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Thermométrie Ultrasonore et caractérisation tissulaire durant les traitements par Ultrasons Focalisés de Haute Intensité par méthodes ultrasonores et élastographie passive

Abstract : High Intensity Focused Ultrasound (HIFU) is a noninvasive technique allowing generating a temperature increase away from the emitter. This technique is already used in clinic for the treatment of many pathologies such as essential tremors, cancers of the prostate, liver, pancreas, uterine fibroid, glaucoma etc. An imaging modality is required to effectively guide and monitor non-invasive HIFU treatments. Today, magnetic resonance imaging (MRI) and B-mode imaging are the main techniques used in combination with HIFU to guide treatment. MRI is superior to ultrasound for visualizing tissue temperature and necrosis, but this technique is extremely expensive and lacks portability, availability, and present compatibility issues with patients and devices. Ultrasonic imaging has advantages in terms of cost and portability, availability and present good spatial and temporal resolution. Conventional ultrasound imaging shows the spatial amplitude distribution of echoes reflected by acoustic impedance inhomogeneities in the medium and is already widely used to guide and monitor HIFU treatments in commercial devices. However, ultrasound imaging provides limited information on the formation of HIFU coagulation necrosis, and doesn’t allow thermometry. In most cases, hyperechos are visible due to microbubbles generated by acoustic cavitation or boiling. However, these hyperechoic zones do not allow precise contouring of the treated area, and they do not appear during the coagulation. Several methods have been proposed to characterize the thermal changes as a function of other parameters, such as ultrasonic backscatter. Tissue elasticity has also been studied, based on the fact that the tissues become stiffer when they are coagulated. In this thesis, we investigated the changes in backscattered energy by tissue treated by ultrasound due to thermal coagulation without the generation of microbubbles between 37°C and 80°C. The measurements, particularly focused on the energy of the backscattered signal, have been performed for the monitoring of treatments between 1 and 2.9 MHz. Quantitative ultrasounds methods have been performed to reveal information on the microstructure of tissues, in order to explain the phenomena inducing the changes in the backscattered signals during a HIFU heating. A chapter focuses on the monitoring of ultrasonic interventions by passive elastography. Finally, the characterization devices and techniques used in these works were used to characterize liver tissue derived from standard hepatectomies and containing primary and secondary tumors in terms of attenuation. The impact of the different attenuations between the two types of tumor tissue was evaluated by numerical simulations of HIFU treatments. The results led to conclusions about the need for different treatments for primary and secondary cancers in the liver
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Victor Barrère. Thermométrie Ultrasonore et caractérisation tissulaire durant les traitements par Ultrasons Focalisés de Haute Intensité par méthodes ultrasonores et élastographie passive. Acoustique [physics.class-ph]. Université de Lyon, 2019. Français. ⟨NNT : 2019LYSE1348⟩. ⟨tel-02612694⟩

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