地面磁共振变间隔自旋回波序列控制及测量方法研究

发布时间：2018-07-07 20:31

  本文选题：地面磁共振 + 自旋回波序列　； 参考：《吉林大学》2017年硕士论文

【摘要】：随着经济和社会高速发展,我国对淡水资源的依赖越来越严重,导致水资源问题越来越突出,尤其是淡水资源缺乏和水灾害频发,这两个问题已严重制约我国经济发展和社会进步。地面磁共振探测技术以其直接定量探测地下水的优势广泛应用于水资源评价和灾害水探查等领域。横向弛豫时间T_2是磁共振信号重要的特征参数,包括丰富的水文地质信息。目前传统地面磁共振探测通常采用单脉冲激发序列测量平均弛豫时间T_2~*,但因地磁场的不均匀性,造成T_2~*远小于横向弛豫时间T_2,因而无法准确计算孔隙度、渗透率和孔径分布等水文地质参数,严重制约了地面磁共振技术的发展。针对在外磁场不均匀情况下单脉冲激发序列无法准确测量T_2~*的问题,本文以精确测量横向弛豫时间T_2为目的,对地面磁共振变间隔自旋回波序列控制及测量方法进行研究。在地面磁共振探测原理的基础上,阐述了地磁场不均匀性对氢原子核横向弛豫过程的影响,并给出T_2~*≤T_2关系。结合单脉冲激发序列,引入自旋回波序列探测方法,为提高横向弛豫时间T_2探测精度,提出变间隔自旋回波序列测量方案。为实现高精度和高效率的磁共振探测,对回波时间和回波个数进行研究,根据磁共振信号衰减特征,设计了按对数等分分布的变间隔回波时间,需设置最小回波时间temin、最大回波时间temax和回波个数N。建立含水层仿真模型进行仿真实验,仿真结果表明:在保证高精度和高效率探测前提下,最佳回波个数N选取7～10个;最大回波时间temax接近或大于真实jk向弛豫时间T_2时,T_2探测精度高。为实现变间隔自旋回波序列探测,在已有磁共振仪器硬件基础上,设计了变间隔自旋回波序列控制软件。本文利用LabVIEW软件平台进行上位机控制软件设计,包括控制参数设置、仪器运行监测、信号波形显示等功能。下位机控制模块基于ARM+CPLD构架,能够产生高精度控制信号。本文设计的变间隔自旋回波序列控制软件具有自动化程度高、操作简易、系统稳定可靠、控制精度和效率高的特点。对变间隔自旋回波序列控制及测量方法进行测试,通过测试结果可得:本文所设计的控制软件可产生高精度发射电流,发射频率误差低于0.06%,发射时间误差范围在1.33%之内,90°脉冲与180°脉冲持续时间关系为τ_(p180)=2τ_(p90),满足自旋回波信号产生原理,并且多次发射电流初始相位一致,使多次探测MRS信号极性相同,保证信号叠加的有效性,从而提高信噪比;高压继电器控制信号切换时间实测值范围为9.45～10ms,有效避免接收线圈中感应高电压对接收系统的冲击;H桥桥臂切换死区时间为10.1μs,与设置值误差为0.5μs,能够保护H逆变桥路安全;整机测试验证了控制软件的稳定性和可靠性,最后通过物理模拟实验,验证了变间隔自旋回波序列测量方法的可行性。
[Abstract]:With the rapid development of economy and society, China's dependence on freshwater resources is becoming more and more serious, leading to more and more serious problems of water resources, especially the shortage of freshwater resources and the frequent occurrence of water disasters. These two problems have seriously restricted China's economic development and social progress. Surface magnetic resonance (Mr) detection technology is widely used in water resources assessment and disaster water exploration for its advantage of direct quantitative detection of groundwater. Transverse relaxation time T _ S _ 2 is an important characteristic parameter of magnetic resonance signal, including abundant hydrogeological information. At present, conventional ground magnetic resonance detection usually uses monopulse excitation sequence to measure the average relaxation time T _ S _ 2. However, due to the nonuniformity of geomagnetic field, T _ 2T _ 2 * is far smaller than the transverse relaxation time T _ 2, so it is impossible to calculate the porosity accurately. The development of surface magnetic resonance technology is seriously restricted by hydrogeological parameters such as permeability and pore size distribution. In order to solve the problem that the order pulse excitation sequence can not accurately measure T _ s _ 2 * in the case of non-uniform external magnetic field, this paper studies the control and measurement method of the ground magnetic resonance variable-interval spin-echo sequence for the purpose of accurately measuring the transverse relaxation time (T _ s _ 2). On the basis of the principle of ground magnetic resonance detection, the influence of magnetic field inhomogeneity on the transverse relaxation process of hydrogen atom is expounded, and the T _ 2T _ 2 * 鈮，

本文编号：2106185