MRI Engineering Core

MRI 工程核心

• 批准号: 10708654
• 负责人: Alan Koretsky
• 金额: $ 191.09万
• 依托单位: NATIONAL INSTITUTE OF NEUROLOGICAL DISORDERS AND STROKE
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/10708654
• 关键词: 3D Print; Acetone; Address; Affect; Amplifiers; Anesthesia procedures; Animals; Brain; Callithrix; Cell Nucleus; Characteristics; Chemicals; Coiled Bodies; Collaborations; Computer software; Coupling; Custom; Data; Data Reporting; Detection; Development; Devices; Electric Capacitance; Electromagnetic Fields; Electronics; Engineering; Evaluation; Feasibility Studies; Feedback; Frequencies; Functional Imaging; Functional Magnetic Resonance Imaging; Future; Generations; Glass; Goals; Head; Heating; Helium; Human; Image; Infrastructure; Institutional Review Boards; Intervention; Kidney; Laboratories; Lead; Length; Liquid substance; Liver; Magnetic Resonance Imaging; Maryland; Measurement; Measures; Mechanics; Modification; Monitor; Mus; National Institute of Mental Health; National Institute of Neurological Disorders and Stroke; Neurosciences; Operative Surgical Procedures; Optics; Patients; Performance; Perfusion Weighted MRI; Pituitary Gland; Policies; Povidone; Principal Investigator; Property; Protocols documentation; Protons; Radiology Specialty; Resolution; Rod; Safety; Schedule; Scheme; Scientist; Shapes; Source; System; Technology; Temperature; Testing; Thermometry; Tissues; Transistors; United States National Institutes of Health; Universities; Validation; Variant; Work; arterial spin labeling; base; brain magnetic resonance imaging; contrast imaging; coronavirus disease; design; experience; experimental study; human imaging; image guided; imaging facilities; imaging system; implementation cost; improved; instrumentation; interest; magnetic field; medical specialties; molecular imaging; neuroimaging; new technology; next generation; notch protein; perfusion imaging; programs; prototype; radio frequency; response; safety testing; seal; simulation; success; transmission process; tumor; voltage

NIH COVID safety policy has severely restricted experimental progress by the MRI Engineering Core. The MRI Engineering core of LFMI (EC) supports new hardware developments for the highest field MRI systems at NIH, an supports specialty projects for other systems. Currently, its main goal is to develop 11.7T human MRI and 17.6T animal MRI in order to perform neuroimaging with superior contrast and resolution. The 11.7T system is awaiting (re-)installation to be scheduled once sufficient liquid Helium for energization is procured, while a feasibility study will be performed for possible installation of the 17.6T animal MRI. The patient table of the 11.7T MRI will need to be modified in the future to accommodate up to 64 receive channels and possibly some additional transmit channels. This requires lengthening of the extension to the table and lengthening of the track system within the magnet bore that supports this assembly. This year, a track extension assembly was completed and tested by temporarily lengthening the table with some rods. Further modifications will have to wait until installation and testing of the existing table configuration has been completed by Siemens. In particular for the 11.7T system, major technical developments are needed to allow transmission and detection of the required RF fields, a difficult and yet unresolved problem. For the initial human studies at 11.7T, we plan to use a general-purpose whole head coil for both transmission and reception. As we plan to use this coil for the initial safety testing required by the FDA, we opted for a relatively simple design that allows for accurate prediction of transmitted electro-magnetic (EM) fields and associated tissue heating from simulations and measurements. We settled on a transmit-receive type 500 MHz inductive birdcage resonator. EM field simulations were performed using commercial software and used to compare measured and simulated frequency response when transmitting and receiving at different combinations of two coupling points. Excellent correspondence between simulations and measurements was observed under various conditions, improving our confidence in our ability to predict coil performance. After additional measurements and simulations with a small spherical test object (phantom) performed last year in our small bore (animal) 11.7T system, this year we repeated these measurements and simulations on our brain phantom, which more closely matches the human head in terms of shape and dielectric and conductive properties. We constructed this phantom by filling a head-shaped glass container with polyvinylpyrrolidone (https://amri.ninds.nih.gov/cgi-bin/phantomrecipe). The evaluation will be part of the FDA IDE submission for the 11.7 T. In the past year, E&M simulations of an inductive birdcage coil and the brain phantom were developed using the Remcom xFDTD program. These simulations were used to compute the spatial variation of SAR in a uniform head phantom that was filled with a conductive dielectric material that simulated the loading of the brain. For validation, these simulations were compared to measurements on the brain phantom. As the human 11.7 T system is not yet available, the coil and phantom were placed in the bore of a 3 T magnet using a custom-built coil cradle so that MR thermometry could be performed on the phantom using the 3 T body coil right after RF heating at the 11.7 T frequency. The head phantom was also equipped with optical temperature probes to monitor selective regions of the phantom during controlled heating and cooling of the phantom. Results so far are encouraging, but it appeared the accuracy of the temperature data is hampered by the instabilities (field drift) of the 3 T scanner. To improve on this, we plan to add a reference chemical to the phantom, which will allow simultaneous measurement of both the background field and the temperature shift. Preliminary experiments with various compounds have shown acetone is the most suitable candidate. The composition of the phantom will need to be adjusted to compensate for the changes in conductivity and permittivity with the addition of acetone. For several years, our lab has been developing on-coil RF amplification technology for multi-channel transmission (also called pTX). Compared to conventional remote voltage-mode RF power amplifiers, on-coil amplification allows better B1 control, reduced load sensitivity, and reduced power losses at a lower implementation cost. In addition, this technology also allows direct sensing of coil current, information that can be used for safety monitoring and feedback. Accurate RF transmit control allows better estimation of increased tissue heating associated with high field MRI. In order to evaluate feasibility and identify potential roadblocks for on-coil pTx at 11.7T, we built an 8-channel 7T prototype using optically controlled current-source RF power amplifiers. To adapt the 7T prototype to work at 11.7T, several issues need to be resolved that relate to the power transistor. Parasitic capacitances in the transistor lead to power loss that is exacerbated at increasing field (=RF frequency). In addition, increased magnetic field also affects transistor performance and power efficiency. To overcome these problems, we started investigating the possibility to improve transistor design beyond capabilities currently available with commercial devices. This is done in collaboration with the University of Maryland, which has experience in transistor design and manufacturing. The Engineering Core also continued its support of the various groups the use MRI at NIH. It developed a variety of mouse coils and RF filters for the Mouse Imaging Facility. Presently all mouse body coils are tuned/matched, and orthogonally arranged saddle pairs and used in transmit/receive (transceiver)-mode with the 7T, 9.4 T and 3 T Bruker systems. Resonant nuclei included 1-H, 13-C, 2-H. Although the crossed coil arrangement for proton and other nuclei (X-nuclei) are theoretically flux decoupled, in practice we found coupling up to -20 dB due to wiring of terminals and other factors. To minimize this coupling further we built bandpass filters for X-nuclei with low insertion loss of typically 0.1 dB and a suppression of 1-H of up to -70 dB. For the 1-H coils we utilized a notch for x with similar scatter parameter characteristics. Those filters are built into the coil, however, to save extra work we made them modular to be placed in the transmission lines and those can be used for all other coils. The loop-windings in the coils were made from either single- or multi-stranded wires that are laid into groves of a 3D printed former developed in-house. The inner bore of the coil former fits around a sealed animal holding container. Various lengths of saddle coils were manufactured to achieve more sensitivity by means of adapted filling factor. For kidney and liver studies shorter coils were utilized. A mouse head coil is also made using crossed saddle loops and is mounted directly on a holding container. Restraining of the animal, temperature control and anesthesia supply is built into that coil arrangement. We also built a mouse body coil with orthogonal loops for x- nuclei in quadrature and linear loops for 1-H. The 1-H loops are 45o rotated. The expected strong coupling was overcome by application of filters as mentioned above. For the 7T in the NMR center arterial spin labeling coils and setup were reengineered to accommodate the new Siemens TERRA system. Lastly, a dual-tuned, 13C 1H head coil for 3T has been completed and integrated into a mechanical assembly for imaging of the human head on a Philips 3T MRI. The coil and its RF interface were tested extensively to develop data for the report being prepared to obtain IRB approval.

NIH 的新冠病毒安全政策严重限制了 MRI 工程核心的实验进展。 LFMI (EC) 的 MRI 工程核心支持 NIH 最高场 MRI 系统的新硬件开发，并支持其他系统的专业项目。目前，其主要目标是开发11.7T人体MRI和17.6T动物MRI，以进行具有卓越对比度和分辨率的神经成像。一旦获得足够的液氦供能，11.7T 系统正在等待（重新）安装，同时将对 17.6T 动物 MRI 的可能安装进行可行性研究。 11.7T MRI 的患者表将来需要进行修改，以容纳多达 64 个接收通道，可能还需要一些额外的传输通道。 这需要加长工作台的延伸部分以及加长支撑该组件的磁体孔内的轨道系统。今年，通过用一些杆暂时加长工作台，完成了轨道延伸组件并进行了测试。进一步的修改必须等到西门子完成现有工作台配置的安装和测试后才能进行。 特别是对于 11.7T 系统，需要进行重大技术开发才能实现所需射频场的传输和检测，这是一个困难且尚未解决的问题。对于 11.7T 的初步人体研究，我们计划使用通用全头线圈进行传输和接收。由于我们计划使用该线圈进行 FDA 要求的初始安全测试，因此我们选择了相对简单的设计，可以通过模拟和测量准确预测传输的电磁 (EM) 场和相关的组织加热。我们选择了发射-接收型 500 MHz 感应鸟笼谐振器。使用商业软件进行电磁场模拟，并用于比较以两个耦合点的不同组合进行发射和接收时的测量频率响应和模拟频率响应。在各种条件下观察到模拟和测量之间的良好对应关系，提高了我们对预测线圈性能的能力的信心。 去年，在我们的小口径（动物）11.7T 系统中使用小型球形测试物体（模型）进行了额外的测量和模拟之后，今年我们在我们的大脑模型上重复了这些测量和模拟，该模型在形状、介电和导电特性方面与人类头部更加接近。我们通过在头形玻璃容器中填充聚乙烯吡咯烷酮来构建这个模型（https://amri.ninds.nih.gov/cgi-bin/phantomrecipe）。该评估将作为 11.7 T 的 FDA IDE 提交的一部分。 去年，使用 Remcom xFDTD 程序开发了感应鸟笼线圈和大脑模型的 E&M 模拟。这些模拟用于计算均匀头部模型中 SAR 的空间变化，该模型填充有模拟大脑负载的导电介电材料。为了验证，这些模拟与大脑模型的测量结果进行了比较。由于人体 11.7 T 系统尚不可用，因此使用定制的线圈支架将线圈和体模放置在 3 T 磁体的孔中，以便在以 11.7 T 频率进行射频加热后，可以立即使用 3 T 身体线圈对体模进行 MR 测温。头部模型还配备了光学温度探头，用于在模型的受控加热和冷却过程中监测模型的选择性区域。迄今为止的结果令人鼓舞，但温度数据的准确性似乎受到 3 T 扫描仪的不稳定性（场漂移）的影响。为了改进这一点，我们计划在模型中添加参考化学品，这将允许同时测量背景场和温度变化。对各种化合物的初步实验表明丙酮是最合适的候选者。需要调整模型的成分，以补偿添加丙酮后电导率和介电常数的变化。 多年来，我们的实验室一直致力于开发用于多通道传输（也称为 pTX）的线圈射频放大技术。与传统的远程电压模式射频功率放大器相比，线圈上放大可以实现更好的 B1 控制、降低负载灵敏度并以更低的实施成本降低功率损耗。此外，该技术还可以直接感测线圈电流，这些信息可用于安全监控和反馈。精确的射频传输控制可以更好地估计与高场 MRI 相关的组织加热增加。为了评估可行性并确定 11.7T 线圈上 pTx 的潜在障碍，我们使用光控电流源射频功率放大器构建了一个 8 通道 7T 原型。 为了使 7T 原型能够在 11.7T 下工作，需要解决与功率晶体管相关的几个问题。晶体管中的寄生电容会导致功率损耗，随着磁场（= RF 频率）的增加，功率损耗会加剧。此外，磁场的增加也会影响晶体管的性能和功率效率。为了克服这些问题，我们开始研究改进晶体管设计的可能性，使其超越目前商用器件的能力。这是与马里兰大学合作完成的，该大学在晶体管设计和制造方面拥有丰富的经验。 工程核心还继续支持 NIH 使用 MRI 的各个团体。它为小鼠成像设备开发了各种小鼠线圈和射频滤波器。目前，所有鼠标体线圈均已调谐/匹配，并正交排列鞍座对，并在 7T、9.4 T 和 3 T Bruker 系统的发射/接收（收发器）模式下使用。共振核包括1-H、13-C、2-H。 尽管质子和其他原子核（X 原子核）的交叉线圈布置理论上是磁通解耦的，但实际上我们发现由于端子接线和其他因素，耦合高达 -20 dB。为了进一步最小化这种耦合，我们为 X 核构建了带通滤波器，其插入损耗通常为 0.1 dB，1-H 抑制高达 -70 dB。对于 1-H 线圈，我们使用具有相似散射参数特征的 x 缺口。这些滤波器内置于线圈中，但是，为了节省额外的工作，我们将它们模块化以放置在传输线中，并且这些滤波器可用于所有其他线圈。线圈中的环形绕组由单股或多股电线制成，这些电线被放置在内部开发的 3D 打印线圈架的树林中。线圈架的内孔安装在密封的动物容纳容器周围。制造了各种长度的鞍形线圈，以通过调整填充系数来实现更高的灵敏度。对于肾脏和肝脏研究，使用较短的线圈。 鼠标头线圈也使用交叉鞍形环制成，并直接安装在容纳容器上。动物的约束、温度控制和麻醉供应都内置于该线圈装置中。我们还构建了一个小鼠身体线圈，其具有用于正交 x 核的正交环和用于 1-H 的线性环。 1-H 环旋转 45o。如上所述，通过应用滤波器克服了预期的强耦合。 对于 NMR 中心的 7T，动脉自旋标记线圈和设置进行了重新设计，以适应新的西门子 TERRA 系统。最后，用于 3T 的双调谐 13C 1H 头部线圈已经完成，并集成到机械组件中，用于在飞利浦 3T MRI 上对人体头部进行成像。 线圈及其射频接口经过了广泛的测试，为正在准备的报告开发数据，以获得 IRB 的批准。