Visualization of Temperature Distribution around Focal Area and Near Fields of High Intensity Focused Ultrasound Using a 3D Measurement System

High intensity focused ultrasound (HIFU) is one of the promising minimally invasive therapeutic methods. Focused ultrasound creates coagulation area with minimally invasiveness. Reduction of thermal damage in non-target area during HIFU therapy as well as effective coagulation of the target area are important. In developing a safe and effective HIFU system and control algorithm for HIFU beam manipulation, experimental evaluation of 3D temperature distribution is required to verify the performance of the system under development. In previous studies, several methods for evaluating 3D temperature distribution were introduced, such as MRI thermometry. This method requires consideration of MRI compatibility of the HIFU system. Other methods including thermocouples and thermochromic liquid crystal (TLC) sheet have been used to visualize temperature distribution in experimental acoustic phantoms. However, there were several limitations in evaluating 3D temperature distribution during HIFU exposure. In this study, a 3D temperature distribution measurement system using micro-capsulated thermochromic liquid crystal (MTLC) was developed. We fabricated an optically transparent temperature sensitive phantom containing MTLC that emits a reflectance spectrum depending on temperature. The 3D temperature distribution was visualized using a light sheet method. The temperature distribution in the optical phantom during HIFU exposure was determined with errors as low as 0.6 degrees C. Using this system, temperature distribution induced by HIFU exposure was visualized using different focusing methods to evaluate their performance.