Two issues hindering the clinical application of image-guided transcostal focused ultrasound surgery (FUS) are the organ motion caused by cardiac and respiratory movements and the presence of the ribcage. Intervening ribs absorb and reflect the majority of ultrasound energy excited by an acoustic source, resulting in insufficient energy delivered to the target organs of the liver, kidney, and pancreas. Localized hot spots also exist at the interfaces between the ribs and soft tissue and in highly absorptive regions such as the skin. The aim of this study is to assess the effects of transmitted beam distortion and frequency-dependent rib heating during trans-costal FUS, and to propose potential solutions to reduce the side effects of rib heating and increase ultrasound efficacy.Direct measurements of the transmitted beam propagation were performed on a porcine rib cage phantom, an epoxy rib cage phantom and an acoustic absorber rib cage phantom, in order of their similarities to the human rib cage. Finite element analysis was used to investigate the rib cage geometry, the position of the target tissue relative to the rib cage, and the geometry and operating frequency of the transducer. Of particular importance, frequency-dependent heating at the target and the intervening ribs were estimated along with experimental verification. The ratio of ultrasonic power density at the target and the ribs, the time-varying spatial distribution of temperature, and the ablated focus of each sonication are regarded as key indicators to determine the optimal frequency. Following that, geometric rib-sparing was evaluated by investigating the operation of 2D matrix arrays to optimize focused beam shape and intensity at target.Trans-costal FUS is most useful in treating tumours that are small and near the surface of the abdominal organs, such as the liver, kidney and pancreas. However, for targets deep inside these organs, severe attenuation of acoustic energy occurs, suggesting that pure ultrasound thermal ablation with different heating patterns will have limited effects in improving the treatment efficacy. Results also demonstrate that the optimal ultrasound frequency is around 0.8 MHz for the configurations considered, but that it may shift to higher frequencies with changes in the axial and lateral positions of the tumours.In this work, I aimed to reduce the side effects of rib heating and increase the ultrasound efficacy at the focal point in trans-costal treatment. However, potential advanced techniques need to be explored for further enhanced localized heating in trans-costal FUS.
|Date of Award||2012|
|Supervisor||Zhihong Huang (Supervisor) & Sandy Cochran (Supervisor)|
- Transthroracic focused ultrasound surgery
- Rib cage
- Sonication protocol