Reduction of flow- and eddy-currents-induced image artifacts in coronary magnetic resonance angiography using a linear centric-encoding SSFP sequence

Coronary magnetic resonance angiography (MRA) acquired using steady-state free precession (SSFP) sequences tends to suffer from image artifacts caused by local magnetic field inhomogeneities. Flow- and gradient-switching-induced eddy currents are important sources of such phase errors, especially under off-resonant conditions. In this study, we propose to reduce these image artifacts by using a linear centric-encoding (LCE) scheme in the phase-encoding (PE) direction. Abrupt change in gradients, including magnitude and polarity between consecutive radiofrequency cycles, is minimized using the LCE scheme. Results from numeric simulations and phantom studies demonstrated that signal oscillation can be markedly reduced using LCE as compared to conventional alternating centric-encoding (ACE) scheme. The image quality of coronary arteries was improved at both 1.5 and 3.0 T using LCE compared to those acquired using ACE PE scheme (1.5 T: ACE/LCE=2.2+/-0.8/3.0+/-0.6, P=.02; 3.0 T: ACE/LCE=2.1+/-1.1/3.0+/-0.8, P=.01). In conclusion, flow- and eddy-currents-induced imaging artifacts in coronary MRA using SSFP sequence can be markedly reduced with LCE acquisition of PE lines.     

SQUID用于铍材残余应力检测的计算机模拟

依据SMM-601型超导量子干涉显微镜检测部分构建计算模型,采用电磁场有限元方法分析了残余应力引起的非磁性金属材料铍电导率变化对超导量子干涉仪(SQUID)检测信号的影响,并讨论了该方法实际应用于非磁性金属材料残余应力检测的条件和可能性。     

Permeability enhancement by induced displacement current in magnetic material with high permittivity

Based on a analytical model, this paper reveals that instead of demagnetization, the induced displacement current in a Mn–Zn ferrite core due to its high permittivity can produce a magnetic field that effectively adds to the incident magnetic field in the low-loss frequency region. This leads to a resultant magnetic field being larger than incident field in magnitude, and results in measured permeability being higher than its intrinsic value in the low-loss frequency region. It is also found that such effect is more pronounced in large core than in small core. With increasing frequency and consequently the core being more lossy, the induced magnetic field due to both conductivity and imaginary part of complex permittivity demagnetize the magnetic material and counteracts the incident magnetic field. In consequence, the measured real part of complex permeability becomes smaller than its intrinsic value. The larger the core dimension, the smaller the real part of complex permeability is. These conclusions based on the numerical analysis are in good agreement with the experimental observation in the aspect of change tendency of permeability versus frequency.     

Kinetic investigation of narrow-bore columns packed with prototype sub-2 μm superficially porous particles with various shell thickness

The recent successful breakthrough of sub-3 μm shell particles in HPLC has triggered considerable research efforts toward the design of new brands of core-shell particles. We investigated the mass transfer mechanism of a few analytes in narrow-bore columns packed with prototype 1.7 μm shell particles, made of 1.0, 1.2, and 1.4 μm solid nonporous cores surrounded by porous shells 350, 250, and 150 nm thick, respectively. Three probe solutes, uracil, naphthalene, and insulin, were chosen to assess the kinetic performance of these columns. Inverse size exclusion chromatography, peak parking experiments, and the numerical integration of the experimental peak profiles were carried out in order to measure the external, internal, and total column porosities, the true bulk diffusion coefficients of these analytes, the height equivalent to a theoretical plate, the longitudinal diffusion term, and the trans-particle mass transfer resistance term. The residual eddy diffusion term was measured by difference. The results show the existence of important trans-column velocity biases (7%) possibly due to the presence of particle multiplets in the slurry mixture used during the packing process. Our results illustrates some of the difficulties encountered by scientists preparing and packing shell particles into narrow-bore columns.     

EAST等离子体击穿阶段真空室上涡流算法研究

托卡马克装置产生等离子体击穿的条件是在真空室区域中达到足够大的环电压以及尽可能大的零场区。在等离子体击穿阶段,由于EAST装置极向场线圈有较大的电流变化,会在真空室上感应出涡流,因此要考虑涡流对环电压和零场区的影响。主要介绍了数值法计算EAST装置真空室上涡流,只有正确计算涡流,才能为EAST实验提供了很好的参考价值。     

磁场渗透导体壳的实验研究

本文叙述了测量阶跃脉冲垂直磁场和水平磁场渗透HL-1装置各种比例导体壳时间常数的方法。对空芯和铁芯变压器情况分别做了实验。给出了相应结果。     

Hybrid magnetic field formulation based on the losses separation method for modified dynamic inverse Jiles-Atherton model

Dynamic formulation based on the losses separation method in conducting media for the inverse Jiles-Atherton model is proposed. This formulation is based on the concept of the Hybrid Magnetic Field model (HMF). The HMF consists of the modification of the effective field by introducing two counter-fields associated, respectively, with the eddy current and excess losses. Such a formulation is characterized by seven parameters with five parameters issued from the quasi-static Jiles-Atherton model. Thus, two new parameters related to these fields are added to that defined in the quasi-static model. The identification of these new parameters is based on the measurements of the volumetric energy density. To validate this formulation, measurements are carried out on grain non-oriented Fe-Si 3% electrical sheets.     

A symmetric BEM–FEM method for an axisymmetric eddy current problem

This paper deals with the numerical solution of the time-harmonic eddy current model in an axisymmetric unbounded domain. To this end, a new symmetric BEM–FEM formulation is derived and also analyzed. Moreover, error estimates for the corresponding discretization are proven.     

粘性流体中涡旋扩散的非牛顿效应

在考虑瞬态过程中动量扩散速度基础上，用拉普拉斯变换法对无限长平板突然起动瞬态动量定解方程进行了求解，并按涡量定义求出涡量分布函数。结果表明伴随动量扩散的波动性，涡量扩散也存在波动性。