Tania VERGARA will defend her PhD Thesis “Metamaterials for image quality enhancement in Ultra-High field Magnetic Resonance Imaging”, on Wednesday, November 17 at 2:00 pm in the Ponte amphitheater, located in front of the Institut Fresnel building on Campus Saint Jêróme, Marseille.
Zoom Link : https://univ-amu-fr.zoom.us/j/99297463191?pwd=ZGNVYnNLZ3hsbUY0RnNuTzZPVFY3UT09
Abstract : Magnetic Resonance Imaging (MRI) is one of the most powerful, non-invasive imaging techniques, allowing the detection of a large range of diseases in the entire human body. The advances in hardware developments have led to higher magnetic fields, boosting the Signal-to-Noise Ratio (SNR) and in some cases tissue contrast, resulting in higher quality images. However, stronger magnetic fields and high electromagnetic permittivity of the human body translate into shorter wavelengths of the Radiofrequency (RF) fields used to form an image. Thus, the human body in the Magnetic Resonance (MR) system becomes non negligible with respect to the incoming RF waves, resulting in a shading effect due to a spatial variation of the RF field magnitude and phase across the Field of View (FOV ). It is vital to address this problem since important structures of the body may be masked on the image. Such effect is already visible when imaging large volumes at 3 Teslas (T) like the abdomen or pelvis. In the most recently available Ultra-High Field (UHF) MRI 7T systems this effect occurs at smaller volumes such as the head, turning the RF inhomogeneity into one of the major factors preventing the full potential of these systems. In the underlying thesis, metamaterials are proposed as a viable approach to mitigate these effects.
Metamaterials are synthetic materials with the ability to modify and manipulate the electromagnetic (EM) fields. They have been extensively studied and exploited
in domains such as optics, acoustics and telecommunications, however in the MRI domain their interest started growing recently. The properties of metamaterials make them remarkable candidates to efficiently redistribute the magnetic fields generated by the existing RF coils, enhancing the signal in the regions with lower signal.
The goal of this thesis was to demonstrate that metamaterials are versatile and can contribute to the improvement of the RF-wavelength-related inhomogeneities in
diverse situations for different field strengths. Three metamaterial applications were explored theoretically with electromagnetic field simulations and experimentally in vitro and in vivo. We explored metamaterials based on coupled wires working under the principle of hybridization modes as well as fractal-inspired metamaterials.
Metamaterials are synthetic materials with the ability to modify and manipulate the electromagnetic (EM) fields. They have been extensively studied and exploited
in domains such as optics, acoustics and telecommunications, however in the MRI domain their interest started growing recently. The properties of metamaterials make them remarkable candidates to efficiently redistribute the magnetic fields generated by the existing RF coils, enhancing the signal in the regions with lower signal.
The goal of this thesis was to demonstrate that metamaterials are versatile and can contribute to the improvement of the RF-wavelength-related inhomogeneities in
diverse situations for different field strengths. Three metamaterial applications were explored theoretically with electromagnetic field simulations and experimentally in vitro and in vivo. We explored metamaterials based on coupled wires working under the principle of hybridization modes as well as fractal-inspired metamaterials.
In a first study for preclinical small-animal MRI, it was demonstrated that a coupled-wire metamaterial expands the imaging area of a mouse multinuclear (multi-frequency) surface coil at 7T to perform proton whole-body imaging with higher SNR than a bird-cage coil and flour images and spectroscopy. Then, this concept was tested on a proton surface coil at 17.2T for rat brain imaging which made it possible to observe the complete head and a larger part of the spinal cord of the rat. These preclinical studies were used as a first step to understand the nature and behavior of metamaterial pads to eventually apply them in a clinical setting. In a second study, we targeted the inhomogeneities at 7T human head. By using fractal-inspired metamaterial pads coupled to a birdcage coil we were able to enhance the two temporal lobes simultaneously. In a third study we used coupled-wire metamaterial pads to mitigate the wavelength reduction effect in the pelvis at 3T.
We have demonstrated the adaptability of metamaterials in different MRI applications, taking them from preclinical to clinical studies at different magnetic fields.
The ideas and prototypes developed in this thesis open the possibility towards further studies an applications in which MRI may greatly benefit from metamaterials. In an environment where MRI field strengths are continuously growing, control of the RF field uniformity has reached primary importance. Metamaterials appear as inexpensive and highly efficient tools for improving the distribution of RF fields, contributing to fully exploit the capabilities of UHF MRI systems.
We have demonstrated the adaptability of metamaterials in different MRI applications, taking them from preclinical to clinical studies at different magnetic fields.
The ideas and prototypes developed in this thesis open the possibility towards further studies an applications in which MRI may greatly benefit from metamaterials. In an environment where MRI field strengths are continuously growing, control of the RF field uniformity has reached primary importance. Metamaterials appear as inexpensive and highly efficient tools for improving the distribution of RF fields, contributing to fully exploit the capabilities of UHF MRI systems.
The jury will be composed by :
- Andrew WEBB, Professor at C.J.Gorter High Field Magnetic Resonance Center, LUMC, Leiden, Netherlands, Reviewer
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Simon LAMBERT, Professor at Laboratoire Ampère, Université Claude Bernard Lyon 1, Lyon, France, Reviewer
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Sophie BRASSELET, Director of Institut Fresnel, Aix-Marseille Univ, Marseille, France, Examiner
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Marie POIRIER-QUINOT, Professor at Laboratoire BioMaps, Université Paris-Saclay, Gif-sur-Yvette, France, Examiner
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Stanislav GLYBOVSKI, Senior Researcher at ITMO University, St Petersburg, Russia, Examiner
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Frank KOBER, Director of recherche at Centre de Résonance Magnétique Biologique et Médicale (CRMBM), Aix-Marseille University, Marseille, France, Supervisor
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Marc DUBOIS, CEO at Multiwave Imaging, Marseille, France, Co-Supervisor
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Redha ABDEDDAÏM, Professor at Institut Fresnel, Aix-Marseille Univ, Marseille, France, Co-Supervisor
Keywords: Magnetic Resonance Imaging, Ultra-high field, Radiofrequency coils, Metamaterials, Fractals, B+1 inhomogeneities, Passive radiofrequency shimming, Signal-to-Noise Ratio
