Collaborative Research: Electromagnetic Field Profile Design for Next-Generation Travelling-Wave MRI

该合作提案开发了一种新的方法，用于在UHF和低微波频率范围内集成电磁（EM）场的暴露和激励，以用于高磁场（B3 T）下的下一代磁共振成像（MRI）。高磁场可提高MRI的信噪比，并伴随着RF频率的增加，这可能会导致患者在孔内产生传播模式。例如，在7 T下，所需的RF频率在300 MHz范围内，而适合人体的孔的直径至少为60 cm，使其高于当加载有身体时被视为波导的孔的至少一种模式的截止频率。行波在患者更舒适的环境、更大的视场、成像到目前为止MRI不可到达的区域以及实现新的空间编码方案和各种模式灵敏度分布方面可能是有利的。然而，重要的是能够设计孔中的模式，以获得更好的激励场分布均匀性和对身体的功率暴露的控制。 智力优势：拟议的研究的主要目标是开发的RF场分布和相关的腔壁表面阻抗和激励探头的设计方法，使非常快速和准确的全波高阶计算电磁模拟的大型体加载腔。几个设计将被制造，表征，并过渡到临床和研究合作者在哈佛大学的成像研究。要解决的具体问题是：（a）了解下一代行波、高场磁共振成像射频电磁场分布设计的基本原则和局限性;（B）开发一种工程方法，通过极其快速和可靠的模拟技术，修改人体负载内腔中的表面阻抗;（c）解决与加载腔集成的适当场分布激励（探针）设计的问题;（d）评估和控制体模、动物或人体内的比吸收率;及（e）为哈佛大学的合作者操作的数部临床及研究磁振造影机推行有关设计（这些推行工作并不获建议的拨款资助）。该项目将研究印刷谐振结构或三维介质金属人工表面阻抗形式的周期性或准周期性表面阻抗结构，以及结合线偶极子和环路的不同类型的激励，贴片天线探头与同轴馈电，以及腔背槽激励器（用于不同模式的多个探头将与开关电路结合）。其他（非MRI）应用的研究结果在加载多模腔包括领域从低功率无线电力输送在封闭的空间，以高功率先进的智能微波炉。基础科学和工程的拟议工作的更广泛影响支持国家的科学和技术优势。预期的结果将提供一种新的医学成像方法，使患者更加舒适，并增加视野，灵敏度和功能。对这种改进的医疗诊断工具的需求日益增长，特别保证了对社会的更广泛影响。由于可能改变使用MRI进行医疗诊断的方式，因此该提案在性质上可能会被认为是变革性的。在本科和研究生水平的多学科教育，跨越高频模拟电路设计，电磁仿真，生物EM和计量学领域，将在科罗拉多州的两个顶级机构的影响，加强现有的核心竞争力。这两个机构的PI一直在积极开展外联活动，并与这项拟议的工作计划有关，为现有的K-12外联活动增加了几个新的模块，数百名中学生参加了电场旅行。在各级开展征聘工作，重点关注任职人数不足的群体，这将继续丰富教育环境。与哈佛和Intermountain Neuroimaging Consortium在医疗应用方面的合作，与法国利莫日大学XLIM的国际合作，以及行业合作伙伴（恩智浦）的合作都体现在免费技术参与和临床研究、学生交流和硬件捐赠上。

This collaborative proposal develops a new method of integrated design of exposure and excitation of electromagnetic (EM) fields in the UHF and low microwave frequency range for next-generation magnetic resonance imaging (MRI) at high magnetic fields (B3T). High magnetic fields improve the signal-to-noise ratio in MRI, and are accompanied by increased RF frequencies, which can lead to propagating modes inside the bore with a patient. For example, at 7T, the required RF frequency is in the 300MHz range, while a bore that fits a human is at least 60cm in diameter, making it above the cutoff frequency for at least one mode of the bore viewed as a waveguide, when loaded with the body. The travelling waves can potentially be advantageous in terms of a more comfortable environment for patients, larger field of view, imaging to-date MRI inaccessible areas, and enabling new spatial encoding schemes and a variety of mode sensitivity profiles. It is important, however, to be able to design the modes in the bore for better excitation field profile uniformity and control of power exposure to the body. Intellectual Merit: The principal goal of the proposed research is development of design methodologies for RF field profiles and associated cavity wall surface impedances and excitation probes, enabled by extremely fast and accurate full-wave higher order computational EM simulations of large body-loaded cavities. Several of the designs will be fabricated, characterized, and transitioned to clinical and research collaborators at Harvard University for imaging research. The specific issues to be addressed are: (a) Understanding the fundamental principles and limitations of radio-frequency electromagnetic field profile design for next-generation travelling-wave, high-field MRI; (b) Developing an engineering approach for modification of surface impedances in body-loaded bore cavities, enabled by extremely fast and reliable simulation techniques; (c) Solving the problem of proper field profile excitation (probe) design integrated with loaded cavity; (d) Evaluation and control of specific absorption rates inside the phantom, animal, or human; and (e) Implementing the designs for several clinical and research MRI machines operated by collaborators at Harvard University (these implementations are not supported by the proposed grant). The project will investigate periodic or quasi-periodic surface impedance structures in the form of printed resonant structures or three-dimensional dielectric-metal artificial surface impedances, and different types of excitations combining wire dipoles and loops, patch-antenna probes with coaxial feeds, and cavity backed slot exciters (multiple probes for different modes will be incorporated with switching circuits). Other (non-MRI) applications of the resulting research in loaded multi-mode cavities include areas from low-power wireless power delivery in closed spaces to high-power advanced smart microwave ovens. Broader impacts of the proposed work on basic science and engineering support the nation's science and technology advantage. The anticipated results will provide a new method of medical imaging with more comfort for patients, and an increased field of view, sensitivity, and functionality. Broader impacts on society are especially warranted by growing needs for such improved medical diagnostic tool. Because of the potential to change the way medical diagnostics using MRI is done in the longer term, the proposal may be considered transformative in its nature. Multi-disciplinary education at the undergraduate and graduate levels, spanning areas of high-frequency analog circuit design, EM simulations, bio-EM, and metrology, will make an impact on two top institutions in the state of Colorado, strengthening the existing core competency. The PIs at both institutions have been active in outreach, and related to this proposed work plan to add several new modules to the existing K-12 outreach, with hundreds of middle-school children on Electric Field Trip visits. A recruiting effort at all levels focusing on underrepresented groups will continue to enrich the educational environments. Collaborations related to medical applications with Harvard and Intermountain Neuroimaging Consortium, international collaboration with XLIM, University of Limoges, in France, and industry partnership (NXP) are evidenced by no-cost technical participation and insertion into clinical studies, student exchanges, and hardware donations.