New impulses in magnetic resonance imaging with metamaterials
Magnetic resonance imaging is by far the most high-performance imaging technology, but it is also the technically most challenging one. New concepts based on the use of metamaterials make it possible to open up new fields of application and overcome the limitations of established procedures.
In medical imaging, magnetic resonance imaging (MRI) has become an indispensable instrument in clinical diagnostics and in certain areas also in therapeutic support. On the one hand, this is due to the seemingly inexhaustible variety of the contrast mechanisms that reach from the presentation of morphological structures (even below the actual resolution limit!) to physiological and physical processes (blood circulation measurements, diffusion processes, elasticity, etc.). On the other hand, an important argument is found in the completely non-invasive functional principle and the harmlessness for the human body in contrast to many other medical imaging methods (such as computer tomography, positron emission tomography, or scintigraphy with their ionizing radiation). However, this is at the expense of the substantial technical effort necessary for high-quality MRI images. MR scanners are by far the most technically sophisticated devices in medical diagnostics. The strong magnetostatic field used to »generate« the macroscopic nuclear magnetization as well as the time-variable magnetic field gradients and the radio frequency waves to »excite« the magnetization interact with the body of the patient and potential other devices located in or close to the MR tomograph. Costly technical solutions are necessary to prevent physical impairments of the patient while providing high quality images. The possibilities of the current art of engineering seem to be exhausted to some extent here, since in some areas there have been no significant improvements for many years now. In this context, it is worth mentioning RF chokes as well as the basic principle of spatial coding in MRI.
New concepts to solve typical problems in MRI can open up new fields of application and also overcome the limitations of existing approaches. In recent years, a new »class« of materials with special properties has attracted attention as these materials can be used to rethink many problems. In medical technology, this class of materials has until now only been studied in individual research groups, while it has not been possible to reach commercial maturity yet. We are talking about so-called metamaterials (MTM).
The spectacular peculiarity of metamaterials is that they make it possible to obtain artificial materials with effective properties that can be tailored to the individual application. Even material properties can be produced that usually do not exist in nature. For RF metamaterials, these macroscopic electromagnetic properties are created by microscopic, usually periodic circuit structures that can be realized with cost-effective standard printed circuit board manufacturing techniques, for example. Some of the electromagnetic effects that can be achieved this way are negative refractive indices that allow for a perfect focusing far beyond the diffraction limit or for frequency bands in which wave propagation is suppressed.
In an internal Fraunhofer research project, the Fraunhofer Institute for Digital Medicine MEVIS and the Fraunhofer Institute for High Frequency Physics and Radar Techniques FHR are cooperating to explore the application possibilities of MTM technologies to improve MR imaging and develop product-related solutions based on this. The goal is to allow for the use of any type of electrical lines within MRI without interferences, while also improving the signal-to-noise ratio in the relevant volume ranges. Moreover, special metamaterial lenses are being developed that might be able to facilitate a fundamentally new MRI concept. Finally, based on the developed solution concepts, the goal is to create a flexible design platform for further MTM applications in the MRI area in the middle and long term.