Methods for abdominal tumor ablation through image-guided focused ultrasound

Abstract

The work described in this thesis was aimed at developing and improving methods for extracorporeal image-guided focused ultrasound ablation of abdominal tumors. Main challenges that hamper the widespread clinical adoption of focused ultrasound therapy for the treatment of abdominal cancers include the shadowing effect of the ribs, respiratory-induced organ motion, and organ perfusion. Therefore, the presented methodology is particularly aimed at providing improvements for intercostal interventions and at increasing the achievable ablation rate. A method is presented for phased arrays that can be used to acoustically detect ribs and to map the relative attenuation that each transducer element encounters in its beam path in realtime. Such a method allows for selection of the optimal sonication position and improvement of the applied sonication strategy on a per-sonication basis without the need for elaborate image analysis procedures, with the purpose of avoiding undesired overheating of the ribs or other attenuating structures in the acoustic beam path. In addition, a novel sonication strategy that is aimed at increasing the ablation rate by making use of the physical principles of shock wave formation is described. This strategy was implemented and tested on a clinical MRI-guided focused ultrasound system, demonstrating that local energy absorption can be increased in a controlled manner while preserving healthy tissue layers in the prefocal zone. Increasing the ablation rate either shortens the intervention, which naturally decreases its costs, or increases the maximum tumor size that can be ablated, potentially broadening the pool of eligible patients. Furthermore, a numerical study aimed at improving the phased array ultrasonic transducer design is presented. This study was aimed at designing a phased array transducer that is optimized for extracorporeal abdominal focused ultrasound applications based on predefined design criteria. Boundary conditions were defined such as technical constraints, cost and complexity of the array, the required acoustic pressure profile of the focal point, the required penetration depth, and beam steering capabilities. A cost-efficient improvement of the phased array transducer can potentially increase the ablation rate, increase the number of eligible patients, and reduce treatment complications. The novel transducer design was compared to current clinically available transducers through acoustic and thermal simulations for the application of intercostal sonications in the human liver. It was shown that the novel transducer design is expected to provide improvements for intercostal applications over the evaluated designs that are currently available in clinical practice. For the sonication geometries considered, both the energy density in the prefocal tissue layers as well as the energy exposure of the ribs were reduced, while a higher degree of focusing was achieved. Therefore, the novel phased array transducer design allows for an increase in the ablation rate or an increase of the total treatment volume, rendering intercostal focused ultrasound ablation therapy in a clinically realistic timeframe more feasible. The thesis concludes with a discussion of potential limitations and improvements to the presented methodology, and an outlook on future perspectives for image-guided focused ultrasound therapy for hepatic and pancreatic cancer.