The circuit described in this study is intended for Magnetic Resonance Imaging (MRI) application. The function of this circuit is to transmit RF energy to the sample and then receive the RF energy. The circuit that does this is called a birdcage coil. This coil is capable of producing a very homogenous B1 field over a large volume; it is this aspect of birdcage coils that make them very favorable for animal/human studies as it is necessary that all nuclei in the volume of the coil are excited by uniform RF energy. At high-field (4.7T) when the power is fed to the coil at a single port the coil unable to produce a homogenous B1 field. However when power is fed at multiple ports the performance of the coil improves. In this paper a study is carried out comparing the performance of the coil when power is fed at a single port and two ports. The advantage of feeding at two ports is that there is sqrt(2) improvement in SNR and the RF power efficiency is doubled. In this work strategies are presented for matching, tuning and isolating the two ports. Also, an attempt is made to fabricate a mechanically rigid coil and interfacing the coil with some additional features that will make the coil easy to use. The homogeneity and SNR of a birdcage coil in linear and quadrature mode loaded with saline, oil and CuSO4 phantom was measured on the bench and the scanner. The coil performance was compared to two other birdcage coils in the lab. It was found that the unshielded trombone coil that was 3 times smaller in volume than the coil presented has 140% higher SNR than the coil presented but the homogenous region of the coil presented is 48% higher than the smaller coil. Lastly on the bench; the SNR of the quadrature coil was 30% higher than the coil in the linear mode.

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Le présent mémoire porte sur la conception et le développement de deux antennes RF utilisées en imagerie par résonance magnétique. Ces antennes ont pour but de guider le futur développement d’une plateforme d’imagerie multi-animal qui servira les chercheurs du nouveau CRCHUM. Plus spécifiquement, ces antennes ont été conçues pour l’imagerie du proton à 1.5T. La première utilise une birdcage de type lowpass pour la partie émettrice et utilise 8 éléments de surface pour la partie réceptrice. La seconde antenne est une birdcage de type lowpass polarisée circulairement qui est utilisée à la fois pour l’émission et pour la réception. Cette dernière a présenté de bonnes performances, générant des images avec un SNR élevé et avec une bonne homogénéité, la rendant une bonne candidate pour la future plateforme. La première a présenté quelques problèmes au niveau de la désyntonisation de la birdcage et du couplage entre les éléments. Dans le cas où ces problèmes venaient à être surmontés, cette antenne aurait l’avantage de pouvoir utiliser des techniques d’imagerie parallèle et possiblement d’avoir un SNR plus élevé. / This master thesis focuses on the design and development of two RF coils used in magnetic resonance imaging. These coils are designed to guide the future development of a multi-animal imaging platform that will serve researchers of the new CRCHUM. More specifically, these coils were designed for proton imaging at 1.5T. The first uses a lowpass birdcage as transmitter and uses 8 surface elements for the receiving part. The second coil is a circularly polarized lowpass birdcage which is used both for transmission and for reception. The latter presented good performances, generating images with high SNR and good homogeneity, making it a good candidate for the future platform. The first one presented a few problems at the detuning of the birdcage and the coupling between the elements. In the case where these problems would be overcome, this coil would have the advantage of being able to use parallel imaging techniques and possibly to have a higher SNR.

Le présent mémoire porte sur la conception et le développement de deux antennes RF utilisées en imagerie par résonance magnétique. Ces antennes ont pour but de guider le futur développement d’une plateforme d’imagerie multi-animal qui servira les chercheurs du nouveau CRCHUM. Plus spécifiquement, ces antennes ont été conçues pour l’imagerie du proton à 1.5T. La première utilise une birdcage de type lowpass pour la partie émettrice et utilise 8 éléments de surface pour la partie réceptrice. La seconde antenne est une birdcage de type lowpass polarisée circulairement qui est utilisée à la fois pour l’émission et pour la réception. Cette dernière a présenté de bonnes performances, générant des images avec un SNR élevé et avec une bonne homogénéité, la rendant une bonne candidate pour la future plateforme. La première a présenté quelques problèmes au niveau de la désyntonisation de la birdcage et du couplage entre les éléments. Dans le cas où ces problèmes venaient à être surmontés, cette antenne aurait l’avantage de pouvoir utiliser des techniques d’imagerie parallèle et possiblement d’avoir un SNR plus élevé. / This master thesis focuses on the design and development of two RF coils used in magnetic resonance imaging. These coils are designed to guide the future development of a multi-animal imaging platform that will serve researchers of the new CRCHUM. More specifically, these coils were designed for proton imaging at 1.5T. The first uses a lowpass birdcage as transmitter and uses 8 surface elements for the receiving part. The second coil is a circularly polarized lowpass birdcage which is used both for transmission and for reception. The latter presented good performances, generating images with high SNR and good homogeneity, making it a good candidate for the future platform. The first one presented a few problems at the detuning of the birdcage and the coupling between the elements. In the case where these problems would be overcome, this coil would have the advantage of being able to use parallel imaging techniques and possibly to have a higher SNR.

The work deals with the design and experimental production of the model type Birdcage coil used for magnetic resonance imaging system (MRI). The work deals with the design and behaviour of the model type Birdcage coil volume designed for imaging system for magnetic resonance imaging (MRI). Volume coils are generally very useful for use in MRI, because in transversal design allow easy access into the cavity when the flow vector of magnetization. The aim of the work was to construct a numerical model of transversal coil type Birdcage of the predefined dimensions. The created the coil tuned the desired resonant frequency having regard to maximize performance in the measured sample migrated. The modeling coil experimentally produced and measurements also confirm the accuracy of the numerical model.

This thesis describes development of a miniature reconfigurable antenna and optimization of a dual tuned birdcage coil. The design goals for the miniature reconfigurable antennas are resonance center frequency of 2.44 GHz, bandwidth of 2.4 GHz - 2.48 GHz, size of 0.8 cm x 1.2 cm, radiation efficiency of 70%, pattern correlation coefficient of 0.3 and input impedance of 50 Ω. The main goals to be achieved from the birdcage coil are the better homogeneity and higher signal to noise ratio than the existing coil. The design and optimization of both antenna and birdcage coil were done using simulation software and MATLAB. Wireless communications have progressed rapidly in last decade and communication devices are becoming smaller and smaller. With miniaturization of devices, dimensions of antennas need to be reduced accordingly. In recent years engineers have not only focused on miniaturization but also on the reconfigurability of the antenna. The functionality and performance of an antenna can be greatly improved by a reconfigurable antenna. However, designing such an antenna can be a tricky task. This thesis addresses issues that are faced during design of such miniature reconfigurable antenna. It also describes design and optimization of such an antenna. The modeled and measured results for the miniature reconfigurable antennas were very close except the built antenna requires frequency tuning and better switching technique. Magnetic resonance imaging (MRI) is an imaging modality that provides high quality images. Radio frequency (RF) coils play an important role in MRI. RF coils act like an antenna that transmits RF energy and receives energy as well. The most commonly-used RF coil for volume imaging is the birdcage coil. This thesis describes an optimization of a birdcage coil that is dual tuned for sodium and hydrogen frequencies. The modeled coil has better performance compared to the existing coil.

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O objetivo deste projeto é o desenvolvimento de transdutores de RF tipo gaiola (birdcage coil) e sela (saddle coil), com desenho especial inovador, que mantém elementos com comprimento elétrico constante, para um campo magnético de 2 Tesla. Este transdutor deve permitir o estudo e também o desenvolvimento de novas metodologias em imagens e espectroscopia por RMN de pequenos primatas como marmosets, exigidos pela interação deste grupo com o programa CInAPCe (Cooperação Interinstitucional de Apoio a Pesquisas sobre o Cérebro). Essas bobinas também se destinam ao uso em metodologias de imagens e espectroscopia ?in vivo? por Ressonância Magnética Nuclear (RMN), que utilizem pulsos de RF caracterizados como Pulsos Adiabáticos. Outra perspectiva é a continuação do desenvolvimento de bobinas de RF que operam segundo as características de bobinas de superfície, que pode resultar deste trabalho. Os resultados nos mostram que para as dimensões e freqüência utilizadas, a interação entre os condutores que compõe os transdutores é mais relevante do que a perda de fase da corrente elétrica devido às diferenças no comprimento elétrico. O cruzamento entre os condutores consiste num bom desenho alternativo, melhorando a homogeneidade de campo de RF e a relação sinal/ruído. / The purpose of the present work is the development of birdcage- and saddlelike RF transducers which were based on a special and innovative approach. The aim of this design is to keep constant the electromagnetic length of its elements, for a magnetic field intensity of 2 Tesla. These resonators will allow both the study and the development of new imaging and spectroscopy methods addressed to the NMR research on small primates such as marmosets. Such studies are part of the cooperation between this group and other participants of the CInAPCe program, Portuguese acronym for Inter-institutional Cooperation to Support Brain Research. Other perspective is the continuing development of RF coils whose characteristics are similar to surface coils, also designed for anatomic specific studies of imaging and in vivo NMR spectroscopy that take advantage from the use of RF pulses characterized as Adiabatic Pulses. From the results it could be verified that, for the frequency and dimensions used, the interaction between the conductive paths present in the transducers is more significant than the electric current phase losses due to differences in electric path. The crossing of the conductive paths constitute a good alternative design, improving the RF field homogeneity and the signal to noise ratio.