The main magnetic fields of mobile nuclear magnetic resonance (NMR) magnets differ from those of conventional NMR and magnetic resonance imaging (MRI) magnets. In the Halbach magnet, the main field B 0 is perpendicular to the longitudinal axis, the symmetry of the current distribution with respect to the symmetry of the magnetic field differs from that in conventional target-field applications, and the current distribution on the coil surface cannot be expressed in terms of periodic basis functions. To obtain the winding pattern of the coil, an efficacious target-field approach. The surface of a coil is divided into small discrete elements, where each element is represented by a magnetic dipole. From the stream function of the elements, the resultant magnetic field is calculated. The optimization strategy follows an objective function defined by the power dissipation or efficiency of the coil. This leads to the optimum stream function on the coil surface, whose contour lines define the winding patterns of the coil. This paper shows winding patterns designed of shim coils for Halbach magnet and illustrates the craft of a shim coil using flexible printed circuit board. The performance of the coils is verified by simulating the fields they produce over the sensitive volume.

Objective: To review the technology on water and fat separation technology using MRI,which guides the selection and application for research in clinical.Methods: the physics and methods of water and fat separation was Introduced in MRI,especial for 2 points and 3 points Dixon method as well as their new improved methods,and how to use the Dixon technology combined with parallel data acquisition and sensing compression data reconstruction method to shorten the scanning time was proposed,which is significant for clinical application.Conclusions: Although some oversea companies supply the software for analysis of the data by Dixon technology,but it is usually not sufficient to satisfy the clinical needs.Therefore,each hospital should develop their own technology by their medical physicists,and these national companies should put enough attention to develop this technology.

本文研究、设计了一个可用于MRI的低噪声前置放大器.提出了一个3元件的噪声匹配网络方案,用以变换50Ω信号源阻抗为FET的最佳源阻抗,以使前置放大器的噪声系数达到最小,并具有可观的增益.实验证明前置放大器输入端在阻抗匹配条件下可获得最大增益但噪声系数不小.用砷化镓场效应管(AsGa-FET)ATF33143作为放大管制作的单级低噪声前置放大器在128 MHz(3 T)频率上用网络分析仪(HP8712C)测量增益可以达到25 dB,用噪声系数分析仪(8970B)测量的噪声系数为0.43 dB.

Amide proton transfer (APT) imaging is capable of detecting mobile cellular proteins and peptides in tumor and monitoring pH effects in stroke, through the saturation transfer between irradiated amide protons and water protons. In this work, four healthy subjects, eight brain tumor patients (four with high-grade glioma, one with lung cancer metastasis, and three with meningioma), and four stroke patients (average 4.3 ± 2.5 days after the onset of the stroke) were scanned at 3 T, using different radiofrequency saturation powers. The APT effect was quantified using the magnetization transfer ratio (MTR) asymmetry at 3.5 ppm with respect to the water resonance. At a saturation power of 2 μT, the measured APT-MRI signal of the normal brain tissue was almost zero, due to the contamination of the negative conventional magnetization transfer ratio asymmetry. This irradiation power caused an optimal hyperintense APT-MRI signal in the tumor and an optimal hypointense signal in the stroke, compared to the normal brain tissue. The results suggest that the saturation power of 2 μT is ideal for APT imaging of these two pathologies at 3 T with the existing clinical hardware.

To shorten scanning time and increase the feasibility of experimental results,dual-window K-space weightednavigator technique(DKWN) on right coronary MR angiography at 3.0 T was implemented.The characteristics of DKWNtechnique and conventional navigator technique with fixed gating window in 25 normal subjects were evaluated.Compared tothe conventional navigator sequence,DKWN sequences increase the scanning efficiency by 30%(p 0.05),whilemaintaining good image quality;further the corresponding scanning time decreases from 2.12 to 1.64 min(p 0.05).Thecoronary artery lengths and diameters do not show statistically significant differences between the two techniques(p 0.05).

In this paper, a low-noise preamplifier for MRI is designed and studied. A noise matching network consisting of three elements is presented. To the single-stage AsGa-FET preamplifier working at 128 MHz, the measured gain through network analyzer (HP8712C) and noise figure through noise figure analyzer (8970B) are 25 and 0.43 dB, respectively.

A method is developed for designing a special iron shielded superconducting magnet for MRI in this paper. The shield is designed as an integral part of the cryostat and high permeability and high saturated magnetization iron material is adopted. This scheme will result in a compact iron shielded magnet. In the presented design, the finite element (FE) method is adopted to calculate the magnetic field produced by superconducting coils and nonlinear iron material. The FE method is incorporated into the simulated annealing method which is employed for corresponding optimization. Therefore, geometrical configurations of both coils and iron shield can be optimized together. This method can deal with discrete design variables which are defined to describe the cable arrangements of coil cross sections. A detailed algorithm of the present design is described and an example for designing a 1.5 T clinical iron shielded magnet for MRI is shown.

This paper presents an approach of active ferromagnetic shimming for C-type permanent magnetic resonance imaging (MRI) magnet. It is designed to reduce inhomogeneity of magnetostatic field of C-type permanent magnet to meet the stringent requirement for MRI applications. An optimal configuration (locations and thicknesses) of active ferromagnetic pieces is generated through calculation according to the initial field map and the demanded final homogeneity specifications. This approach uses a minimisation technique which makes the sum of squared magnetic moment minimum to restrict the amount of the active ferromagnetic material used and the maximal thickness of pieces stacked at each hole location in the shimming boards. Simulation and experimental results verify that the method is valid and efficient.

A novel target-field approach for designing bi-planar gradient and shim coils of restricted radius for use with permanent-magnet MRI systems is presented in this article. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic held whose z-component follows a chosen spherical harmonic function. Mathematical expressions that relate the harmonic terms in the surface current distribution to spherical harmonic terms in the field expansion are established and then used in a simple matrix inversion approach to design a range of shim coils. The expressions providing a spherical harmonic decomposition of the field components produced by a particular current distribution are novel. The results can be used to design bi-planar shim coils of restricted radius that will generate a field variation that follows a certain spherical harmonic over a reasonably-sized volume. A stream function was utilized to obtain the discrete wire distribution on the coil plane. This method does not require the setting of the target-field points. Through an analysis of the matrix equations in terms of condition numbers, we show that this novel approach has no ill-conditioned problems. (C) 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 37B: 29-38, 2010

To shorten scanning time and increase the feasibility of experimental results, we performed right coronary artery magnetic resonance angiography (CMRA) at 3.0 T using dual acceptance window weighting function in 25 normal subjects. We examined these subjects using conventional navigator with fixed gating window and 6 dual acceptance window weighted gating (DAWG) sequences with different central weighted ratio (CWR). Compared with the conventional navigator sequence, DAWG sequences with CWRs of 20% and 25% increased the scanning efficiency by 30% and 26% respectively (P<.05), while maintaining good image quality; further the corresponding scanning time decreased from 2.12-1.64 and 1.69 min, respectively (P<.05). However, CWRs less than 15% caused image degradation to some extent. The coronary artery lengths and diameters did not show statistically significant differences between the two techniques (P>.05). Briefly, to avoid the problems caused by low navigator efficiency and to maintain comparable image quality, the weighted gating parameters of 3 mm width central acceptance window and 15 mm width outer acceptance window with CWR between 20% and 25% are recommended for right CMRA at 3 T.

利用线电流和多层介质板模型以及镜像理论定量解析永磁MRI磁体系统中梯度线圈电流在铁磁极板中产生的磁场H和感应场B,磁极板中的这些磁通是产生涡流伪影的根源.为了充分抑制涡流,应该充分减小这些磁通,通过定量计算揭示"抗涡流板"的导磁率必须高于极板材料工业纯铁的导磁率,并且要有足够的厚度以保证其远离饱和,这些结论为进一步提高永磁MRI磁体系统技术水平指明了努力的方向.

In this paper the analytical expressions for the magnetic field H and induction B in iron-pole plates generated by MRI gradient coil are given using line-current and the multilayer dielectric plate model with the mirror-image method. Eddy current emanates from the magnetic flux in the iron-pole plates. In order to fully suppress the eddy current, this magnetic flux should be fully eliminated. The research results indicate the magnetic permeability of the resist-eddy plate must be bigger than that of magnetic pole material, i.e. pure iron, and that the resist-eddy plate should be thick enough to be far away from its magnetic saturation.

For a superconducting magnet of magnetic resonance imaging (MRI), the novel approach presented in this paper allows the design of cylindrical gradient and shim coils of finite length. The method is based on identification of the weighting of harmonic components in the current distribution that will generate a magnetic field whose z-component follows a chosen spherical harmonic function. Mathematical expressions which relate the harmonic terms in the cylindrical current distribution to spherical harmonic terms in the field expansion are established. Thus a simple matrix inversion approach can be used to design a shim coil of any order pure harmonic. The expressions providing a spherical harmonic decomposition of the field components produced by a particular cylindrical current distribution are novel. A stream function was applied to obtain the discrete wire distribution on the cylindrical-surface. This method does not require the setting of the target-field points. The discussion referring to matrix equations in terms of condition numbers proves that this novel approach has no ill-conditioned problems. The results also indicate that it can be used to design cylindrical-surface shim coils of finite length that will generate a field variation which follows a particular spherical harmonic over a reasonably large-sized volume.

Objective To design an optimization method for a novel iron shielded MR superconducting magnet used iron shield as the warm wall of the Dewar. Methods Weighting method based on the design target was used to construct objective function which was optimized in simulated annealing algorithm (SAA). The finite element method (FEM) was adopted in the method to calculate the magnetic field produced by the iron shielded superconducting magnet. Results A compact 1.5T iron shielded superconducting magnet for the whole body MR imaging was designed with the presented method. The length of the magnet was 1.6 m, the magnetic field homogeneity (peak to peak relative deviation) in the sphere of 400 mm diameter at the isocenter of the magnet was 2.2 ppm, the 5 Gs line was 7.1 m in axial direction and 5.3 m in radial direction. Conclusion The method for designing compact iron shielded superconducting magnet of MR imaging is valid.

Based on the sequential quadratic programming（SQP） method,a new approach is presented in this paper to gain a uniform magnetic field for a permanent MRI magnet with biplanar poles.First,the adopted shimming piece is modeled as a magnetic dipole moment to calculate its effect on the background field over the imaging region of interest.Then,the SQP method is utilized to determine the ideal solution for the shimming equation.Finally,the ideal solution is discrete,and the quantization error control technique is used for special cases.This new method helps to reduce the inhomogeneity from 1234.5 ppm to 21.4 ppm over a 36 cm diameter spherical volume（DSV）,within hours in practical shimming work.

For the inhomogeneous magnetic field MR imager, the polarization field is virtually uniform within horizontal degrees of freedom and decreases monotonically in a direction away from the surface. The authors describe the imaging principle, and present requirements and constraints by terms of key points of physical design such as 90° excitation pulse,180° refocused pulse, encoding manner and pulse sequences. In addition, it reveals that the echo time is restricted by the diffusion effect on signal amplitude due to the huge static field gradient. The data evaluated from diffusion not only restrict the usable TE, but also determine the strength of encoding gradient Gx, Gz and RF excitation field B1 and virtually usable RF pulse. The time spent for scanning one slice of human brain and inability of covering whole brain prohibit such MR imager from surgical intervention. It indicates that the advantage of the unilateral MR imager is portable. It can be used for arbitrary large sample to render MRI analysis of surface-layer in a non-invasive manner.

The enigma of the wave-particle duality of photon has remained unimpressively explained for a century since Einstein presents the concept of the photon in 1905. This article establishes a classical geometric structure model of a single photon based on field matter, educes a formula for the size of a photon; assumes that there only are two kinds of photon of right hand and left hand circular polarized, and suggests the frequency. of photon polarization rotated to be its spin frequency. It ascribes the wavelike of photon to its spin motion and the particle-like to its translation motion. From the point of photon particle instead of wave view to re-analyze Young's double-slit interference and polarizer experiments, gives reasonable mechanism. It defines the phase velocity and the group velocity of a photon. It gives a unified and consistent understanding of quantum particle of light and classical electromagnetic waves field. Evidently, such a precisely defined conceptual model is reasonable, objective and easy to accept for classical physicists.

A new approach has been presented in the paper to optimize the longitudinal gra- dient coil performance. First, traditional spherical harmonic target fleld method is deduced, the relation between the magnetic fleld and the current distribution is described by a matrix equa- tion. Then, simulated annealing method is introduced to the optimization procedure, and those high order coe-cients which are used to vanish become the variables designed for optimization. Finally, stream function method is used to transform the current density into discrete gradient coils. Comparison between traditional method and the optimized method shows that the in- homogeneity in the region of interest can be reduced from 13.59% to 4.9%, the coil e-ciency is increased from 11.56mT/m/A to 18.07mT/m/A, and the minimum distance of the discrete current wire is raised from 0.95mm to 2.02mm. DOI: 10.2529/PIERS061003221205

In this paper an approach of ferromagnetic shimming for permanent MRI magnet is presented. It is designed to reduce unhomogeneity of magnetostatic fleld of permanent magnet to meet the stringent requirement for MRI applications. An optimal conflguration of ferromagnetic pieces is generated through calculation according to the initial fleld map and the demanded flnal homogeneity speciflcations. This approach uses a minimization technique that makes the sum of squared magnetic moment minimum to restrict the amount of the ferromagnetic material used and the maximal thickness of shim pieces stacked at each position on the shimming boards. Simulation results verify that the method is efiective and e-cient. DOI: 10.2529/PIERS060826010243 As well known there are two kinds of MRI (1) magnets: closed cylindrical superconductive(SC) magnet and open biplanar-pole permanent magnet such as C-shaped one. The open space of the C-magnet helps the patient overcome any feelings of claustrophobia that may be experienced in a closed designed magnet. Both superconductive and permanent magnet must be shimmed to reduce the unhomogeneity of the magnetic fleld in the working magnetic fleld volume to within a predetermined speciflcation, i.e., within a few parts per million for use in medical diagnosis. However, to the permanent magnet due to approximation in the design as well as magnetizing and fabricating tolerances, the flnal homogeneity of main magnetic fleld is often far away from the acceptable level. Therefore, shim technology is of major importance in the design and manufacture of permanent MRI magnet. Many papers (2{4) and patents (5,6) published focus on passive steel shimming of the SC magnet. As for active coil shimming of the SC magnet, likewise there are many papers (7,8) published. Recently target fleld method (9{11) has been used for active shimming of SC magnet. Conventional electromagnetic shimming follows the approach based on representing the fleld as a spherical harmonics series: