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 diculty 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 signi®cant di€erence 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 Information Sciences 123 (2000) 271±280 www.elsevier.com/locate/ins * Corresponding author. Fax: +1-409-772-0751. E-mail address: [email protected] (H. Ying). 0020-0255/00/$ see front matter Ó 2000 Published by Elsevier Science Inc. All rights reserved. PII: S 0 0 2 0 0 2 5 5 ( 9 9 ) 0 0 1 3 4 6 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 ®rst laser control system that is guided by noninvasive ultrasonic measurement in real-time. Ó 2000 Published by Elsevier Science Inc. All rights reserved.