Ultrasound Thermometry for HIFU-Therapy

Abstract

High-Intensity Focused Ultrasound (HIFU) is an alternative tumour therapy with the ability for non-invasive thermal ablation of tissue. For a safe application, the heat deposition needs to be monitored over time, which is currently done with Magnetic Resonance Imaging. Ultrasound (US) based monitoring is a promising alternative, as it is less expensive and allows the use of a single device for both therapy and monitoring. In this work, a method for spatial and temporal US thermometry has been investigated based on simulation studies and in-vitro measurements. The chosen approach is based on the approximately linear dependence between temperature and speed of sound (SoS) in tissue for a given temperature range. By tracking the speckles of successive B-images, the possibility of detecting local changes in SoS and therefore in temperature is given. A speckle tracking algorithm was implemented for 2D and 3D US thermometry using a spatial compounding method to reduce artifacts. The algorithm was experimentally validated in an agar-based phantom and in porcine tissue for temperature rises up to 8°C. We used a focusing single element US transducer as therapeutic probe, a linear (/matrix array) transducer with 128 (/32(tm) 32) elements for imaging and thermocouples for validation and calibration. In all experiments, both computational and in-vitro, we succeeded in monitoring the thermal induced SoS changes over time. The in-vitro measurements were in good agreement with the simulation results and the thermocouple measurements (rms temperature difference = 0.53 °C, rms correlation coefficient = 0. 96).