Optical Diagnostics of a High Intensity Focused Ultrasound (HIFU) Field: Temperature and Acoustic Streaming Measurements

Abstract

High Intensity Focused Ultrasound (HIFU) is an active topic of research for the non-invasive treatment of cancerous tumors by thermal ablation. The ability to estimate the HIFU field is essential for the safety and the accurate quantification of the experiments. The available lab measurement techniques such as thermocouples and pressure probes are invasive which limits the accuracy and the spatial resolution of the measurements, and most techniques measure the field variable at one point. Optical measurement techniques, on the other hand, provide non-invasive, fast, and spatially resolved measurement. In this work, optical methods will be used to provide measurements for two quantities of interest in the HIFU field: temperature and acoustic streaming velocity.

In the thermometry part of this work, two novel optical methods of measuring temperature are proposed. The first method provides a 1-D temperature profile in the radial direction without the need for any calibration, while the second one provides a radial and axial scan of the temperature field but requires calibration of the imaging system.

In the acoustic streaming part, Particle Image Velocimetry (PIV) was used to measure the streaming velocity from pulses of HIFU in a confined liquid space, which is designed to mimic the effect of HIFU in small cavities in the human body. The effects of HIFU power, frequency, sonication time, cavity size, and the presence of a close boundary downstream to the focal point are presented.

While most of the experimental effort in the focused ultrasound field is concentrated on the clinical studies of their possible medical applications, many publications indicated that our understanding of the physical behavior of the HIFU field is not complete yet and needs more works. the methods developed by this work can enhance this side. Also, the proposed thermometry methods can be used to measure the temperature of any localized heating source whose temperature profile can be described by a Gaussian profile.