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

该合作提案为下一代高磁场（B3T）磁共振成像（MRI）开发了一种集成设计UHF和低微波频率范围内电磁（EM）场暴露和激发的新方法。高磁场提高了MRI的信噪比，并伴随着射频频率的增加，这可能导致患者在孔内传播模式。例如，在7T时，所需的射频频率在300MHz范围内，而适合人体的孔直径至少为60cm，当装载在身体上时，至少有一种模式的孔被视为波导，使其高于截止频率。行波在为患者提供更舒适的环境、更大的视野、成像迄今为止MRI无法到达的区域、实现新的空间编码方案和各种模式灵敏度剖面等方面具有潜在的优势。然而，重要的是，能够在膛内设计模式，以获得更好的激励场分布均匀性和控制功率暴露到身体。智力优势：提出的研究的主要目标是开发射频场轮廓和相关腔壁表面阻抗和激励探头的设计方法，通过对大型体载腔进行极快速和准确的全波高阶计算EM模拟。其中一些设计将被制作、表征，并移交给哈佛大学的临床和研究合作者进行成像研究。需要解决的具体问题是：(a)了解下一代行波高场核磁共振成像射频电磁场剖面设计的基本原理和局限性；(b)发展一种工程方法，利用极其快速和可靠的模拟技术，改变载体孔洞的表面阻抗；(c)解决与加载腔相结合的场廓形激励（探头）设计问题；(d)评价和控制幻体、动物或人体内的特定吸收率；(e)实施由哈佛大学合作者操作的几台临床和研究MRI机器的设计（这些实施不受拟议拨款的支持）。该项目将研究周期或准周期表面阻抗结构，以印刷谐振结构或三维介电-金属人工表面阻抗的形式，以及结合线偶极子和环路的不同类型的激励，同轴馈电的贴片天线探针和腔背槽激励器（不同模式的多个探针将与开关电路结合）。研究结果在负载多模腔中的其他（非mri）应用包括从封闭空间的低功率无线供电到高功率先进智能微波炉等领域。拟议工作对基础科学和工程的更广泛影响支持了国家的科技优势。预期的结果将提供一种新的医学成像方法，使患者更舒适，并增加视野，灵敏度和功能。对这种改进的医疗诊断工具日益增长的需求尤其保证对社会产生更广泛的影响。从长远来看，由于有可能改变使用核磁共振成像进行医学诊断的方式，因此该提议可能被认为是本质上的变革。本科和研究生阶段的多学科教育，涵盖高频模拟电路设计、电磁仿真、生物电磁和计量学等领域，将对科罗拉多州的两所顶尖院校产生影响，加强现有的核心竞争力。这两个机构的pi一直在积极开展外展活动，并与拟议的工作计划有关，在现有的K-12外展活动中增加几个新模块，有数百名中学生参加实地考察活动。在各级以代表性不足的群体为重点的征聘工作将继续丰富教育环境。与哈佛大学和山间神经成像联盟的医疗应用相关的合作，与XLIM，法国利摩日大学的国际合作，以及行业伙伴关系（恩智浦）的无成本技术参与和插入临床研究，学生交流和硬件捐赠证明。

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.