MRI Engineering Core

National Institute Of Neurological Disorders And Stroke

Investigators

Linked publications, trials & patents

Abstract

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.