高场磁共振下并行激发技术的相关研究

发布时间：2018-07-05 09:38

本文选题：磁共振成像 + 射频场不均匀性　；参考：《浙江大学》2012年博士论文

【摘要】：磁共振成像技术是一种非介入式探测技术,有着能够反映诸多结构功能等生理差异与变化的多重对比机制,在当今的临床诊断和医学研究中有着极其重要的意义。自磁共振成像技术应用以来,为满足人们对成像高分辨率、高信噪比的需求,扫描仪的主磁场场强不断提升,对应的射频脉冲载波频率也随之提高。其结果是短波长的射频场与负载的耦合效应增强,导致射频能量在空间上的不均匀传递、从而对空间内质子的不均匀激发。作为磁共振信号源的质子激发的不均匀性的直接后果是,成像结果不能准确反映其真实信息,从而无法提供用于临床诊断和研究的可靠参照。针对该问题,本文以实现高场下射频场的不均匀性补偿、从而快速高质量成像为目标,从激发脉冲序列设计、线圈阵列的结构设计和实际K空间轨迹测量这三个方面展开研究工作。 (1)关于激发脉冲序列设计为实现高场下三维目标区域内质子的均匀快速激发,提出了一种基于并行激发技术的激发脉冲序列的优化设计方法。该方法通过优化K空间轨迹的设计来优化脉冲序列时长、激发效果以及临床安全性,并已经高场下的建模仿真验证了其有效性。更具体的,该方法通过寻找优化的方式来限定K空间轨迹的分布范围、定义对K空间内的欠采样,来缩短脉冲序列时长；由于该方法所定义的K空间轨迹分布范围与激发目标特定相关,因此对于在该分布范围内采样的K空间轨迹、针对性的设计射频脉冲即能保证实现期望的激发效果；此外,该方法放开了对于三维K空间轨迹类型选择的限制,允许在设计中选择最优轨迹类型来减小激发所需射频脉冲的幅值与能量,从而提升了临床安全性。 (2)关于线圈阵列的结构设计为在保证成像质量前提下充分发挥三维并行激发技术的加速作用,提出了一种用于并行激发技术第三维激发加速的线圈阵列的初步设计方法与评估方法。此前尚未有关于第三维扩展的线圈阵列的报道,是由本文首次提出的创新设计,因此在设计中借鉴了用于磁共振并行成像技术的线圈阵列的设计思路。经高场下的建模仿真验证了该设计方法与评估方法的有效性。启用此类线圈阵列作为发射线圈,结合以本文提出的用于三维空间选择性激励的序列设计方法,能够有效的缩短激发脉冲序列时长,从而提高成像时间分辨率 (3)关于实际K空间轨迹测量为确保并行激发脉冲序列对目标区域质子的准确激发,研究了用于解决激发K空间轨迹形变问题的实际K空间轨迹测量方法。在永磁磁共振扫描仪上实验测试了经典的序列方式测量方法,结合理论与实验分析其优缺点,并通过将实际测得的K空间轨迹与理想轨迹进行成像重建的结果比较,验证了序列方式测量方法的有效性。
[Abstract]:Magnetic resonance imaging (MRI) is a kind of non-interventional detection technology, which can reflect many physiological differences and changes, such as structure and function. It is of great significance in clinical diagnosis and medical research. Since the application of magnetic resonance imaging technology, in order to meet the needs of high resolution and high signal-to-noise ratio (SNR) of imaging, the magnetic field intensity of scanner has been increasing continuously, and the corresponding frequency of RF pulse carrier has also been increased. The result is that the coupling effect between the RF field and the load at short wavelength is enhanced, which leads to the non-uniform transfer of RF energy in space and thus the inhomogeneous excitation of protons in the space. The direct consequence of the inhomogeneity of proton excitation as a source of magnetic resonance signal is that the imaging results can not accurately reflect its true information and therefore can not provide a reliable reference for clinical diagnosis and research. Aiming at this problem, the aim of this paper is to realize the compensation of the nonuniformity of RF field under high field, so that the fast and high quality imaging can be achieved, and the pulse sequence is designed from the excitation pulse sequence. The structure design of coil array and the measurement of actual K space trajectory are studied. (1) the design of excitation pulse sequence is to realize uniform and fast proton excitation in three dimensional target region under high field. An optimal design method of excitation pulse sequence based on parallel excitation technique is proposed. This method optimizes the design of K space locus to optimize the pulse sequence duration, excitation effect and clinical safety, and its effectiveness has been verified by modeling and simulation under high field. More specifically, the method limits the distribution range of K space locus by looking for an optimization method, and defines the undersampling in K space to shorten the pulse sequence time. Because the range of K space trajectory defined by this method is related to the specific target of excitation, the targeted design of radio frequency pulse can guarantee the desired excitation effect for the K space trajectory sampled in the range of this distribution. The method loosens the restrictions on the choice of three dimensional K space trajectory types and allows the optimal trajectory type to be selected in the design to reduce the amplitude and energy of the RF pulses required for excitation. Therefore, the clinical safety is improved. (2) the structure of coil array is designed to give full play to the acceleration of 3D parallel excitation technology under the premise of guaranteeing imaging quality. This paper presents a preliminary design and evaluation method of coil array for the 3D excitation acceleration of parallel excitation technology. There has been no previous report on the third extended coil array, which is the first innovative design proposed in this paper. Therefore, the design idea of the coil array used in the magnetic resonance imaging technology is used in the design. The effectiveness of the design method and evaluation method is verified by modeling and simulation under high field. Using this kind of coil array as transmitting coil, combining with the sequence design method proposed in this paper for three-dimensional selective excitation, it can effectively shorten the duration of excitation pulse sequence. So as to improve the imaging time resolution (3) about the actual K space trajectory measurement, in order to ensure the accurate excitation of the protons in the target region by the parallel excitation pulse sequence, A real K space trajectory measurement method is studied to solve the problem of trajectory deformation in excited K space. The classical sequential measurement method is tested on the permanent magnet magnetic resonance scanner. The advantages and disadvantages of the method are analyzed by combining the theory and experiment, and the results of imaging reconstruction are compared between the measured K space trajectory and the ideal track. The validity of the sequential measurement method is verified.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2012
【分类号】：R310

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