【作者】 杨帆；

【导师】 蒋瑜；

【作者基本信息】 华东师范大学 ， 无线电物理， 2020， 硕士

【摘要】 在磁共振成像实验中需要梯度磁场的切换,而这会形成涡流场,对成像区域内的真实梯度磁场造成干扰,频率编码梯度和相位编码梯度方向上的涡流场会给采集到的信号带来相位误差,使重建的图像发生畸变、形成伪影;选层梯度方向上的涡流场会造成层面选择的不准确。梯度波形预补偿是解决涡流问题的一种常用技术,可以使用模拟或数字方法实现。模拟预补偿方案灵活性差,波形输出和预设值误差较大;传统数字波形预补偿方案又存在补偿波形精度不够、波形输出延时不稳定等问题。针对这些问题,本文基于实验室研制的高精度梯度卡,设计了一种新的具有数字预补偿功能的梯度方案。使用现场可编程门阵列(field programmable gate array,FPGA)对序列设计的标准梯度波形进行实时计算得到补偿波形,将标准梯度波形和补偿波形分别送入两片独立的数模转换器(digital-to-analog converter,DAC),对模拟信号进行叠加产生补偿后波形;采用高速时钟和同步触发设计方法,对包含五组时间常数的预补偿波形进行了实时计算。最终在20 bit的DAC基础上实现了优于23 bit的高精度数字涡流预补偿方法;大大缩短了预补偿波形相对触发信号的延时,并保证每次的延时恒定不变。本梯度方案与商业磁共振成像系统实现了兼容,进行了波形预补偿和水模成像实验。实验中,对现有涡流测量序列进行改进,在每次采集后施加剩磁修正梯度,得到了更准确的测量结果;提出了一种新的补偿参数迭代方法,通过分段的迭代过程,避免一次测量就确定补偿波形形状,可以求解出更合适的补偿参数。实验结果表明,通过本方案的应用,梯度板卡输出预补偿波形的精度显著提高,有效降低了涡流,提升了成像效果。更多还原

【Abstract】 In the magnetic resonance imaging experiment,switching the gradient magnetic field is required,and this produces an eddy current field,which will interfere with the real gradient magnetic field in the imaging region.The eddy current field in the directions of frequency encoding and phase encoding causes phase errors to the signal,then reconstructed image is distorted,and artifacts are formed.Slice selection is inaccurate due to the eddy current field in this direction.Gradient waveform precompensation is a common technique for solving eddy current problems,which can be implemented using analog or digital methods.The analog pre-compensation method has poor flexibility,and the error between the waveform output and the ideal waveform is large.The traditional digital waveform pre-compensation method has problems such as insufficient accuracy of the compensated waveform and unstable waveform output delay.To solve these problems,a new gradient scheme with digital pre-compensation based on the high-precision gradient card was designed.Digital pre-compensation waveform was calculated from standard gradient waveform by field programmable gate array(FPGA).Then the original waveform and pre-compensation waveform were transmitted into two separated digital-to-analog converters(DAC)and transformed to analog signals respectively.Finally,compensated waveform was generated by adding the two analog waveforms.Besides,the pre-compensation waveform described by five various exponential decay terms was calculated timely using high-speed clock and synchronization trigger method.Digital pre-compensation based on two 20 bit DACs achieved higher resolution,and were more precise than 23 bit resolution.Meanwhile,this method considerably reduced delay time between compensated waveform and trigger signal as well as keep it constant.The gradient scheme was compatible with commercial magnetic resonance imaging systems,and waveform pre-compensation experiments and water phantom imaging experiments were performed.During the experiment,the existing eddy current measurement sequence was improved,and the remanence correction gradient was applied after each acquisition to obtain more accurate measurement results.A new iterative method for compensation parameters was proposed.By segmenting the iteration process,the shape of the compensation waveform was not determined by the first eddy-measurement,which resulted in more suitable compensation parameter solution.The experiment result shows that this scheme significantly improves the accuracy of the pre-compensated waveform,which effectively reduce the eddy current and improve the effect of image formation.更多还原