Real-time ultrasound-guided fuzzy control of tissue coagulation progress during laser heating

Laser coagulation is a minimally invasive therapy that utilizes laser energy to thermally kill benign and malignant lesions such as cancers, at the temperature range of 55–85°C. It is of clinical importance to control the laser deposition into the tissue in such a way that the lesion will be destroyed while the surrounding healthy tissue will remain intact. However, a primary technical difficulty in achieving this goal lies in the fact that the relationship between the delivered laser energy and the tissue damage is nonlinear and time-varying, which cannot be accurately predicted or rigorously modeled due to the significant difference in various physical properties of even similar tissues. In this paper, we present a novel real-time ultrasound-guided fuzzy laser control system for coagulation. Current status of tissue coagulation depth, noninvasively measured by an innovative ultrasound system that we recently developed, was fed into a fuzzy proportional-derivative (PD) controller, which periodically adjusted output power of a 1064 nm Nd:YAG laser. The ultrasound-guided system was tested in 21 in vitro experiments in which fresh sheep liver samples were irradiated by the laser with a coagulation setpoint ranging from 4 to 14 mm with a 2 mm increment. We provide analytical analysis and design of the fuzzy controller, which turns out to be an inherently nonlinear PD controller with self-tuning variable gains. We also present the hardware and software implementation of the entire measurement and control system. Our control system is unique, and it is the first laser control system that is guided by noninvasive ultrasonic measurement in real-time.