Eye-guided laser surgery

The application of lasers in microsurgery and in particular in neurosurgery offers many advantages such as, e.g., hemostasis, no-touch (i.e. aseptic) procedures, and pinpoint accuracy. However, these advantages can often not be fully exploited since adequate laser beam delivery techniques are not available. Traditionally micromanipulators attached to the surgical microscope and also laser-handpieces are used. Such techniques require that one hand of the surgeon is used to direct the laser beam, leaving only one hand free for the adjustments of the microscope, suctioning, and dissection. Moreover, precise and continuous aiming along complex targets in deep confined anatomical spaces is extremely difficult with such systems. In this work, we present a novel technique for laser surgery which allows to overcome the basic pitfalls of the traditional delivery techniques. The idea is to slave the laser position to the surgeons line of sight. Such an eye guided laser surgery allows to free completely the surgeon's hands and enables him to easily guide the laser along any target line in the object field of the microscope by simply looking at it. The goal of this thesis work consisted in demonstrating the technical feasibility of this idea by developing an experimental prototype. In a first step the problems of eye tracking on an operating microscope are addressed. Among the different possibilities to detect the position of gaze, we have chosen a video based technique. A special eyepiece allowing to obtain the infrared image of the surgeon's eye and an optimized infrared illumination source allowing to enhance the contrast of the eye image are described. The second step consisted in critically reviewing the different solutions to couple a laser beam to the operating microscope and in discussing all aspects concerning the system soft- and hardware. The technical performances of this first eye-guided laser aiming system are then analyzed. The results confirm that the speed of response of the system is sufficient for microsurgery. The resolution of the laser aiming in the operating field depends on the magnification of the microscope. A value of 62 micrometers is achieved at the highest magnification. A second part of the thesis is concerned with the study of some aspects of human vision relevant to the ability of the surgeon to control the laser with his regard. The most important issues in this context are saccadic eye movements, the fixation characteristics and ocular dominance. They have been investigated by measuring the corresponding characteristics of test persons on the eye guided laser system. The eye movements during laser aiming consists of a succession of rapid saccades and longer fixations of the eye. The dynamics of these jerks constitutes a problem with respect to laser dosimetry. We demonstrate that adequate filtering allows to obtain a smooth laser aiming with a constant laser energy deposition. The stability of fixation is analyzed with respect to ocular dominance and it is shown that there is no disturbing influence on the ability to maintain stable laser aiming. The system is slightly sensitive to head motions of the surgeon. A 3-D mathematical model has been developed to describe this sensitivity. The model has been verified experimentally. It constitutes to our knowledge the first description of the sensitivity of the eye tracking technique to head motions. It allows to optimize the parameters of the prototype in order to limit the effect of head movements on the accuracy of the eye tracking. The experiments on eye guided laser aiming with test persons also revealed that untrained persons are able to perform complex laser aiming and point addressing tasks with this first prototype with a minimum of training. The use of the eye guided laser system in real operating conditions is described in the last part of this work. Different surgeons were able to perform the first eye guided laser assisted microvascular anastomoses in vivo on the rat. One of them used the system for the first time and his successful interventions clearly demonstrated the important ergonomic advantages of this highly evolved man-machine interface.

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