Automatic Focus Adjustment for Single-Spot Tissue Temperature Control in Robotic Laser Surgery

This paper reports on a study whose goal is to control the tissue temperature at a specific spot during laser surgery, for the purpose of, inducing coagulation or sealing blood vessels. We propose a solution that relies on the automatic adjustment of the laser focus (and thus how concentrated the laser beam is), combined with the use of an infrared thermal camera for non-contact temperature monitoring. One of the main challenges in the control of thermal laser-tissue interactions is that these interactions can be hard to predict due to the inherent variability in the molecular composition of biological tissue. To tackle this challenge, we explore two different control approaches: (1) a model-less controller using a Proportional-Integral (PI) formulation, whose gains are set via a tuning procedure performed on laboratory-made tissue phantoms; and (2) a model-based controller using an adaptive formulation that makes it robust to tissue variability. We report on experiments, performed on four types of tissue specimens, showing that both controllers can consistently achieve temperature tracking with a Root-Mean-Square Error (RMSE) <inline-formula> <tex-math notation="LaTeX">$\approx$ </tex-math></inline-formula> 1 °C.