Radu, Xavier
[UCL]
(eng)
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.


Bibliographic reference |
Radu, Xavier. Metamaterial devoted to magnetic resonance imaging : numerical analysis and experimental validation. Prom. : Craeye, Christophe |
Permanent URL |
http://hdl.handle.net/2078.1/28537 |