MRI Engineering Core

LFMI(EC)的MRI工程核心支持NIH最高场强MRI系统的新硬件开发，并支持其他系统的专业项目。目前，它的主要目标是开发11.7T人类MRI和17.6T动物MRI，以便进行具有优越对比度和分辨率的神经成像。 11.7T人体系统的通电等待了两年多的时间才能获得氦。这种情况在2023年得到解决，并于6月份开始冷却系统。该系统预计将于2023年10月投入使用，届时电子设备的最终安装将开始。预计该系统将在2024年初准备好进行核磁共振扫描。 由工程核心负责的11.7T人体系统的关键部件之一是射频系统，包括射频发射器和探测器。在先前开发的音量发送器上实现失谐电路以允许其用于发送接收模式方面取得了进展。这允许与提供比体积检测器更高的灵敏度的接收检测器阵列联合操作。一个32通道的接收器探测器正在建设中，它的六个通道已经建造和测试。全面的32声道能力预计将在2024年初，届时它可以与11.7T扫描仪内的可调谐音量发射器一起测试和使用。 为了允许人体扫描，必须确保射频传输不会导致组织加热超过1摄氏度。由于11.7T的磁共振成像会引起高度不均匀的射频场和相关的组织加热，因此需要进行模拟和测量来估计安全操作极限。在2023年期间，工程核心在这些方面取得了进展，并开发了一种具有类似于人头的电磁特性的测试体模。此外，模体的化学成分提供了磁共振参考信号，使人们能够根据温度相关的共振频率变化准确地估计温度。初始实验将在3T下使用11.7T类射频辐射(即500 MHz频率)进行。 几年来，我们实验室一直在开发用于多通道传输的线圈上射频放大技术(也称为PTX)。这项技术对11.7T的人类磁共振成像非常重要，因为它可以改进对传输场及其相关组织加热的控制。7T的原型系统于2022年进行了测试。2023年，工程核心研究了11.7T PTX的关键部件，并表征了磁场对射频放大器中功率晶体管工作特性的影响。这为我们设计了一种高效的原型提供了信息，我们计划在2023年底生产。 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导致功率损失，这种损失随着磁场(=射频频率)的增加而加剧。此外，磁场的增大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。这项研究是与马里兰大学合作完成的，马里兰大学在晶体管设计和制造方面有经验。 工程核心还继续支持国家卫生研究院使用核磁共振的各个小组。它为鼠标成像设备开发了各种鼠标线圈和射频过滤器。目前，所有鼠标身体线圈都是调谐/匹配的，并垂直排列鞍形对，用于7T、9.4T和3T Bruker系统的发送/接收(收发器)模式。共振核包括1-H、13-C、2-H。 在测试先前开发的用于3T的组合式13C-1H磁头线圈的安全性方面做了大量的工作。这需要进行大量的射频场模拟，并仔细检查3T扫描仪的安全功能和射频校准程序。在射频测试和安全委员会评估之间进行了几次迭代，2023年夏天，磁头线圈获得了人类使用的批准。

The MRI Engineering core of LFMI (EC) supports new hardware developments for the highest field MRI systems at NIH, and 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. Energization of the 11.7T human system was waiting over two years for Helium to become available. This situation got resolved in 2023, and cool down of the system started in June. The system is expected to be at field in October 2023, when final installation of the electronics will commence. It is anticipated that the system will be ready for MRI scanning in early 2024. One of the key parts of the 11.7T human system that is under the responsibility of the Engineering Core is the RF system, including RF transmitters and detectors. Progress has been made on implementing detuning circuitry on the previously developed volume transmitter to allow it to be used in transmit receive mode. This allows joint operation with receive detector arrays that provide improved sensitivity over volume detectors. A 32-channel receiver detector is under construction, and six of its channels have been constructed and tested. Full, 32-channel capability is expected in early 2024, when it can be tested and used with the detuneable volume transmitter inside the 11.7T scanner. To allow human scanning, it must be ensured that RF transmission does not cause tissue heating more than 1 degree Celsius. As MRI at 11.7T cause highly inhomogeneous RF fields and associated tissue heating, simulations and measurements need to be performed to estimate safe operating limits. During 2023, the Engineering Core has made progress on these aspects and has developed a test phantom with electromagnetic properties similar to the human head. In addition, the phantoms chemical composition provide MRI reference signals that enable accurate estimation of temperature based on temperature-dependent resonance frequency shifts. Initial experiments will be performed at 3T using 11.7T-like RF irradadiation (i.e. 500 MHz frequency). For several years, our lab has been developing on-coil RF amplification technology for multi-channel transmission (also called pTx). This technology is important for human MRI at 11.7 T as it allows improved control over the transmission field and its associated tissue heating. A prototype system for 7T was tested in 2022. In 2023, the Engineering Core has explored the critical components for pTx at 11.7T, and characterized the effect of the magnetic field on the operational characteristic of the power transistor in the RF amplifier. This has informed on the design of an efficient prototype, which we plan to manufacture in late 2023. 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. Substantial effort was made in testing the safety of the previously developed combined 13C-1H head coil for 3T. This required extensive RF field simulations and close examination of the safety features and RF calibration procedures on the 3T scanner. Several iterations occurred between RF testing and safety committee evaluation, and in summer 2023 the head coil received approval for human use.