Measurements of temperature increase induced on a tissue-mimicking material by a clinical US-guided HIFU system

A framework for the evaluation of temperature increase in a tissue-mimicking material (TMM) induced by a clinical Ultrasound-guided High Intensity Focused Ultrasound (US-guided HIFU) system was developed. HIFU procedures are minimally invasive treatments that achieve deep tumor ablation, with the sparing of normal tissues, through thermal or mechanical effects induced by a HIFU beam generated with a focused transducer. Temperature evaluation is therefore crucial for the assurance of patient safety and treatment effectiveness. Nevertheless, it is a very difficult task on the US-guided systems, where high-pressure fields are involved. As far as we know, this study is the first attempt of temperature evaluation on a clinical US-guided HIFU system. Temperature evaluation was performed at typical clinical settings (between 80 W and 400 W, for 3s sonications) by the use of needle thermocouples connected to a voltmeter and inserted in a polyacrylamide gel phantom, prepared in-house to reproduce soft tissue behavior. Data sampling was performed with the use of acquisition software developed with LabView, while US-imaging was used to verify the position of the thermocouple. Typical rising curves of temperature were recovered, and rapid decrease was found when the HIFU field turned off. The highest temperature increases were concentrated inside the geometrical focus and were higher than 55 Celsius degrees at all power outputs. Repetition of measurements was not possible after sonications at the highest power outputs (400W). The absolute temperature of 98 Celsius degrees was never exceeded.