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Novel materials to improve magnetic resonance imaging

Periodic Reporting for period 3 - NOMA-MRI (Novel materials to improve magnetic resonance imaging)

Reporting period: 2018-11-01 to 2020-04-30

There are two overall aims of this proposal: the first is to improve MRI (both on clinical 3T and 1.5T systems as well as very high field scanners at 7T and above) using high permittivity materials, and the second is to develop an entirely new type of transmission/detection device for MRI based on reconfigurable plasmas. In the first aim mathematical tools will be developed to determine the best geometry and material composition for different clinical applications, to use these materials in challenging clinical cases such as prostate and fetal imaging, and to extend their use to improve image quality on low-field systems that are typically found in developing countries. The aim is to improve diagnostic image quality across the board at all field strengths. Since the materials developed can be implemented irrespective of vendor, a major aim is the distribution of knowledge world-wide. In the second aim, a totally new concept in MR transmission and signal acquisition will be explored using a reconfigurable plasma. The possibility to make an RF coil essentially invisible, and to be able to configure an array of plasma elements in any desired geometry opens up an entirely new area to be explored in MRI.
"The first major aim of the project was to design new and efficient algorithms for being able to calculate how dielectric materials could be tailored in three dimensions to produce specific signal intensities within the image. In a paper entitled ""High-Permittivity Pad Design for Dielectric Shimming in Magnetic Resonance Imaging using Projection Based Model Reduction and a 3D Nonlinear Inversion Scheme"" which has been submitted to IEEE Transactions on Medical Imaging, we presented a fast and efficient method for designing dielectric pads. The domain decomposition method of our previous work was taken as a starting point, and we showed that the order of this model can be reduced to a great extent by employing a projection based model order reduction technique, and can subsequently be used to construct an automated optimization method. For the specific problem of imaging the cerebellum at 7T which was considered in this paper, the previously presented domain decomposition based model with an order of approximately 27,000 was successfully reduced to a system of order 500, thereby reducing the model order by a factor of 54. Having these reduced-order models available, B+ field responses can now be evaluated in only a fraction of a second for each pad realization at the cost of a very small reduction in accuracy. We showed how this approach could very rapidly produce convergence on a pad designed to image the cerebellum on a commercial 7 T scanner.

The second major aim was to investigate new materials with very high permittivities, and to assess their use in improving clinical MRI. In this area major progress has been made using blocks of lead zirconate titanate with a relative permittivity of 1070 to improve the quality of spinal cord imaging on a commercial 3 Tesla scanner. This work was recently published in Magnetic Resonance in Medicine, the most commonly read technical MRI journal. Electromagnetic simulations and the in vivo data acquired in this study confirm that very high permittivity ceramic blocks, made of lead zirconate titanate, improve the transmit efficiency in the spine. No adverse effects on the receive sensitivity of the array were found in this study. The locally enhanced receive sensitivity in combination with the focused transmit profile results in highersignal-to-noise in the spine region, leading to an increased image quality. Importantly, with regard to the overall aim of the project of enabling quality enhancement on systems which are not state-of-the-art, this study showed the feasibility of “upgrading” single channel transmit hardware at 3T with high permittivity PZT blocks to support spine applications. Image quality is significantly improved and power deposition is reduced with respect to a basic quadrature setup.

An interesting new project and collaboration was formed in mid-2016 with the ceramics group at ITMO University in Russia. They are able to form ceramics with very high permittivities and relatively low losses. In collaboration we were able to produce radiofrequency coils for very high frequency high resolution microimaging at 17 Tesla. This work was presented at the PIERS Conference in Paris in late 2016.

The final aim of the project is to investigate new types of detectors for MRI which use plasmas rather than the conventional copper conductors. Progress in this work has been achieved in collaboration with new partners at the University of Wurzburg in Germany. A paper has been submitted on this work, in which we have shown the first demonstration of plasma-based antennas for MR imaging on a commercial 3 Tesla system. The measured B1+ field of the single monopole antenna show similar behavior as a conventional metal antenna.

New materials developed during this project so far have been provided to many sites around the world. In 2016 and 2017 these include the University of Cardiff Imaging Center, King's College London, and the new 7 Tesla facility in Glasgow."
"One of the main aims of the project is to determine how new materials can be used to improve the image quality at relatively low field, i.e. on clinical 1.5 Tesla scanners. Our results have shown that this is only possible using materials with extremely high permittivities, i.e. 3500 and above. Although we are pursing this approach, we have also started to design new materials based on the principles of metasurfaces and metamaterias. Initially we developed these for 7 Tesla scanning of the brain. In a paper entitled ""Flexible and compact hybrid metasurfaces for enhanced ultra high field in vivo magnetic resonance imaging"" which was published in Nature Scientific Reports in 2017, we desinged a new hybrid metasurface structure, comprising a two-dimensional metamaterial surface and a very high permittivity dielectric substrate, that has been designed to enhance the local performance of an ultra-high field MRI scanner. This new flexible and compact resonant structure is the first metasurface which can be integrated with multi-element close-fitting receive coil arrays that are used for all clinical MRI scans. We demonstrated the utility of the metasurface acquiring in-vivo human brain images and proton MR spectra with enhanced local sensitivity on a commercial 7 Tesla system.

The results of this study showed that the metasurface acted as a ""super high permittivity"" material, and so we have initialized experiments using this approach at 1.5 Tesla. Since a major goal of the project is to make sure that image enhancements can be achieved on much older systems that are not state-of-the-art and cannot be upgraded due to financial reasons, we have partnered with the Medical School in Alexandroupolis in Greece to prototype experiments on their 1.5 Tesla system. These experiments have been performed in collaboration with new partners in St.Petersburg with whom we have collaborated under the auspices of the ERC grant. Initial results have shown that two dimensional metasurfaces can also be extremely useful in terms of enhancing image quality on older 1.5 Tesla systems. This work has been published in a series of conference abstracts and a paper has been submitted."
Images acquired using a plasma coil at 3 Tesla
Illustration of the improvement in image quality in the cerebellum using new dielectric pads
Increase in transmit efficiency at 3T using new PZT blocks
Signal enhancement produced by a new metasurface at 1.5 Tesla