In protein NMR spectroscopy, homonuclear mixing pulses are used to reveal correlations amongst chemically bonded nuclear spins. / Engineering and Applied Sciences

The focus of this thesis lies on the development, and implementation, of parallel transmission (pTx) techniques in magnetic resonance imaging for flip-angle homogenization throughout the human brain at ultra-high field. In order to allow in-vivo demonstrations, a conservative yet viable safety concept is introduced to control the absorbed radiofrequency (RF) power . Subsequently, novel methods for local SAR control and non-selective RF pulse-design are investigated. The impact of these short and energy-efficient waveforms, referred to as kT-points, is first demonstrated in the context of the small-tip-angle domain. Targeting a larger scope of applications, the kT-points design is then generalized to encompass large flip angle excitations and inversions. This concept is applied to one of the most commonly used T1-weighted sequences in neuroimaging. Results thus obtained at 7 Tesla are compared to images acquired with a clinical setup at 3 Tesla, validating the principles of the kT-points method and demonstrating that pTx-enabled ultra-high field systems can also be competitive in the context of T1-weighted imaging. Finally, simplifications in the global design of the pTx-implementation are studied in order to obtain a more cost-effective solution.

The International Thermonuclear Experimental Reactor (ITER) will be a major step towards controlled energy fusion in tokamaks. Operation of the hot confined plasma inside the tokamak will have to be optimized and simulations will have to prove that each pulse conducted is feasible under the operational limits of the reactor. For such purpose a Pulse Design Simulator is developed at ITER. This workflow lacks a transport model to simulate the dynamics of the plasma caused by micro-instabilities driven by turbulences. The purpose of this thesis is the adaptation of such model into the Integrated Modelling and Analysis Suite (IMAS), namely mapping the inputs and outputs of an existing code for its integration to the workflow. This work presents a fast 1D core transport code capable of simulating the evolution of the poloidal flux, the temperature evolution of both ions and electrons and the particle density transport. The model is coupled to a neural network regression of the transport model QuaLiKiz for the computation of first-principle based turbulent heat and particle transport coefficients. / Internationella Termonukleära Experimentella Reaktorn (ITER) kommer att vara ett stort steg mot kontrollerad energifusion i tokamaker. Driften av det varma, instängda plasmaet inne i tokamaken måste optimeras, och simuleringar måste bevisa att varje pulsskötning är genomförbar inom reaktorns driftgränser. För detta ändamål utvecklas en pulsdessignsimulator vid ITER. Denna arbetsflöde saknar en transportmodell för att simulera plasmaets dynamik orsakad av mikroinstabiliteter drivna av turbulenser. Syftet med denna avhandling är anpassningen av en sådan modell till Integrated Modelling and Analysis Suite (IMAS), nämligen att kartlägga in- och utdata av en befintlig kod för dess integration i arbetsflödet. Denna arbete presenterar en snabb 1D-kärntransportkod som kan simulera utvecklingen av den poloidala flödet, temperaturutvecklingen för både joner och elektroner samt partikeltäthetstransporten. Modellen är kopplad till en neural nätverksregression av transportmodellen QuaLiKiz för beräkning av första principbaserade turbulenta värme- och partikeltransportkoefficienter.

L'objectif de cette thèse repose sur le développement et la mise en œuvre des techniques de transmission parallèle (pTx) en Imagerie par Résonance Magnétique pour homogénéiser l’excitation des spins dans le cerveau humain à ultra-haut champ. Afin de permettre des démonstrations in-vivo, un concept de sécurité conservateur mais viable est introduit pour le contrôle de la puissance de la radiofréquence (RF) transmise. Par la suite, de nouvelles méthodes de minimisation du Taux d’Absorption Spécifique local et de conception d’impulsions RF non-sélectives sont investiguées. L’impact de ces impulsions courtes et relativement peu énergétiques, appelées « kT-points », est d'abord démontré dans l’approximation des petits angles de bascule de l’aimantation. Pour cibler un éventail d’applications plus large, la conception de type kT-points est ensuite généralisée en englobant les excitations à grand angle de bascule et les inversions. Cette méthode est appliquée à l'une des séquences pondérées en T1 les plus couramment utilisées en neuro-imagerie. Les résultats ainsi obtenus à 7 Tesla sont comparés à des images acquises avec une configuration clinique à 3 Tesla. Les principes de la méthode sont ainsi validés et démonstration est faite que la transmission parallèle permet aux systèmes à très haut champ d’être aussi compétitifs en imagerie pondérée en T1. Enfin, des simplifications dans la conception globale de la pTx sont étudiées pour un meilleur rapport coût-efficacité des solutions proposées. / The focus of this thesis lies on the development, and implementation, of parallel transmission (pTx) techniques in magnetic resonance imaging for flip-angle homogenization throughout the human brain at ultra-high field. In order to allow in-vivo demonstrations, a conservative yet viable safety concept is introduced to control the absorbed radiofrequency (RF) power . Subsequently, novel methods for local SAR control and non-selective RF pulse-design are investigated. The impact of these short and energy-efficient waveforms, referred to as kT-points, is first demonstrated in the context of the small-tip-angle domain. Targeting a larger scope of applications, the kT-points design is then generalized to encompass large flip angle excitations and inversions. This concept is applied to one of the most commonly used T1-weighted sequences in neuroimaging. Results thus obtained at 7 Tesla are compared to images acquired with a clinical setup at 3 Tesla, validating the principles of the kT-points method and demonstrating that pTx-enabled ultra-high field systems can also be competitive in the context of T1-weighted imaging. Finally, simplifications in the global design of the pTx-implementation are studied in order to obtain a more cost-effective solution.