【作者】 李龙；

【导师】 张立； 周雷；

【作者基本信息】 西安电子科技大学 ， 电子信息硕士（专业学位）， 2024， 硕士

【摘要】 磁共振作为主流的医学影像模态,是基于磁共振物理和电磁理论的尖端电磁成像技术,随着对成像质量的求进一步提升,现今已向7T及以上超高场强发展。超高场磁共振能够有更清晰的分辨率、更快的成像速度和更高的成像对比度等优点,但是用于自旋质子激励的射频场工作频率也随之提高,进而磁共振系统中电磁波波长接近人体尺寸,由此带来的相位延迟和反射造成了驻波效应,造成了磁共振射频场（B₁⁺）的不均匀性。而具有高介电常数的人工电磁材料凭借其性能稳定,成本较低等优势能够有力解决驻波产的了伪影问题。因此本文聚焦人工电磁材料在超高场磁共振射频场调控中的应用研究。本文首先针对高介电常数介质板和人工电磁材料开展了国内外研究现状的调研,并对磁共振成像系统的基本原理做了说明。进一步对高介电常数介质材料的电磁场调控基本原理进行了阐释。在这些理论基础之上,本文分别针对经典驻波磁共振成像系统和行波磁共振成像系统设计了两种具有高介电常数的人工电磁材料并进行了仿真和磁共振成像实验验证。经典的磁共振射频系统中多采用容积发射线圈进行射频场发射,主要特征为成像物体位于谐振器的感应近场区,且电磁场呈现驻波特性。经典的人工电磁材料的设计方法是基于辐射场的散射系数的计算得到其等效介电常数,但不能很好反映其在感应近场区域的电特性。针对该问题提出了微带线谐振法并设计了一种可以在容积发射线圈感应近场区工作,具有和高介电常数介质板相同B₁⁺场调控作用的人工电磁材料。该人工电磁材料的相对介电常数为78.4,具有和高介电常数介质板在近场区具有相同电特性,并通过电磁仿真和经典驻波磁共振系统实验验证其在负载中达到了和介质板相同的B₁⁺场调控的效果。相较于经典的磁共振容积发射线圈系统,行波磁共振射频系统更有利于避免驻波效应带来的B₁⁺场不均匀性问题。行波磁共振系统是利用磁体和梯度线圈内孔径组成的金属圆柱空腔结构作为传输辐射场的行波波导,针对现有的行波系统中天线与负载之间波阻不匹配造成的伪影问题,设计了一种用于行波天线和人体负载之间波阻匹配的环状人工电磁材料,创新性的将人工电磁材料应用于调控行波磁共振系统的电磁波行为。通过电磁仿真和行波磁共振系统实验验证了其波阻匹配的有效性,相较于不加载调控材料的情况,人体负载人体头部区域的功率沉积提升了2.8倍,负载头部伪影区域的B₁⁺场幅值提升了3倍。显著提升了B₁⁺场平均幅值,改善了原来波阻不匹配造成的伪影区域并有效提升了磁共振成像的质量。更多还原

【Abstract】 Magnetic resonance is one of the mainstream medical imaging modalities,which is an advanced electromagnetic imaging technology based on magnetic resonance physics and electromagnetic theory.With the further improvement of the demand for imaging quality,it has now developed towards 7T ultra-high field strength.Ultra-high field imaging can have advantages such as clearer resolution,faster imaging speed,and higher imaging contrast.However,the operating frequency of the RF field used for spin proton excitation has also increased.Furthermore,the electromagnetic wave length in the magnetic resonance system approaches the size of the human body,resulting in phase delay and reflection that cause standing wave effects,leading to the inhomogeneity of the magnetic resonance radio frequency field（B₁⁺）.Artificial dielectric materials with high dielectric constants,with their advantages of stable performance and low cost,effectively solve the problem of standing wave artifacts and significantly improve the quality of magnetic resonance imaging.Therefore,this thesis aims to conduct research on the application of artificial dielectric materials in ultra-high field magnetic resonance technology.This thesis first investigates the current research status of high dielectric constant dielectric plates and artificial dielectric materials at home and abroad.Then,it explains the working principle of the imaging process in magnetic resonance imaging systems,and explains the mechanism of high dielectric constant dielectric materials on loads by establishing a spherical scattering model.On this basis,two artificial dielectric materials with high dielectric constants were designed for classical standing wave magnetic resonance imaging systems and traveling wave magnetic resonance imaging systems,and simulation and experimental verification were conducted.In classical magnetic resonance radio frequency systems,volumetric emission coils are often used for radio frequency field emission.The main feature is that the imaging object is located in the induction near-field region of the resonator,and the electromagnetic field exhibits standing wave characteristics.The classic design method is based on calculating the scattering coefficient of the radiation field to obtain the equivalent dielectric constant of artificial dielectric materials,which cannot well reflect their electrical characteristics in the induced near-field region.A microstrip line resonance method was proposed to address this issue,and an artificial dielectric material with the same B₁⁺field control effect as a high dielectric constant dielectric plate was designed to operate in the near-field induction region of the volumetric emission coil.The relative dielectric constant of this artificial dielectric material is 78.4,which has the same electrical characteristics as a high dielectric constant dielectric plate in the near-field region.And through electromagnetic simulation and classical standing wave magnetic resonance system experiments,it was verified that it achieved the same B₁⁺field control effect as the dielectric plate in the load.Compared to the classical magnetic resonance volumetric emission coil system,the traveling wave magnetic resonance RF system is more conducive to avoiding the B₁⁺field inhomogeneity problem caused by the standing wave effect.The traveling wave magnetic resonance system utilizes a metal cylindrical cavity structure composed of a magnet and a gradient coil inner aperture as a traveling wave waveguide for transmitting radiation fields.In response to the problem of wave resistance mismatch between the antenna and the load in existing traveling wave systems,a ring-shaped artificial dielectric material is designed for wave resistance matching between the traveling wave antenna and the human body load.The innovative application of artificial dielectric materials to regulate the electromagnetic wave behavior of the traveling wave magnetic resonance system.The effectiveness of its wave resistance matching has been verified through electromagnetic simulation and traveling wave magnetic resonance system experiments.Compared to the situation without loading control materials,the power deposition in the human head area under load has increased by 2.8 times,and the B₁⁺field amplitude in the artifact area under load has increased by 3 times.Significantly increased the average amplitude of B₁⁺field,improved the artifacts caused by the original wave impedance mismatch,and effectively improved the quality of magnetic resonance imaging.更多还原