共查询到17条相似文献,搜索用时 203 毫秒
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目标场方法由Turner于1986年提出,到目前为止该方法已成为核磁共振成像系统中匀场、梯度线圈设计的一种主流方法。各种改进的目标场方法纷纷涌现。为解决线圈设计中的有限尺寸问题,Forbes和Crozier提出了一种新方法,该方法预先通过三角函数来约束线圈面上的电流密度分布,为克服方程求解过程中的病态问题,对电流密度表达式中的待定系数采用最小均方差(least square,LS)和L2范数相结合的方法来进行估计,成功得到了有限长匀场线圈的设计结果。该文采用最小均方差和L1范数结合的方法来对面电流密度分解表达式中的未知系数进行估计,并将该方法应用于核磁共振成像(magnetic resonance imaging,MRI)系统的匀场线圈设计中。系统主磁体长度为3 m,室温孔直径为1.6 m,目标区域(region of interest,ROI)直径为0.4 m。算例采用Matlab仿真平台,结果表明该文提出的LS-L1目标场方法相比于传统的LS-L2目标场方法磁场偏差更小、效果更好。 相似文献
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核磁共振成像,要求磁体有尽可能高的均匀磁场.本文介绍使用匀场线圈,调整永磁体磁场均匀度的原理和方法.文中给出调试时,使用计算机程序的框图,并用一次调试实例,说明测试磁场的方法,问题的关键和改进意见. 相似文献
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磁体系统是核磁共振成像装置主要部件之一。它要求在一个大范围空间内产生高度均匀的磁场,磁场强度在0.1~2.0特(1特=10000高斯),磁场均匀度要求10~(-4)~10~(-6)。实现这样的磁场可以有三条途径:(1)常规电磁磁体;(2)超导电磁体;(3)永磁磁体。前两种磁体系统已在核磁共振成像装置中获得广泛应用,后一磁体系统原先只在一些小型核磁共振波谱仪中应中,最近才开始在核磁共振成像装置中采用。未能广泛采用的原因可能是:(1)永磁大型磁体系统成本较高;(2)永磁磁体系统较重;(3)技术上有一定难度。但是,永磁磁体系统有它一定的特点,即它没有昂贵和复杂的附加设备,如高稳定度大功率励磁电源。因而它的操作维护比较简便,深受使用者欢迎。虽然永磁磁体的一次投资较大,但比常规电磁体节约能源,节省 相似文献
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采用目标场方法设计了一组超导核磁共振成像系统梯度线圈.该梯度线圈固定在半径为0.8 m、长3m的圆柱形骨架上,目标区域位于该圆柱中心,半径为0.2 m,梯度场强为40 mT/m.在该方法中,电流密度在柱坐标系下被分解为三角级数之和,接着通过Biot-Savart定律求出轴向磁场强度表达式,再通过基于目标场的Tikhonov正则化算法把待定三角系数的求解问题转化成线性方程组的求解,并通过流函数技术得到线圈的实际绕线分布,最后进行磁场偏差分析,分析结果表明,所设计线圈的磁场偏差远远小于通常所要求的5%.此外,该方法不仅适用于柱面梯度线圈设计,对于超导磁共振成像系统的有源匀场线圈设计也同样适用. 相似文献
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An improved functional approach for designing MRI (magnetic resonance imaging) main magnets with active shielding is presented.
By nulling one or two external moments as well as a certain series of internal moments of the magnetic field, new designs
with improved shielding in combination with or without shorter magnet lengths are obtained. The improved method can be employed
to design short and practical superconducting magnets at any given field strength. The resulting designs yield the desired
field homogeneity inside the region of interest without using superconducting shim coils. This approach requires only a modest
amount of computing power. One of the design steps, a contour plot of the continuous current solutions, can be utilized to
study stretch goals for favorable design parameters. 相似文献
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The homogeneity and stability of the static magnetic field are of paramount importance to the accuracy of MR procedures that are sensitive to phase errors and magnetic field inhomogeneity. It is shown that intense gradient utilization in clinical horizontal-bore superconducting MR scanners of three different vendors results in main magnetic fields that vary on a long time scale both spatially and temporally by amounts of order 0.8–2.5 ppm. The observed spatial changes have linear and quadratic variations that are strongest along the z direction. It is shown that the effect of such variations is of sufficient magnitude to completely obfuscate thermal phase shifts measured by proton-resonance frequency-shift MR thermometry and certainly affect accuracy. In addition, field variations cause signal loss and line-broadening in MR spectroscopy, as exemplified by a fourfold line-broadening of metabolites over the course of a 45 min human brain study. The field variations are consistent with resistive heating of the magnet structures. It is concluded that correction strategies are required to compensate for these spatial and temporal field drifts for phase-sensitive MR protocols. It is demonstrated that serial field mapping and phased difference imaging correction protocols can substantially compensate for the drift effects observed in the MR thermometry and spectroscopy experiments. 相似文献
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External magnetic disturbance influences the quality of the image obtained with a magnetic resonance imaging (MRI) system because it causes magnetic resonance frequency drift. When a superconducting magnet in persistent current mode operation is exposed to external magnetic disturbance coil current varies to keep linkage flux constant. Conventional analysis is usually formulated with constant current and does not provide correct evaluation of drift of magnetic field in the magnet. This paper proposes a method of numerical analysis of magnetic field considering conservation of linkage flux in a superconducting magnet under a persistent current mode operation. This method is applied to evaluate drift of the magnetic field in a superconducting magnet for magnetic resonance imaging due to external magnetic disturbance. Analysis results show good agreement with the experimental results. 相似文献
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螺线管线圈磁场的高精度计算方法及其比较 总被引:5,自引:0,他引:5
在某些场合,需要对螺线管线圈产生的磁场进行高精度计算。例如超导核磁共振成像磁体,要求在中心为0.35-0.5米的球径区域内产生的磁场,其不均匀度小于几个ppm,因此磁场的计算精度至少应不低于七位有效数字。 相似文献
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P. Desgoutte L. Chaabane B. Favre A. Deguin C. Lapray A. Briguet 《Magma (New York, N.Y.)》1997,5(1):53-57
In magnetic resonance imaging and spectroscopy, knowledge of the magnetic field gradient behavior is very important. This
work describes a simple way to characterize the temporal and spatial dependence of the main magnetic field when a gradient
is switched. Records are performed with a home-built magnetometer. This device is controlled by a personal computer for recording
and processing the NMR signals from an array of small probes spatially distributed and switched by the magnetometer. We present
results of measurements on a 2-T superconducting magnet. These results show the residual defects of an active shielded gradient
coils system. 相似文献
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Gordon DeMeester Michael Morich Alex Byrne Frank Davies David Melotte Adrian Thomas 《Magma (New York, N.Y.)》2002,13(3):193-198
The desire to improve patient comfort and acceptance of high field magnetic resonance imaging (MRI) systems results in new approaches to magnet and other critical MRI component design. One such approach, based upon an anatomical field of view analysis, is presented in this paper. The approach is utilized to define imaging volume requirements for short, high field, solenoid magnet designs that maintain whole body imaging capability. A short magnet design with excellent magnetic field homogeneity is presented accordingly. Combined with a novel flared gradient coil the overall system achieves improvements in openness. 相似文献