Pulsed magnetic field gradients generated by gradient coils are widely used in signal location in magnetic resonance imaging (MRI). However, gradient coils can also induce eddy currents in final magnetic field in the nearby conducting structures which lead to distortion and artifact in images, misguiding clinical diagnosis. We tried in our laboratory to measure the magnetic field of gradient-induced eddy current in 1.5 T superconducting magnetic resonance imaging device; and extracted key parameters including amplitude and time constant of exponential terms according to inductance-resistance series mathematical module. These parameters of both self-induced component and crossing component are useful to design digital filters to implement pulse pre-emphasize to reshape the waveform. A measure device that is a basement equipped with phantoms and receiving coils was designed and placed in the isocenter of the magnetic field. By applying testing sequence, contrast experiments were carried out in a superconducting magnet before and after eddy current compensation. Sets of one dimension signal were obtained as raw data to calculate gradient-induced eddy currents. Curve fitting by least squares method was also done to match inductance-resistance series module. The results also illustrated that pulse pre-emphasize measurement with digital filter was correct and effective in reducing eddy current effect. Pre-emphasize waveform was developed based on system function. The usefulness of pre-emphasize measurement in reducing eddy current was confirmed and the improvement was also presented. All these are valuable for reducing artifact in magnetic resonance imaging device.
脉冲梯度磁场由梯度线圈产生，广泛用于磁共振成像设备的信号定位系统中。但该脉冲也会同时在线圈周围导体结构中感应出涡流场，从而导致总磁场改变，使图像出现变形或伪影，影响临床诊断。本文通过实验测量了 1.5 T 超导磁共振成像设备中脉冲梯度导致的涡流场；根据电感-电阻串联模型，分析了其数学模型，该模型表明涡流磁场可以由独立项和交叉项组成；然后本文定量测定了每项指数函数的振幅和时间常数等关键参数，为设计数字滤波器以进行预加重和波形整形提供了依据。本文涉及的测量是在设计的专用工装上进行，它由基于中心对称的六个方向上的水模和各自的射频接收线圈构成，通过加载测试序列可以得到有无涡流的原始一维数据，经计算可将其转化为有意义的采样数据。根据本文研究提及的数学模型，将采样数据用最小二乘法拟合后表明，数据符合电感-电阻串联模型，涡流效应确实存在。通过对比实验同时也进一步证实，用数字滤波器对脉冲进行预加重后，涡流场被有效减小。本文建立了系统函数的数学模型，验证了预加重在减小涡流上的实用价值，提出了改进办法，为磁共振设备减轻涡流效应提供了参考。.
Keywords: eddy current; gradient; magnetic resonance imaging.