonance imaging the spatial resolution is not determined by diffraction; rather, it is limited by magnetic field sensitivity, and so can in principle go well below the optical wavelength. The sensitivity of magnetic resonance imaging has recently been improved enough to image single cells, and magnetic resonance force microscopy has succeeded in detecting single electrons and small nuclear spin ensembles. However, this technique currently requires cryogenic temperatures, which limit most potential biological applications. Alternatively, single-electron spin states can be detected optically, even at room temperature in some systems. Here we show how magneto-optical spin detection can be used to determine the location of a spin associated with a single nitrogen-vacancy centre in diamond with nanometre resolution under ambient conditions. By placing these nitrogen-vacancy spins in functionalized diamond nanocrystals, biologically specific magnetofluorescent spin markers can be produced. Significantly, we show that this nanometre-scale resolution can be achieved without any probes located closer than typical cell dimensions. Furthermore, we demonstrate the use of a single diamond spin as a scanning probe magnetometer to map nanoscale magnetic field variations. The potential impact of single-spin imaging at room temperature is far-reaching. It could lead to the capability to probe biologically relevant spins in living cells.     

Pressure gradient evolution in the near-Earth magnetotail at the arrival of BBFs

Using in situ observations from THEMIS A, D and E during the 2008-2011 tail season, we present a statistical study of the evolution of pressure gradients in the near-Earth tail during bursty bulk flow （BBF） convection. We identified 138 substorm BBFs and 2,197 non-substorm BBFs for this study. We found that both the pressure and the Bz component of the magnetic field were enhanced at the arrival of BBFs at the spacecraft locations. We suggest that the increase of Bz during non-substorm BBFs is associated with flux pile-up. However, the much stronger enhancement of Bz during substorm BBFs implies the occurrence of magnetic field dipolarization which is caused by both the flux pile-up process and near-Earth current disruption. Furthermore, a bow-wave-like high pressure appears to be formed at the arrival of substorm BBFs, which is responsible for the formation of region-1-sense FACs. The azimuthal pressure gradient associated with the arrival of substorm BBFs lasts for about 5 min. The enhanced pressure gradient associated with the bow wave is caused by the braking and diversion of the Earthward flow in the inner plasma sheet. The results from this statistical study suggest that the braking and azimuthal diversion of BBFs may commonly create azimuthal pressure gradients, which are related to the forrnation of the FAC of the substorm current wedge.     

Thermoluminescence of Al2O3 crystals grown by temperature gradient techniques

Color center concentrations of the Al2O3 crystal grown by temperature gradient techniques were calculated from Gaussian fits to absorption spectra. The concentrations for F- and F + -centers at 204, 232, and 255 nm were determined to be 1.361×10 17 , 0.098×10 17 , and 0.325×10 17cm-3 , respectively. Studies have shown that the thermoluminescence (TL) glow curve exhibits a prominent 450 K peak that matches well the first-order fitting curve. The thermal activation energy E and frequency factor s of the trap were determined to be 0.94456±0.00545 eV and 5.8703×10 11 s-1 . With this theoretical analysis, a simple one-trap/one-center TL model is presented to provide a theoretical explanation of the TL process.     

Low-temperature magnetic properties of horse spleen ferritin

The magnetic properties of the antiferromagnetic cores in ferritin are of importance in the construction and improvement of ferritin-based magnetic resonance imaging systems and their application to environmental magnetism. In this study, we carry out integrated magnetic and transmission electron microscopy analyses of horse spleen ferritin (HoSF) to understand the relationships between the magnetic behavior of HoSF and temperature, applied field and grain-size distributions. The R-value from the Wohlfarth-Cisowski test for the investigated sample at 5 K was 0.46, indicating very weak magnetostatic interactions among the nanoparticles of HoSF. The nanoparticles of HoSF show superparamagnetic properties at room temperature, while below the blocking temperature of T _b ≈ 12 K it has a net magnetic moment that comes from the uncompensated spins of the nanoparticle surface or spin-canting. The thermal relaxation process of HoSF follows the Néel-Arrhenius expression. From low-temperature AC susceptibility data, we calculated the effective magnetic anisotropy energy E _a=(5.52±0.16)×10^?21 J; the effective magnetic anisotropy energy constant K _eff =(4.65±0.14)×10⁴ J/m³ and the pre-exponential frequency factor f ₀=(4.52±2.93)×10¹¹ Hz. These values are useful in understanding the magnetic behavior of the antiferromagnetic nanoparticles and their potential application in biomedical technology.     

交流磁场对Al-Fe合金凝固组织的影响

研究了交流磁场对Al-Fe合金中含铁相形态及分布的影响.近共晶Al-1.99%Fe(质量分数,下同)和亚共晶Al-0.90%Fe两种铝合金在3℃/min的冷却速率凝固过程中,施加强度为0.3 T,频率为20 Hz的交流磁场,并与未施加磁场的试样进行对比.实验结果表明,在这两种合金中,无论是先析出Al3Fe相还是先析出-αAl,施加交流磁场后,Al3Fe相均向试样的中心处富集.这是由于Al3Fe相的磁化率大于熔融铝基体的磁化率,使得Al3Fe相和铝基体相比受到指向样品轴线的更大的电磁力,从而导致其向试样中心处聚集.     

超磁致伸缩驱动微定位平台的动态特性研究

针对超磁致伸缩驱动器(Giant Magnetostrictive Actuator,简称 GMA)磁滞非线性和动态特性难以精确描述的问题,以基于GMA的二自由度精密微定位平台为对象,对其进行磁滞非线性和动力学建模并仿真分析。根据Jiles-Atherton磁滞模型、磁致伸缩模型、磁滞非线性圧磁方程和驱动器的结构动力学原理,建立了组合的微定位平台磁滞非线性动态模型;通过模型计算出输出位移的大小;另外对四种数学模型进行动态特性仿真分析。研究结果表明:仿真位移曲线与实验测得输出位移曲线基本一致,验证了所建模型的正确性;且优化了系统的设计参数,对改善GMA的动态性能和稳态输出性能提供了依据。