MRI Engineering Core

National Institute Of Neurological Disorders And Stroke

Investigators

Linked publications, trials & patents

Abstract

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.