This thesis presents technical and methodological developments aiming tooffer a viable alternative for the treatment of digestive cancers (rectum and esophagus). Compared to the standard methods of therapy, the high intensity contact ultrasound guided by MRI is a less invasive approach. MRI offers 2 advantages: good spatial resolution, and real-time temperature control. This treatment method requires efficacy and safety. Three prototypes of RF coil integrated with ultrasound transducers were built in order to increase the spatial and temporal resolution ofthe MR images, and the accuracy of the temperature measurement. The integrated coils showed a better sensitivity compared to a standard extracorporeal coil. Anatomical (voxel 0.4x0.4x5 mm3)and thermometry (voxel 0.75x0.75x8 mm3, 2s/image) high resolution MR images were acquired in-vivo. The temperature was measured, within a radius of 20 mm from the balloon, with a standard deviation <1°C. The flow artifacts caused by the water circulating inside the cooling balloon could be shifted out of the region of interest. The temperature evolution was controlled automatically, at different depths, with one control point per beam. The controller showed a good accuracy during in-vivo experiments (standard deviation less than 5%). The phased-arrayultra sound transducer permits the successive activation of multiple beams during the same dynamic of sonication. Simulations were conducted in order to offer an optimal treatment planning for a defined tumor. A new design of ultrasound transducer with 256 elements with revolution symmetry, based on a natural geometrical focalization, was proposed.

Cette thèse présente des développements techniques et méthodologiques visant une alternative viable pour le traitement des cancers digestifs (rectum, œsophage). Par rapport aux méthodes standards de thérapie, les ultrasons de contact de haute intensité sous guidage IRM sont une approche moins invasive. L’IRM offre 2 avantages: bonne résolution spatiale et contrôle en temps réel de la température. Cette méthode de traitement demande efficacité et sécurité. Trois prototypes d’antenne RF intégrées à des transducteurs ultrasonores ont été réalisés afin d’améliorer la résolution spatiale et temporelle des images IRM et la précision de la mesure de température. Les antennes intégrées ont montré une meilleure sensibilité par rapport à une antenne extra corporelle standard. Des images IRM haute résolution, anatomiques (voxel0.4x0.4x5 mm3) et de thermométrie (voxel 0.75x0.75x8 mm3, 2s/image) ont été acquises in-vivo.La température a été mesurée, dans un rayon de 20 mm au-delà du ballon, avec un écart type<1°C. Les artéfacts de flux causés par l’eau circulante à l’intérieur du ballon de refroidissement ont pu être projetés hors de la région d’intérêt. L’évolution de la température a été contrôlée automatiquement, à des profondeurs variables, avec un point de contrôle par faisceau. Le contrôleur a montré une grande précision in-vivo (écart type <5%). Le transducteur ultrasonore matriciel permet d’activer successivement plusieurs faisceaux pendant la même dynamique de tir.Des simulations ont été conduites afin de proposer une planification du traitement optimale pour une tumeur désignée. Un nouveau concept de sonde ultrasonore à 256 éléments avec focalisation géométrique naturelle a été proposé. / This thesis presents technical and methodological developments aiming tooffer a viable alternative for the treatment of digestive cancers (rectum and esophagus). Compared to the standard methods of therapy, the high intensity contact ultrasound guided by MRI is a less invasive approach. MRI offers 2 advantages: good spatial resolution, and real-time temperature control. This treatment method requires efficacy and safety. Three prototypes of RF coil integrated with ultrasound transducers were built in order to increase the spatial and temporal resolution ofthe MR images, and the accuracy of the temperature measurement. The integrated coils showed a better sensitivity compared to a standard extracorporeal coil. Anatomical (voxel 0.4x0.4x5 mm3)and thermometry (voxel 0.75x0.75x8 mm3, 2s/image) high resolution MR images were acquired in-vivo. The temperature was measured, within a radius of 20 mm from the balloon, with a standard deviation <1°C. The flow artifacts caused by the water circulating inside the cooling balloon could be shifted out of the region of interest. The temperature evolution was controlled automatically, at different depths, with one control point per beam. The controller showed a good accuracy during in-vivo experiments (standard deviation less than 5%). The phased-arrayultra sound transducer permits the successive activation of multiple beams during the same dynamic of sonication. Simulations were conducted in order to offer an optimal treatment planning for a defined tumor. A new design of ultrasound transducer with 256 elements with revolution symmetry, based on a natural geometrical focalization, was proposed.