Ultrasound monitoring of temperature change during interstitial laser thermotherapy of liver: An in vitro study

Abstract

Laser interstitial thermotherapy (LITT) is an internal ablation therapy consists of the percutaneous or intraoperative insertion of laser fibers directly into the liver tumor with maximum diameter of 5 cm. It is very important to control temperature increasing non invasively, because on high temperatures, tissue carbonization occurs and it can damage normal tissues. In this research, pixel shift changes on ultrasound B-mode images with temperature were measured. LITT in vitro was performed on 11 freshly sheep liver tissues using a Nd:YAG laser with a bare-tip optical fiber. Invasive temperature monitoring was performed during heating and cooling down by fixing micro thermocouples on tissue. At the same time, noninvasive temperature monitoring was performed with ultrasound B-mode images. The speed of sound variations with the temperature changes are create virtual shifts in scattering positions and time shifts into the received echo signals. It can locally cause pixel shifts on B-mode images. These pixel shifts were measured by echo tracking algorithm. Linear and nonlinear regression analysis between independent variable (temperature changes) and dependent variable (pixel shift on images) were performed. It was shown that with correlation coefficient of 0.892, cubic function was suitable. In this method, because of bubbles formation and tissue carbonization, monitoring of more than 100ºC, was difficult. This method could be used for noninvasive temperature monitoring for a large number of patient, during LITT.