Experimentos de Imagens por Ressonância Magnética (IRM) em pequenos animais, assim como em humanos, exigem um conjunto especifico de bobinas de Radiofrequência (RF) para maximizar ambos a homogeneidade de campo durante a transmissão e a Relação Sinal Ruído (RSR) durante a recepção. As geometrias mais comuns de bobinas transmissoras utilizadas em sistemas de humanos são as bobinas tipo gaiola ou Birdcage Coil. Dentre as geometrias de bobinas receptoras, o conceito de bobina tipo Phased Array é amplamente utilizado em aplicações que necessitam de alta RSR em uma grande região de interesse, além de permitirem obter imagens com metodologias de aquisição paralela. Este trabalho descreve o desenvolvimento de um conjunto de bobinas transmissoras e receptoras especificamente projetadas para a aquisição de imagens do cérebro de ratos para o estudo do hipocampo. As geometrias de bobinas transmissoras estudadas foram dois Birdcages com 8 e 16 condutores e a geometria proposta por nós chamada Double Crossed Saddle (DCS Coil). Para a recepção desenvolvemos uma bobina de superfície com dois loops e um Phased Array com dois canais de recepção. Os resultados confirmam que dentre as bobinas transmissoras desenvolvidas a geometria do tipo Birdcage com 16 condutores é a mais homogênea, produzindo campos de RF com alta uniformidade em regiões de interesse de até 80% do diâmetro interno das bobinas. No entanto, o elevado número de capacitores em sua estrutura faz com que a geometria DCS coil, devido à sua simplicidade e reduzido número de capacitores, represente uma alternativa em experimentos onde as condições de carga da amostra possam variar. Dentre as geometrias de receptoras estudadas a bobina de superfície obteve maior desempenho em termos de RSR em comparação com o Phased Array de 2 canais. A comparação dos resultados utilizando bobinas específicas para a transmissão e recepção com uma bobina volumétrica operando como transmissora e receptora simultaneamente comprova a superioridade em termo de RSR dos sistemas que utilizam bobinas dedicadas, sendo confirmados através de imagens in vivo do cérebro de ratos, possibilitando aquisições com mesma resolução e RSR em um tempo reduzido de experimento. / Magnetic Resonance Imaging (MRI) experiments on small animals, as well as in human, require a specific RF coil set in order to maximize the Radiofrequency (RF) field homogeneity during transmission and Signal-to-Noise Ratio (SNR) during reception. The most common geometries of RF transmitter coil used in human systems are the well known Birdcage resonators. Among the receiver coils geometry the concept of NMR Phased Arrays or multi channel coils is widely employed in applications that need a high SNR in a large region of interest (ROI), further allowing parallel imaging acquisition methodologies. The work reported here describes the development of a transmit-only and receive-only RF coil set actively detuned specifically designed to MRI acquisition of rats brain for purposes of neuroscience studies. The transmitter geometries developed were two Birdcages with 8 and 16 rungs and our proposed geometry named Double Crossed Saddle (DCS). For reception we developed one common surface coil made of two turn loops and a 2-channel Phased Array, both actively detuned during reception. The results have confirmed that the 16 rungs Birdcage are superior among other transmit coils in producing homogeneous RF field inside a ROI of 80% of coil´s inner diameter. However, the simplicity and reduced number of capacitors makes the DCS coil a good choice in experiments with different samples and load conditions. Among the receive coils developed, the surface coil showed a better SNR in comparison with the 2-channel Phased array, which has the advantage of producing a large area with high SNR. The SNR of both surface coil and 2-channel array was compared with a transceiver Saddle Crossed coil, available at our lab, specific designed to obtain rat brain images. These results have corroborated that transmit-only and receive-only RF coils have best performance than transceiver volume coils for obtain MRI images of rats brain, allowing image acquisition with same resolution and reduced scan time.

Experimentos de Imagens por Ressonância Magnética (IRM) em pequenos animais, assim como em humanos, exigem um conjunto especifico de bobinas de Radiofrequência (RF) para maximizar ambos a homogeneidade de campo durante a transmissão e a Relação Sinal Ruído (RSR) durante a recepção. As geometrias mais comuns de bobinas transmissoras utilizadas em sistemas de humanos são as bobinas tipo gaiola ou Birdcage Coil. Dentre as geometrias de bobinas receptoras, o conceito de bobina tipo Phased Array é amplamente utilizado em aplicações que necessitam de alta RSR em uma grande região de interesse, além de permitirem obter imagens com metodologias de aquisição paralela. Este trabalho descreve o desenvolvimento de um conjunto de bobinas transmissoras e receptoras especificamente projetadas para a aquisição de imagens do cérebro de ratos para o estudo do hipocampo. As geometrias de bobinas transmissoras estudadas foram dois Birdcages com 8 e 16 condutores e a geometria proposta por nós chamada Double Crossed Saddle (DCS Coil). Para a recepção desenvolvemos uma bobina de superfície com dois loops e um Phased Array com dois canais de recepção. Os resultados confirmam que dentre as bobinas transmissoras desenvolvidas a geometria do tipo Birdcage com 16 condutores é a mais homogênea, produzindo campos de RF com alta uniformidade em regiões de interesse de até 80% do diâmetro interno das bobinas. No entanto, o elevado número de capacitores em sua estrutura faz com que a geometria DCS coil, devido à sua simplicidade e reduzido número de capacitores, represente uma alternativa em experimentos onde as condições de carga da amostra possam variar. Dentre as geometrias de receptoras estudadas a bobina de superfície obteve maior desempenho em termos de RSR em comparação com o Phased Array de 2 canais. A comparação dos resultados utilizando bobinas específicas para a transmissão e recepção com uma bobina volumétrica operando como transmissora e receptora simultaneamente comprova a superioridade em termo de RSR dos sistemas que utilizam bobinas dedicadas, sendo confirmados através de imagens in vivo do cérebro de ratos, possibilitando aquisições com mesma resolução e RSR em um tempo reduzido de experimento. / Magnetic Resonance Imaging (MRI) experiments on small animals, as well as in human, require a specific RF coil set in order to maximize the Radiofrequency (RF) field homogeneity during transmission and Signal-to-Noise Ratio (SNR) during reception. The most common geometries of RF transmitter coil used in human systems are the well known Birdcage resonators. Among the receiver coils geometry the concept of NMR Phased Arrays or multi channel coils is widely employed in applications that need a high SNR in a large region of interest (ROI), further allowing parallel imaging acquisition methodologies. The work reported here describes the development of a transmit-only and receive-only RF coil set actively detuned specifically designed to MRI acquisition of rats brain for purposes of neuroscience studies. The transmitter geometries developed were two Birdcages with 8 and 16 rungs and our proposed geometry named Double Crossed Saddle (DCS). For reception we developed one common surface coil made of two turn loops and a 2-channel Phased Array, both actively detuned during reception. The results have confirmed that the 16 rungs Birdcage are superior among other transmit coils in producing homogeneous RF field inside a ROI of 80% of coil´s inner diameter. However, the simplicity and reduced number of capacitors makes the DCS coil a good choice in experiments with different samples and load conditions. Among the receive coils developed, the surface coil showed a better SNR in comparison with the 2-channel Phased array, which has the advantage of producing a large area with high SNR. The SNR of both surface coil and 2-channel array was compared with a transceiver Saddle Crossed coil, available at our lab, specific designed to obtain rat brain images. These results have corroborated that transmit-only and receive-only RF coils have best performance than transceiver volume coils for obtain MRI images of rats brain, allowing image acquisition with same resolution and reduced scan time.

Atherosclerotic plaques in the bifurcation of the carotid artery vessels can pose a significant stroke risk from stenosis, thrombosis and emboli, or plaque rupture. However, the possibility of the latter depends on the structure of the plaque and its stability. So far, the assessment of such depositions, and the evaluation of the risk they pose, is not satisfactory with 3 Tesla black blood imaging. It is expected that the SNR increase at 7 Tesla, together with an appropriate and patient-safe RF coil, will result in higher resolution images that would help in better assessing the composition of atherosclerotic plaques in vessel walls. A custom-built neck array was designed and constructed, with the aim of investigating the benefits of the higher field strength using DANTE-prepared black blood imaging. A 4-channel transmit array was designed to generate the required <b>B</b>^&plus;<sub style='position: relative; left: -.5em;'>1</sub> field for the DANTE module to be used. A separate close fitting 4-channel receive array was preferred for improved SNR and parallel (receive) imaging. Geometric, active, passive as well as preamp decoupling schemes were employed for adequate isolation between the arrays and their channels. Electromagnetic simulation software, Semcad X (SPEAG, Zurich), was used for safety assessment with human phantoms (Virtual population). The <b>E</b> fields for 1 W transmission per channel were calculated for each element for a worst case SAR estimation. The transmission power limits per channel were set according to the 10g SAR limit set in IEC 60601. For simulation validation, temperature measurements and surface heat mapping were performed on a meat phantom. Finally, a healthy male subject was scanned using a protocol consisting of <b>B</b>₁ mapping, RF shimming at an ROI, and 2D and 3D DANTE prepared Gradient Echo (GRE). The worst-case heating scenario, as defined in the methods section, generated a maximum local SAR of 7.65 W/kg for 1 Watt per channel input. Thus, for 1st level mode (20W/kg max), the power limit was set at 2.6 W per channel. The heating profile was similar to that simulated and the measured temperature increase was within a &plusmn;10&percnt; margin relative to the simulation. The global SAR power limit per channel was found to be higher (i.e. more allowed power) than the worst case local SAR power limit, and thus did not impose additional power penalty. The resolution achieved was 0.6 mm isotropic for the 3D protocol and 0.6 by 0.6 by 2.5 mm for the 2D protocol. The average SNR was measured within the vessel wall location of the two carotid arteries and found to be 27&plusmn;6 for the DANTE images and for the static tissue closer to the skin the SNR was 55&plusmn;2. In conclusion, a 4Tx/4Rx coil was designed to target the carotid arteries operating under pTx mode and a black blood imaging sequence was implemented for blood signal suppression and vessel wall imaging. The initial results from the subject and phantom imaging show satisfactory blood suppression and spatial resolution.

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.

Le projet Iseult-Inumac mené au sein d’un consortium franco-allemand vise à développer les techniques associées à l’imagerie par résonance magnétique à ultra haut champ, notamment à travers la construction d’un imageur à 11,7 teslas. La résolution accrue du nouvel imageur devrait apporter une meilleure compréhension du système nerveux et de ses dysfonctionnements pour aboutir à un dépistage plus précoce des pathologies telles que la maladie d’Alzheimer, les accidents vasculaires cérébraux ou les tumeurs.La principale difficulté technique du projet, en dehors de la construction de l’aimant statique, réside dans la capacité d’émettre une onde électromagnétique d’amplitude uniforme dans tout le cerveau du patient : problème de l’inhomogénéité de B1 (ou radiofréquence). Une solution proposée consiste à utiliser une antenne réseau à transmission parallèle et des séquences d’excitation IRM spécifiques dites de compensation.Cette thèse se concentre sur la conception des antennes réseaux à transmission parallèle pour l’IRM à 7 et 11,7 T. Un prototype à 8 canaux indépendants utilisant une technologie innovante de résonateurs linéaires est présentée pour l’IRM 7 T. Il a été conçu par la simulation numérique. Un protocole de validation a été développé pour la caractérisation et l’analyse des performances des antennes réseaux. Le succès de cette validation a permis de développer par la simulation une nouvelle antenne à 12 canaux et une stratégie de pilotage innovante permettant de n’utiliser que 8 chaînes de pilotage au lieu de 12. Enfin, la même méthodologie a été appliquée pour concevoir et prédire les performances d’une antenne réseau à 11,7 T. / The Iseult-Inumac project led by a franco-german consortium aims at developing the ultra-high field MRI technologies, especially with the building of an 11.7 tesla scanner. The high resolution of this new scanner may improve the understanding of human nervous system and its dysfunctions in order to detect pathologies like the Alzheimer disease, strokes or tumors earlier.The biggest technical difficulty, besides the building of the static magnet itself, lies in the emission of a uniform electromagnetic wave in the entire brain of the patient. It is usually referred as to B1 (or RF) inhomogeneity issues. A solution consists in using a phased array RF coil and specific MRI sequences for shimming in a parallel transmit approach.This thesis deals with the study of parallel transmit phased arrays RF coils for 7 T and 11.7 T MRI. An eight independent channels prototype-coil that uses a new planar strip array technology is described for 7 T MRI. It was designed based on numerical simulation. A validation methodology has been developed for characterization and performance analysis of transmit phased arrays. After a successful validation of this first prototype, a more efficient 12-channel coil and a new driving strategy that only requires 8 driving channels instead of 12 were designed. Finally, the same methodology was applied to a phased array RF coil for 11.7 T MRI.

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.

Die vorliegende Arbeit untersucht die grundsätzliche Machbarkeit, Radiofrequenzimpulse (RF) der Ultrahochfeld (UHF) Magnetresonanztomographie (MRT) (B0≥7.0T) für therapeutische Verfahren wie die RF Hyperthermie oder die lokalisierte Freigabe von Wirkstoffträgern und Markern zu nutzen. Im Rahmen der Arbeit wurde ein 8-Kanal Sened/Empfangsapplikator entwickelt, der bei einer Protonenfrequenz von 298MHz operiert. Mit diesem weltweit ersten System konnte in der Arbeit experimentell bewiesen werden, dass die entwickelte Hardware sowohl zielgerichtete lokalisierte RF Erwärmung als auch MR Bildgebung und MR Thermometrie (MRTh) realisiert. Mit den zusätzlichen Freiheitsgraden (Phase, Amplitude) eines mehrkanaligen Sendesystems konnte aufgezeigt werden, dass der Ort der thermischen Dosierung gezielt verändert bzw. festgelegt werden kann. In realitätsnahen Temperatursimulationen mit numerischen Modellen des Menschen, wird in der Arbeit aufgezeigt, dass mittels des entwickelten Hybridaufbaus eine kontrollierte und lokalisierte thermische Dosierung im Zentrum des menschlichen Kopfes erzeugt werden kann. Nach der erfolgreichen Durchführung dieser Machbarkeitsstudie wurden in theoretischen Überlegungen, numerischen Simulationen und in ersten grundlegenden experimentellen Versuchen die elektromagnetischen Gegebenheiten von MRT und lokal induzierter RF Hyperthermie für Frequenzen größer als 298MHz untersucht. In einem Frequenzbereich bis zu 1.44GHz konnte der Energiefokus mit Hilfe spezialisierter RF Antennenkonfigurationen entscheidend weiter verkleinert werden, sodass Temperaturkegeldurchmesser von wenigen Millimetern erreicht wurden. Gleichzeitig konnte gezeigt werden, dass die vorgestellten Konzepte ausreichende Signalstärke der zirkular polarisierten Spinanregungsfelder bei akzeptabler oberflächlicher Energieabsorption erzeugen, um eine potentielle Machbarkeit von in vivo MRT bei B0=33.8T oder in vivo Elektronenspinresonanz (ESR) im L-Band zu demonstrieren. / The presented work details the basic feasibility of using radiofrequency (RF) fields generated by ultrahigh field (UHF) magnetic resonance (MR) (B0≥7.0T) systems for therapeutic applications such as RF hyperthermia and targeted drug delivery. A truly hybrid 8-channel transmit/receive applicator operating at the 7.0T proton MR frequency of 298MHz has been developed. Experimental verification conducted in this work demonstrated that the hybrid applicator supports targeted RF heating, MR imaging and MR thermometry (MRTh). The approach offers extra degrees of freedom (RF phase, RF amplitude) that afford deliberate changes in the location and thermal dose of targeted RF induced heating. High spatial and temporal MR temperature mapping can be achieved due to intrinsic signal-to-noise ratio (SNR) gain of UHF MR together with the enhanced parallel imaging performance inherent to the multi-channel receive architecture used. Temperature simulations in human voxel models revealed that the proposed hybrid setup is capable to deposit a controlled and localized RF induced thermal dose in the center of the human brain. After demonstrating basic feasibility, theoretical considerations and proof-of-principle experiments were conducted for RF frequencies of up to 1.44GHz to explore electrodynamic constraints for MRI and targeted RF heating applications for a frequency range larger than 298MHz. For this frequency regime a significant reduction in the effective area of energy absorption was observed when using dedicated RF antenna arrays proposed and developed in this work. Based upon this initial experience it is safe to conclude that the presented concepts generate sufficient signal strength for the circular polarized spin excitation fields with acceptable specific absorption rate (SAR) on the surface, to render in vivo MRI at B0=33.8T or in vivo electron paramagnetic resonance (EPR) at L-Band feasible.