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

Laser coagulation is a minimally invasive therapy that utilizes laser energ y to thermally kill benign and malignant lesions such as cancers, at the te mperature range of 55-85 degrees 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 th at the relationship between the delivered laser energy and the tissue damag e is nonlinear and time-varying, which cannot be accurately predicted or ri gorously modeled due to the significant difference in various physical prop erties of even similar tissues. Tn this paper, we present a novel real-time ultrasound-guided fuzzy laser control system for coagulation. Current stat us of tissue coagulation depth, noninvasively measured by an innovative ult rasound system that we recently developed, was fed into a fuzzy proportiona l-derivative (PD) controller, which periodically adjusted output power of a 1064 nm Nd:YAG laser. The ultrasound-guided system was tested in 21 in vit ro experiments in which fresh sheep liver samples were irradiated by the la ser with a coagulation setpoint ranging from 4 to 14 mm with a 2 mm increme nt. We provide analytical analysis and design of the fuzzy controller, whic h turns out to be an inherently nonlinear PD controller with self-tuning va riable gains. We also present the hardware and software implementation of t he entire measurement and control system. Our control system is unique. and it is the first laser control system that is guided by noninvasive ultraso nic measurement in real-time. (C) 2000 Published by Elsevier Science Inc. A ll rights reserved.