核磁共振系统的数字化研究
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摘要
核磁共振(NMR)技术作为一种研究物质结构的重要工具,在物质检测和医学影像等领域中得到了广泛的应用。与此同时,其应用领域的拓展和科学研究的深入,又对核磁共振系统提出了更高的要求。本论文主要针对目前常规商业化NMR谱仪存在的问题,开展了谱仪技术的数字化研究,在保证谱仪功能和性能的前提下,对谱仪结构进行了优化,降低了设计成本。该研究工作为NMR设备的普及奠定了基础。
论文的主要内容如下:
1.核磁共振系统数字化研究的趋势。首先,介绍了核磁共振技术的发展,指出应用数字化技术是谱仪发展的一个重要方向。在此基础上,针对本论文的主要工作论述了数字化研究的意义。
2.基于USB总线的一体化核磁共振谱仪控制台的数字化研究。首先,在总结常规核磁共振谱仪结构的基础上,提出了全数字、一体化的谱仪控制台的设计思想。然后,详细介绍了该谱仪控制台的设计思路和硬件结构。该谱仪控制台采用USB总线,实现了外置式架构。此外,该谱仪控制台将控制/通讯部分、脉冲序列控制部分、射频发射部分与信号接收部分集成于一块板卡之上,还可以实现两种工作模式。最后,采用常规NMR序列实验较好地验证了整套系统的性能。可以看到,在谱仪结构简化的同时保证了性能指标,增强了灵活性,拓宽了应用范围。
3.多通道磁共振信号接收方法的数字化研究。首先,讨论了近年来随着相控阵和并行成像技术的飞速发展对信号采集系统的要求。然后,针对现有技术的不足,提出了F_TDM多通道接收方法,并进行了理论分析。最后,我们设计了基于PCI总线的四通道F_TDM数字接收机,通过成像实验论证了该方法的可行性。
4.选择性激发脉冲的数字化研究。为提高射频功率放大器的效率,降低仪器的成本,我们提出了正负相位组合(P/N)选择性激发脉冲。首先,我们将P/N脉冲展开成傅立叶级数,分析了P/N脉冲和软脉冲在选择性激发上的等效性。然后通过密度矩阵的方法模拟了P/N脉冲的激发带宽曲线,并与软脉冲的激发带宽曲线作了比较。最后通过实验测量了P/N脉冲的激发带宽曲线,并给出了采用P/N脉冲选层获得的多层SE2D图像。
5.磁共振系统数字化研究的总结与展望。总结了本论文的主要研究工作及其应用方向,对目前工作中存在的问题进行了分析并指出了改进的方向。
Nuclear Magnetic Resonance (NMR) technique has become one of the most important tools for probing the material structure, and has been widely used in the fields of substance analysis and medical imaging. Simultaneously, the development of the application field and scientific research in NMR has put high demands on its system technology research. To solve the problems existing in standard commercial spectrometers, we focus our research on the digital investigation of NMR system. This research can optimize the structure and reduce the cost of the system without performance lost. We believe it laid a foundation for the popularization of NMR spectrometers.
The main contents in this dissertation are as follows:
1. The trend in the digitalization of NMR system. To begin with, we provided an overview of the development of NMR technique. Then we dwelt on the significance of the research work.
2. The digital investigation of integrated NMR spectrometer based on USB interface. Firstly, the structure of standard commercial spectrometers was introduced. Then, a complete digital integrated NMR spectrometer was designed. The spectrometer, in which the control/communication unit, pulse generator, RF waveform transmitter and digital receiver are integrated on one PCB card, can works in two modes. Its hardware structure is independent of PC and greatly simplified without the loss of flexibility and performance.
3. The digital investigation of F_TDM receiver for parallel acquisition of MRI. In recent years, the rapid development of parallel acquisition in magnetic resonance imaging (MRI) has put high demands on receiver technology. Firstly, the disadvantages of the existing technology were pointed out. Then, an alternative technique F_TDM, which minimizes physical dimensions and system complexity, was proposed and the principle of this technique was discussed. Finally, a PCI-based four-channel F_TDM receiver was designed for verifying and demonstrating the scheme and the performance of this proposed technique.
4. The investigation of selective-excitation radiofrequency (RF) pulse for MRI. A selective-excitation RF pulse that uses hard pulses composed of a sequence of composite pulses with positive and negative phases (P/N pulse) was proposed herein. It can be used for imaging studies to improve the efficiency of amplification at the lowest cost. Fourier series have been first used to analyze the equivalence between the proposed P/N pulse and the conventional soft pulse on selective excitation. Subsequently, computer simulations based on density-matrix theory were used to compare the excitation profiles of both the soft and the P/N pulses. In addition, the excitation profiles of the P/N pulses have been measured experimentally and several slices of images have been obtained as proofs by using the multislice SE2D sequence through replacement of the conventional soft pulse by the proposed P/N pulse.
5. The summary and prospect of the digitalization of NMR system. We analyzed the existing problems in present works and pointed out the future research direction.
论文的主要内容如下:
1.核磁共振系统数字化研究的趋势。首先,介绍了核磁共振技术的发展,指出应用数字化技术是谱仪发展的一个重要方向。在此基础上,针对本论文的主要工作论述了数字化研究的意义。
2.基于USB总线的一体化核磁共振谱仪控制台的数字化研究。首先,在总结常规核磁共振谱仪结构的基础上,提出了全数字、一体化的谱仪控制台的设计思想。然后,详细介绍了该谱仪控制台的设计思路和硬件结构。该谱仪控制台采用USB总线,实现了外置式架构。此外,该谱仪控制台将控制/通讯部分、脉冲序列控制部分、射频发射部分与信号接收部分集成于一块板卡之上,还可以实现两种工作模式。最后,采用常规NMR序列实验较好地验证了整套系统的性能。可以看到,在谱仪结构简化的同时保证了性能指标,增强了灵活性,拓宽了应用范围。
3.多通道磁共振信号接收方法的数字化研究。首先,讨论了近年来随着相控阵和并行成像技术的飞速发展对信号采集系统的要求。然后,针对现有技术的不足,提出了F_TDM多通道接收方法,并进行了理论分析。最后,我们设计了基于PCI总线的四通道F_TDM数字接收机,通过成像实验论证了该方法的可行性。
4.选择性激发脉冲的数字化研究。为提高射频功率放大器的效率,降低仪器的成本,我们提出了正负相位组合(P/N)选择性激发脉冲。首先,我们将P/N脉冲展开成傅立叶级数,分析了P/N脉冲和软脉冲在选择性激发上的等效性。然后通过密度矩阵的方法模拟了P/N脉冲的激发带宽曲线,并与软脉冲的激发带宽曲线作了比较。最后通过实验测量了P/N脉冲的激发带宽曲线,并给出了采用P/N脉冲选层获得的多层SE2D图像。
5.磁共振系统数字化研究的总结与展望。总结了本论文的主要研究工作及其应用方向,对目前工作中存在的问题进行了分析并指出了改进的方向。
Nuclear Magnetic Resonance (NMR) technique has become one of the most important tools for probing the material structure, and has been widely used in the fields of substance analysis and medical imaging. Simultaneously, the development of the application field and scientific research in NMR has put high demands on its system technology research. To solve the problems existing in standard commercial spectrometers, we focus our research on the digital investigation of NMR system. This research can optimize the structure and reduce the cost of the system without performance lost. We believe it laid a foundation for the popularization of NMR spectrometers.
The main contents in this dissertation are as follows:
1. The trend in the digitalization of NMR system. To begin with, we provided an overview of the development of NMR technique. Then we dwelt on the significance of the research work.
2. The digital investigation of integrated NMR spectrometer based on USB interface. Firstly, the structure of standard commercial spectrometers was introduced. Then, a complete digital integrated NMR spectrometer was designed. The spectrometer, in which the control/communication unit, pulse generator, RF waveform transmitter and digital receiver are integrated on one PCB card, can works in two modes. Its hardware structure is independent of PC and greatly simplified without the loss of flexibility and performance.
3. The digital investigation of F_TDM receiver for parallel acquisition of MRI. In recent years, the rapid development of parallel acquisition in magnetic resonance imaging (MRI) has put high demands on receiver technology. Firstly, the disadvantages of the existing technology were pointed out. Then, an alternative technique F_TDM, which minimizes physical dimensions and system complexity, was proposed and the principle of this technique was discussed. Finally, a PCI-based four-channel F_TDM receiver was designed for verifying and demonstrating the scheme and the performance of this proposed technique.
4. The investigation of selective-excitation radiofrequency (RF) pulse for MRI. A selective-excitation RF pulse that uses hard pulses composed of a sequence of composite pulses with positive and negative phases (P/N pulse) was proposed herein. It can be used for imaging studies to improve the efficiency of amplification at the lowest cost. Fourier series have been first used to analyze the equivalence between the proposed P/N pulse and the conventional soft pulse on selective excitation. Subsequently, computer simulations based on density-matrix theory were used to compare the excitation profiles of both the soft and the P/N pulses. In addition, the excitation profiles of the P/N pulses have been measured experimentally and several slices of images have been obtained as proofs by using the multislice SE2D sequence through replacement of the conventional soft pulse by the proposed P/N pulse.
5. The summary and prospect of the digitalization of NMR system. We analyzed the existing problems in present works and pointed out the future research direction.
引文
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[9]Ernst RR(1971)Fourier difference spectroscopy.J.Magn.Reson.4:280-296
[10]Damadian RV(1971)Tumor detection by nuclear magnetic resonance.Science 171:1151
[11]Lauterbur PC(1973)Image formation by induced local interactions:examples employing nuclear magnetic resonance.Nature 242:190-191
[12]Kumar A,Welti D,Ernst RR(1975)NMR Fourier zeugmatography.J.Magn.Reson.18:69-83
[13]Mansfielf P(1977)Multi-planar image formation using NMR spin-ethos.J.Phys.C:Solid State Physics 10:L55-L58
[14]俎栋林(2004)核磁共振成像学,高等教育出版社,北京
[15]Damadian(1972)Apparatus and method for detecting cancer in tissue.US Patent No.3789832
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