Static Magnetic Field Intensity Research Articles

Effects of main magnetic field time-variation upon mr images in permanent magnet mri system

The effect of the time-variation of the static magnetic field in the permanent magnet MRI system on the image quality is studied, together with the method to eliminate the effect. The permanent magnet MRI system has a larger-time-variation of the static magnetic field than the superconducting magnet MRI system due to the temperature variation of the permanent magnet used to produce the static field. Thus, one of the important problems inherent to the permanent magnet MRI system is to examine the time-variation of the static magnetic field intensity within the imaging time on the image quality. However, no detailed report has been made on this effect. This paper shows that the effect of such variation on the image can be represented by the point-spread function. Through the analysis of this point-spread function, it is shown that the effect of the time-variation of the static magnetic field is negligible in the ordinary imaging condition. Furthermore, a method is presented to cope with a time-variation of the static magnetic field, which is produced by a sudden change of the hardware environment and is much larger than the variation in the usual imaging condition. The variation of the static magnetic field in this method is measured during the imaging, and the effect of the variation is eliminated. The effectiveness of the method is verified based on the signal data obtained from the actual system.

4899109 Method and apparatus for automated magnetic field shimming in magnetic resonance spectroscopic imaging

An automated magnetic field shimming process for magnetic resonance spectroscopic imaging (MRSI) uses doubly phase encoded NMR spin echo responses in conjunction with multiple fourier transformation to derive a spectroscopic plot for each voxel in a three-dimensional array of voxels in an image volume (including portions of a living subject) which is to be subsequently subjected to MRSI using NMR FID or spin echo responses. The thus derived hydrogen peak frequency is taken as a measurement of existing static magnetic field intensity within that voxel and adjusted shim coil currents are calculated so as to reduce or minimize nonuniformity of the static magnetic field. A nonexistent pseudo shim coil having an assumed uniform shim contribution may enhance the resulting field homogeneity. The auto shimming procedure may be applied iteratively as required to achieve the predetermined degree of field uniformity.

THEORETICAL ANALYSIS OF FREE ELECTRON LASER AMPLIFIER

In this paper, a detailed analysis on the operating mechanism and parameter properties of free electron laser amplifier (FELA) in the case of small signal is presented. The results show that FELA is in essence the interaction between the electron beam and the fast cyclotron wave. The cyclotron resonance condition of electron motion in the uniform axial and periodic transverse static magnetic fields significantly influences the interaction process. We have calculated numerically the central operating frequency, band width, small signal gain and wave number of the amplified electromagnetic wave as functions of the parameters of the amplifier (such as the voltage and current density of the electron beam, the intensities of axial and transverse static magnetic field). The way to optimize the design of FELA is also suggested. It is possible to obtain a small signal gain of 10-2 cm-1 with enough band width in the millimeter and submillimeter range of weve length following a proper design.

Resonance of a wire antenna near the lower hybrid frequency in an RF-generated magnetoplasma

The lower hybrid resonance (LHR) of a wire antenna immersed in an RF-generated laboratory plasma was observed with a frequency sweeping admittance bridge. It is shown that the resonance frequency observed in the experiment agrees with the LHR frequency calculated from an ion density and a static magnetic field intensity.

General purpose computer program for exact esr spectrum calculations with applications to vanadium chelates

A computer program has been developed for the calculation of ESR spectra directly from spin and radiation matrices. Positions of lines in terms of the static magnetic field and relative line intensities are computed and plotted as Dirac δ functions. Powder spectra are obtained as a summation of single crystal data calculated for “well-chosen” orientations. The integrated powder spectrum envelope or its derivatives are obtained as output plots, with ad hoc line-shape functions being selected as justified by empirical data. The program requires the following as basic input data: Terms of a suitable Hamiltonian, parameter values ( A, D, g, S, I, etc.), and orientation directions. Provisions are made to alter the accuracy of the program for rapid operation, where “higher order” effects are of no interest, and to ignore forbidden transitions completely if so desired. This program is used to synthesize and study the powder spectrum of a vanadyl chelate (axial symmetry: S = 1 2 I = 7 2 Limitations and extensions of the program are discussed with particular reference to vanadium chelates.

Plasma acceleration using a high frequency field and a static magnetic field

The acceleration of a plasma by an intense electromagnetic field and an inhomogeneous static magnetic field was studied. Numerical calculations based on simplified plasma models showed that plasma ions can be accelerated up to 100 keV along the magnetic axis by the use of 1 MW of microwave power and an accelerating structure such as a resonant cavity having a Q-value of 100. Experiments were performed using a 2 MW S-band magnetron, resonant and nonresonant accelerating structures and an external plasma source. In the case of low plasma density (n < 1010 cm-3), the electrons only were accelerated to 100 keV or higher in the neighbourhood of the electron cyclotron resonance. In the case of high plasma density (n similar 1012-1013 cm-3), ions were accelerated together with electrons and the maximum energy exceeded 100 keV. The dependence of ion energy on the static magnetic field intensity disagreed, however, with the theoretical prediction that the acceleration is most effective in the electron cyclotron resonance region.

QUANTUM THEORY OF GALVANOMAGNETIC PHENOMENA IN ELECTRON-PHONON SYSTEM

摘要 本文利用量子场论方法,统一地研究了电声子系统的交流磁阻、霍耳效应与迴旋共振现象。获得了与磁场强度H和外电场频率ω有关的弛豫时间、有效质量及迴旋共振线型的严格表达式。在我们的结果中,特别考虑了电子间库仑作用的集体效应、磁场引起的电子轨道运动量子化效应及交变电场的影响。 Abstract By making use of the method of quantum field theory, the A.C. magnetoresistance, the Hall effect, and the cyclotron resonance phenomena in electron-phonon system are investigated. The relaxation times and effective masses, which depend on the frequency of external electric field ω and intensity of static magnetic field H, and the strict expression for the line shape of cyclotron resonance are obtained in this way. In our work, the effect of Coulumb interaction between electrons, the effect of quantization of orbital motion, and the influence of alternating electric field have been taken into account. 作者及机构信息 李吉士, 张思远, 章思俊 Authors and contacts LI JI-SHI, ZHANG SI-YUAN, ZHANG SI-JUN 参考文献 [1] 施引文献

Ion Cyclotron Resonance in a Slightly Ionized Gas

A 130-ma arc discharge in hydrogen at a pressure of 0 to 5 x 10/sup -2/ Torr, in a static magnetic field of 2 to 5 kOe, is studied in a cylindrical discharge tube. A r-f coil around the tube produces a small axial r-f magnetic field and an azimuthal r-f electric field. The r-f power absorption by the discharge is measured as a fanction of the static magnetic field intensity, and is found to reach a maximum at the proton cyclotron resonant frequency. This frequency is used to calculate the proton collision frequency as a function of pressure. It is found that the collision frequency is almost constant below about 2.8 x 10/sup -2/ Torr, but that the collision frequency may be proportional to the pressure from 2.8to 5.0 x 10/sup -2/ Torr. (T.F.H.)

Ferrite Phase Shifters in Rectangular Wave Guide

The problem of the propagation of electromagnetic energy down an infinitely long rectangular wave guide partially filled by a ferrite slab is solved. The solution is expressed in the form of a transcendental equation involving the propagation constant. Calculations are carried out for a lossless ferrite, and the phase constant is evaluated as a function of the appropriate parameters, namely, the ferrite slab thickness, the lateral position of the slab in the guide, and the applied transverse static magnetic field intensity. The results are plotted in graphic form for values of the static magnetic field in the region of ferrite saturation both above and below ferromagnetic resonance. The electromagnetic field configurations within the wave guide are plotted in detail for values of the parameters of practical interest. Two versions of the nonreciprocal phase shifter are discussed. The first consists of a single slab placed asymmetrically in the wave guide, while the second consists of two symmetrically placed slabs with antiparallel static magnetic fields imposed. The nonreciprocal nature is summarized in terms of the differential phase shift β+-β− which is pertinent in applications of this phase shifter.