Several medical and surgical fields are concerned by thermal actions of lasers. However, quantification of laser thermal action on biological tissue is not currently performed at the time of treatment. We propose to use a self-tuning control system, in order to improve the safety and efficiency of medical lasers. This study aims to develop a control strategy for real-time control of coagulation using a millisecond pulsed Nd:YAG laser. Temperature evolution was used as a relevant parameter since it is a key factor for thermal damage. A control law was stated using the dynamic programming algorithm which is an optimization technique based on the Bellman's optimization principle. Because the control law was depending on tissue parameters, a parametric adaptive algorithm, based on the least square method, was developed to estimate in real-time the tissue parameters altered by thermal action. Finally, the structure of the self-tuning algorithm was elaborated by linking the dynamic programming algorithm and the parametric adaptive algorithm. The self-tuning control was tested using computer simulation. The results shows that sequences of pulses are producing well-controlled thermal action and that unexpected perturbations are taken into account.
An open-loop temperature control was introduced to control evolution of the maximum
temperature on the tissue surface to be within upper and lower limits. For this purpose, the temperature
evolutions of sample shots were analyzed and optimal sequences of laser pulses were computed. The
1.06 tm pulsed Nd:YAG laser was used and the thermal camera measured temperature. Experiment
on animals in vivo and in vitro was performed to test the technique. Upper and lower temperature
limits during laser irradiation were set below 100 °C since thermal coagulation was ofprimary concern.
Usually, difference between the upper and lower limits was set to 1 5°C during experiment. However,
this difference depended on the laser specifications such as power, pulse width, and repetition rates, as
well as on tissue properties. Coagulation studies showed a clear relation of temperature versus
cogulation depth. Therefore, the heating temperature and the duration time can be used as primary
parameters instead of laser power and exposure time or energy.