0.35T磁共振系统上层选轮廓的仿真与矫正研究

发布时间：2018-11-06 08:41

【摘要】：磁共振成像(MRI)技术因其能精确细致的反应人体组织结构而在近年来得到快速发展。MRI领域内先进的成像技术大多以超导磁共振系统为平台来实现,但是受限于超导MRI设备高昂的装配和维护费用,我国医疗机构体系内超导MRI设备的普及率还比较低。相对的,永磁型低场磁共振系统借由其相对较低的价格在国内得以迅速普及。永磁低场磁共振系统存在着信噪比和稳定性都相对较低的问题,所以一些重要技术也因此难以在这种设备上完美实现。准确的成像层面激发是保证磁共振成像质量的重要因素。在磁共振成像中,射频激发脉冲(RF)与层选梯度共同作用激发层选轮廓内的氢核,完成对成像层面的选择。然而在实际的磁共振系统中,由于RF发射系统与梯度系统无法保证时间上的完全一致,会使实际获得的层选轮廓不够理想,进而导致得到的图像质量的下降。本论文主要针对实验室所使用的OPER-0.35T开放式永磁磁共振系统中存在的激发脉冲与层选梯度不同步问题进行了研究并提出和验证了解决方案。主要内容如下：1.对0.35T系统RF激发脉冲与层选梯度之间相对延迟如何测量的问题的研究。设计了通过对选定序列(UTE序列)的理论层选轮廓与实际层选轮廓进行对比匹配来得到相对延迟的方案。2.层选轮廓仿真算法的实现。研究了Shinnar-Le Roux(SLR)理论的数理原理,并以此为基础设计了层选轮廓的仿真算法,该算法能够仿真出各种相对延迟情况下的RF脉冲与层选梯度所产生的层选轮廓情况。3.通过实际实验验证了该方法的有效性。依据所测得延迟对扫描序列进行补偿修改,对比修改后和未修改两种序列实际扫描所得到的图像,验证了补偿后的序列能够有效提高激发层面的准确性并在一定程度上提高图像信噪比。
[Abstract]:Magnetic resonance imaging (MRI) technology has been developed rapidly in recent years because of its precise and meticulous response to human tissue structure. Most of the advanced imaging techniques in the field of MRI are realized on the platform of superconducting magnetic resonance system. However, due to the high cost of assembly and maintenance of superconducting MRI equipment, the prevalence of superconducting MRI equipment in medical institutions in China is relatively low. In contrast, the permanent magnet low-field magnetic resonance system by its relatively low prices in the country has been rapidly popularized. The low field magnetic resonance system has the problems of relatively low signal-to-noise ratio (SNR) and low stability, so it is difficult for some important technologies to be realized perfectly in this kind of equipment. Accurate level excitation is an important factor to ensure the quality of Mr imaging. In magnetic resonance imaging, radio-frequency pulsed (RF) and the selective gradient work together to excite the hydrogen nuclei in the profile of the layer to complete the selection of the imaging layer. However, in the actual magnetic resonance system, because the RF emission system and the gradient system can not guarantee the complete consistency of the time, it will make the actual layer selection contour not ideal, which will lead to the deterioration of the image quality. In this paper, we mainly focus on the OPER-0.35T open permanent magnet magnetic resonance system, which is used in laboratory. The problem of asynchronous excitation pulse and stratified gradient is studied, and the solution is proposed and verified. The main contents are as follows: 1. How to measure the relative delay between the RF excitation pulse and the layer gradient in 0.35T system is studied. The scheme of relative delay is designed by comparing the theoretical layer selection contour of selected sequence (UTE sequence) with the actual layer selection contour. 2. The realization of layer selection contour simulation algorithm. This paper studies the mathematical principle of Shinnar-Le Roux (SLR) theory, and designs a simulation algorithm based on this theory. The algorithm can simulate the profile of layer selection generated by RF pulse and layer selection gradient under various relative delays. 3. The effectiveness of the method is verified by practical experiments. According to the measured delay, the scan sequence is compensated and modified, and the images obtained from the actual scanning of the modified and unmodified sequences are compared. It is verified that the compensated sequence can effectively improve the accuracy of the excitation level and improve the image SNR to a certain extent.
【学位授予单位】：浙江大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：R445.2

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