RF coils based on low-profile electromagnetic band gap (EBG)-structures for the usage in magnetic resonance imaging (MRI)

在DFG项目的第一阶段，已经提出了用于7Tesla MRI的新型多通道射频线圈，并使用近场测量系统进行了积极的评估。每个元件包括具有大约三个半波长的电长度的弯曲偶极子和其后面的屏蔽板，以便主要在患者(传输病例)的方向上产生磁场或主要从那里接收磁场。屏蔽要么是长度优化的金属板，要么是形成高阻抗表面(HIS)的扁平电磁带隙结构。在前一种情况下，本征共振基板可以改善患者体内的电磁场。另一方面，HIS屏蔽实施了同相镜像电流，从而提高了B1效率(每接受功率的磁场幅度)。此外，由于结构化亚表面的带隙特性，相邻的线圈元件彼此之间的隔离度更高。在DFG项目的第二阶段，将在7特斯拉扫描仪上对可用的线圈进行广泛的测试。最初，这种评估包括单一元素，通过使用在电磁特性上模拟人体的所谓体模进行测试。随后，将建立更复杂的多元素线圈系统(用于膝盖或头部成像的4通道设置和用于腹部和背部成像的8通道阵列)。最后，使用本项目创新的多通道线圈对上述身体部位进行活体测量。在使用7-特斯拉扫描仪进行测试的同时，还应根据与实际评估相关的结果，按照先前的创新射频线圈方法进行进一步的研究。我们想要探索一种与位置相关的屏蔽体表面阻抗。最初将考虑极端情况PEC(均匀金属)和PMC(结构化His Meta-Surface)。这一概念代表了在人体内产生核磁共振优化电磁场的进一步探索的自由度。除了上述偶极子元件，环形天线(磁偶极子)将被认为高于他的屏蔽，并特别适用于身体的部分，在那里这种环形拓扑结构提供了优势，例如用于乳房成像。最后，将研究电偶极子和磁偶极子的组合，即将已建立的弯曲偶极子与环形天线组合，从而产生切向和法向磁场分量。这两个分量的正确相移导致产生圆极化近场的单个元件。

During the first period of this DFG project novel multi-channel radio frequency coils for 7Tesla MRI have been proposed and were positively evaluated using a near-field measurement system. Each element comprises of a meandered dipole with an electrical length of approximately three half wave lengths and a shielding plate behind it in order to primarily generate the magnetic field in the direction of the patient (transmit case) or primarily receive from there. The shielding is either a length optimized metallic plate or a flat electromagnetic bandgap structure forming a high impedance surface (HIS). In the former case, the eigen-resonant base plate results in improved electro-magnetic fields in the body of the patient. The HIS shield on the other hand enforces in-phase mirror-currents that lead to an improved B1 efficiency (magnetic field amplitude per accepted power). Furthermore adjacent coil elements are more isolated from each other due to the band-gap characteristic of the structured meta-surface. In the second phase of the DFG project, the available coils are to be tested extensively on a 7-Tesla scanner. Initially, this evaluation includes single elements, which are tested by utilizing so-called phantoms that mimic the human body in its electromagnetic properties. Subsequently, more complex multi-element coil-systems will be established (4-channel setups for knee or head imaging and 8-channel arrays for the abdominal and back imaging). Finally, in-vivo measurements of the above mentioned body parts shall be carried out with the innovative multi-channel coils of this project. Parallel to the tests with the 7-Tesla scanner further research shall be carried out following the previous innovative RF coil approaches, also based on the findings related to the practical evaluation. We would like to explore a location-dependent surface impedance of the shield. Initially the extreme cases PEC (homogeneous metal) and PMC (structured HIS meta-surface) will be considered. This concept represents a further unexplored degree of freedom in the generation of MRI-optimized electromagnetic fields in the human body. In addition to the above-mentioned dipole elements ring antennas (magnetic dipoles) will be considered above HIS shields, and applied especially in parts of the body, where such a ring topology offers advantages, such as for breast imaging. Finally, combinations of electric and magnetic dipoles will be examined, i.e., the established meander dipoles are combined with ring antennas, so that both, tangential and normal magnetic field components are generated. A correct phasing of these two components results in a single element that produces a circularly polarized magnetic near field.