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1、中国科学技术大学博士学位论文一种新型MRI谱仪的设计及关键技术研究姓名：潘文宇申请学位级别：博士专业：模式识别与智能系统指导教师：周荷琴2011-04摘 要 I 摘摘 要要 磁共振成像（Magnetic Resonance Imaging，MRI）是医学影像领域中的一种新型成像方法，它利用射频脉冲使磁场中的氢核共振产生信号，经计算机处理而成像。MRI 具有成像参数多、对比度高、可任意方位断层、无骨伪影干扰、对人体无电离辐射伤害等优点，目前已成为临床检查中应用最广泛的先进技术之一。 MRI 系统主要由主计算机、谱仪、磁体、射频线圈、梯度线圈和功率放大器等组成。其中，谱仪（Spectrometer
2、）是整个系统的核心部件，它负责系统中各种射频和梯度信号的产生、发射、接收和处理。谱仪的技术含量很高，其功能的丰富性决定了对用户需求的支持是否完善， 其性能的优劣直接影响到磁共振成像的质量。商用 MRI 谱仪只有少数几家国外公司可以生产，国内磁共振成像系统中的谱仪主要依靠进口，价格昂贵，国产的谱仪产品尚不成熟，与国外先进水平相比仍有一定的差距。因此，研究和设计具有自主知识产权的高性能、低成本的谱仪，对于带动 MRI 系统整体技术水平的提高和促进 MRI 设备的进一步普及具有非常重要的意义。 本文在深入理解 MRI 原理的基础上， 结合前期对谱仪设计相关技术的研究，提出了一种基于 FPGA、DSP
3、、ADC、DAC 等 IC 芯片的新型数字化谱仪设计方案。在文章中，首先介绍了谱仪的总体设计方案，然后对射频发生器、梯度发生器、射频接收器等模块的设计方法以及关键的信号处理技术进行了详细介绍。归纳起来，本文主要完成了以下有创新性的工作： 1) 在对 MRI 谱仪关键技术长期研究的基础上，提出一种新型数字化谱仪的总体设计方案。在硬件系统设计上，以高性能 FPGA、DSP、ADC、DAC 等IC 芯片构造谱仪的硬件平台，形成了体积小、结构精简、稳定性好、定时精度高、成本低的一体化谱仪系统。软件系统主要包括进行命令和数据处理的核心模块和其他外围程序模块，它与谱仪的硬件系统相配套，能够实现用户的多种成
4、像需求，获取高质量的磁共振图像。所设计的谱仪既能用于低场永磁型 MRI 系统，又能用于高场超导型 MRI 系统。 2) 提出一种谱仪数字射频发生器设计方法。利用多功能、高性能的正交数字上变频芯片完成频率合成、数字正交调制、D/A 转换等功能，可以快速改变基带波形、调制频率、幅度和相位，具有精度高、稳定性好、频率覆盖范围广、集成度高、成本低等特点，可满足主流 MRI 系统的需求。 3) 设计了谱仪的梯度发生器， 提出一种基于高性能 DSP 的梯度计算模块设计方法。梯度计算有关的旋转、预加重、匀场补偿等功能全部由一片 DSP 来实摘 要 II 现，充分利用了 DSP 运算速度快、精度高、编程方便的
5、优点，且算法通用性好，具有良好的可移植性，是一种实用的梯度计算解决方案。 4) 提出一种谱仪射频接收器设计方法。 采用多功能接收芯片完成 A/D 转换和数字正交解调等功能，具有解调相位准确、集成度高、稳定性好、成本低等特点。 利用 FPGA 中进行数字磁共振信号的多级滤波和抽取处理， 实现了基于常规滤波器的算法，提出了基于多级小波分解的算法，两种算法各有所长，均能较好地完成对磁共振信号的预处理工作，得到成像所需的 K 空间信号，可根据实际需要灵活选择。 关键词：关键词：磁共振成像，谱仪，直接数字频率合成，正交幅度调制，梯度计算， 正交解调，多级滤波抽取，小波分解 Abstract III AB
6、STRACT Magnetic resonance imaging (MRI) is a new imaging method in the field of medical imaging. It uses radio frequency (RF) pulses to excite the hydrogen nucleuses thus producing nuclear resonance signals, which are then processed in computer to obtain image. With the advantages of multi-parameter
7、s imaging, high contrast, arbitrary slice imaging, no bone artifacts disturbance, and non-invasive etc, MRI has become one of the most widely used technologies. MRI system consists of computer, spectrometer, magnet, RF coils, gradient coils and power amplifiers. As the core of the whole MRI system,
8、Spectrometer is in charge of generating, sending, receiving and processing of all kinds of RF signals and gradient signals. Spectrometer is a high technology device, whose functional divisity determines the ability of meeting users requirements and whose performances directly influence images qualit
9、y. Currently, commercial MRI spectrometer can only be produced by a very few foreign companies so that most spectrometers of Chinese MRI systems are imported at high price. With the fact that our domestic spectrometer falls far behind foreign countries, it is rather important to research and design
10、our own spectromer with high performances and low cost. Upon the fully understandings in MRI principles and previous deep explorations into spectrometer design, a new MRI spectrometer based on FPGA, DSP, ADC, DAC and other IC chips is proposed. In this thesis, we firtly introduce the overall design
11、of our MRI spectrometer. Then, we will dicuss every particular module and its key techniques in the spectrometer, including RF generator, gradient generator and RF receiver. In sum, our work is organized as follows: 1) A new digital MRI design is proposed. The hardware system is based on high perfor
12、mance FPGA, DSP, ADC, DAC and other IC chips. The design has many advantages such as small size, simple structure, low cost, high stability and timmer precision. On the other side, software system includes core modules used for command and data processing and other peripheral modules. Cooperating wi
13、th hardware system, software system can satisfy many imaging requirements from users and obtain high quality images. Our spectrometer is applicable to both low field permanent magnet MRI system and high field superconduction MRI system. 2) Propose a solution for digital radio frequency generator of
14、MRI spectrometer. Using digital-up-convertor chips with high performance and multi-function, we can Abstract IV accomplish frequency synthesis, digital quadrature modulation and D/A conversion. Such solution has many properties such as fast modifying base-belt waveform data, frequency modulation, am
15、plitude and phase modulation, high precision and stability, broad frqency convering, high integrated level, low cost and good compatibility. 3) Design a gradient generator based on high performance DSP. All gradient calculation including rotation, pre-emphasis and shim are implemented in a single DS
16、P. Such solution takes fully advantage of DSPs properties such as high speed and precision, easily programming, algorithm compatibility and code reusability. 4) Design a RF receiver of MRI spectrometer. We use multi-functional receiving chip to accomplish A/D conversion and digital quadrature demodulation so that it has low cost, high demodulation phase precision, integrated level and stablility. By extracting and multiplely filtering the acquired nuclear magnetic signals, we have implemen
