Metamaterial devoted to magnetic resonance imaging : numerical analysis and experimental validation
(2009)
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- Authors
- Supervisors
- Craeye, Christophe
- Abstract
- (en) Recent years have seen a growing interest in a new type of structures, often periodic, called "metamaterials". These new artificial materials exhibit many new appealing properties not found in nature, and open many new possibili- ties in the design of microwave and optical components. The most spectacular property of metamaterials is their ability to manipulate the near-field of an ob- ject and to image it with sub-wavelength resolution. Besides this, among medical imaging techniques, Magnetic Resonance Imaging (MRI) has become increasingly competitive, its main advantage be- ing its non-ionizing property. This radio-frequency technique appears to be a very interesting field of investigation for metamaterials due to its narrow bandwidth. This imaging technique requires constant improvements of the imaging quality in order to better understand the human body mechanisms and also provide better and faster diagnosis. The imaging process in MRI does not involve any optical means such as focusing or collimation. The objective of the present thesis is to demonstrate numerically and ex- perimentally that the ability of a metamaterial to manipulate the near-field can be used to realize new devices devoted to deep internal imaging or to concen- trate MR signal in order to improve the signal-to-noise ratio of MR images. Both the numerical simulation and the experimental aspects are approached in this work. In a first instance, the numerical tools developed to study metamateri- als are presented. The basics of the Method of Moments is recalled. The MoM enables to study finite or infinite periodic structures made of metal and/or dielectrics. Since it requires to mesh only the surface of the struc- tures, this method is particularly suited for the simulation in open-space of periodic structures like metamaterials. Several numerical techniques that can be combined with the MoM are presented, namely: the Array Scanning Method which is used to compute the response of a periodic structure to a non-periodic excitation and the Macro Basis Function approach which enables to accelerate the analysis of large periodic structures of finite extent. In a second time, these numerical methods are applied to the analysis and design of two types of metamaterials that can be used in 3Tesla MRI. The most promising structure is the wire medium which has the ability to trans- fer or concentrate the magnetic field onto significant distance with low losses. Straight wire media are investigated numerically from the point of view of their coefficient of transmission and eigenmodes. Special attention is given to the recuperation of the imaged fields, through the insertion of the receiv- ing coil inside the metamaterial. Convergent geometries are numerically ana- lyzed in order to determine their capacity to improve the RF-homogeneity of MR images by concentrating the magnetic field. Finally, curved wire media are investigated, the obtained results indicate that flexible endoscopic devices based on wire medium metamaterials may be envisaged. Finally, we present the medical images obtained in clinical condition with the different types of wire media. These results confirm the ability of wire media to transfer MR images with low losses and open the possibility to build new MRI devices devoted to internal imaging.
- Affiliations
Citations
Radu, X. (2009). Metamaterial devoted to magnetic resonance imaging : numerical analysis and experimental validation. https://hdl.handle.net/2078.5/128517