Non-invasive tissue erosion by high-intensity focused ultrasound (HIFU)
Lim, Joshua Jun Jie
Date of Issue2017
School of Mechanical and Aerospace Engineering
High-Intensity Focused Ultrasound (HIFU) is the usage of soundwaves for medical applications. It focuses Ultrasound waves to converge at a point of treatment to deliver energies in the form of heat or agitation. By adjusting the appropriate parameters, lesions or cavitation can be produced accordingly. This is done by the mechanism of thermal or mechanical ablation. Different Medical Treatments can be then recreated by controlling HIFU under certain conditions. The main focus of this project will be the mechanical mechanism, specifically erosion on soft tissue. Multi-Frequency excitations, specifically Single & Dual Frequency excitations will be studied and compared to record and evaluate their corresponding erosional effects. Dual Frequency has exhibited a better erosional effectiveness in terms of area and volume. The understating of the underlying mechanism behind erosion will be done by the monitoring of bubble activities of the different waveforms via Passive Cavitation Detection, the stronger erosional effect exhibited by Dual Frequency was also due to a higher overall cavitation threshold of the waveform. Additionally, by comparing the Inertial Cavitation of Dual Frequency with the Stable Cavitation of Single Frequency, Inertial Cavitation was stronger. The processing of the bubble activities will be done by Passive Cavitation Detection (PCD). Data will be digitally processed to be represented in the form of spectrograms. Further evaluation will be done based on the data to identify the erosional mechanism in multi-frequency excitations. For the erosion of soft tissue to be carried out, cavitation is a necessary process. 2 types of cavitation, namely Stable & Inertial Cavitation are observed in both Single & Dual Frequency Excitations. Using a spectrogram, different types of cavitation can be identified within a certain frequency window. In terms of Stable Cavitation, spectrogram can reveal the presence of harmonics in a repeating signal. It can be identified that in Single & Dual Frequency excitations, there is a primary or rather a more dominant type of cavitation. Single Frequency has a stronger Stable Cavitation as compared to Inertial Cavitation. Conversely, Dual Frequency has a stronger Inertial Cavitation as compared to Stable Cavitation. By comparing Dual Frequency’s Inertial Cavitation against Single Frequency’s Stable Cavitation, it was found that Dual Frequency Inertial Cavitation exhibited a larger sound pressure level. Therefore, Dual Frequency is a much more preferred method due to its better efficiency in erosional effects as a result of the enhanced cavitation. However, much can be done to investigate the bubble dynamics and other parameters in dual frequency to realise its relation to inertial cavitation. Further investigations need to be done in higher multi-frequency excitations and their corresponding cavitation effects. This is to fully utilise and configure HIFU to deliver a precise & higher range of medical treatments in addition to energy efficient.
Final Year Project (FYP)
Nanyang Technological University