Eye-guided laser surgery | EPFL Graph Search

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.

Compact fast-steering tip/tilt laser scanner for high power material processing applications

Moustapha Hafez

The common configuration for deflecting laser beams in two dimensions is based on two single-axis galvanometric scanners placed perpendicular to each other. Over recent years, these fast scanners have proved to be both very reliable and very accurate. They are widely available in the marketplace and have wide-ranging technical specifications. However, such a configuration is rather bulky and can not satisfy the needs of industry which seeks compact solutions for an easy integration of these scanning heads into their production lines. In addition, two separate mirrors leads to optical aberrations when these scanning heads are used in conjunction with scanning lenses, which are specially designed for a single entrance pupil. Therefore, a compact fast-steering tip/tilt laser scanner with a single mirror would act as a key component in diverse applications, e.g. in material processing, astronomy, intersatellite laser communications, imaging systems, bio-medical and ophthalmologic applications. The compact two-axis tilt laser scanner presented in this work can perform a variety of functions such as image tracking, beam stabilization and alignment, line-of-sight pointing, and scene scanning. Designing such a scanner required a multi-disciplinary approach involving optics, mechanics, electromagnetics, sensors and control theories. The scanner is composed of one single mirror with a large active area and has a single point of rotation for the two axes of tilt. Moreover, the center of gravity coincides with the center of rotation to reach optimum dynamic behavior. The scanner's compact dimensions allow an easy integration in the various optical systems. Furthermore, the mirror is easily interchangeable which gives greater flexibility in the choice of the laser source. Another major advantage of the proposed design is its ability to be scaled down to miniaturized versions. The scaling depends mainly on the active surface of the mirror, which is generally specified by the application. The larger the mirror, the higher its inertia and the larger are the actuators in order to guarantee high performance. Two pairs of push-pull linear electromagnetic actuators are used to drive the mirror of the scanner. The mirror suspension is based on a sliding bearing composed of a cone-ball contact with optimized friction and wear behavior. A magnetic pre-load force is used to hold the mirror and to give sufficient rotational stiffness. Furthermore, a position transducer based on a photodetector and a miniature laser pen is integrated in the module and is used for closed loop feedback positioning. The mirror can be tilted around both axes by ± 61mrad (± 3.5°) with an accuracy better than 50μrad. Moreover, a differential resolution in the order of 2.5μrad, and settling times for maximum tip and tilt deflections of 10ms and 14ms respectively are achieved. The overall volume of the scanner is 30 x 40 x 50 mm3, and its total weight does not exceed 90g. Design, simulations and experimental investigations were carried out in this work in order to optimize the different components of the tip/tilt scanner. These components are: the motors, the bearing system, the position transducer and the feedback control loop. An analytical model that allows an optimization of the geometrical parameters for an electromagnetic actuator based on rectangular coils and magnets is proposed. This model proved to be very reliable and can be extended to the design of a variety of electromagnetic driving systems. In addition, an extensive analysis of the beam path distortions generated when a single mirror is pivoted around two axes is covered. Furthermore, correcting equations used to compensate for the systematic errors resulting from this configuration are proposed. The tip/tilt scanner is suited to a variety of applications in industrial material laser processing. These applications are not just restricted to conventional processes like welding, cutting, drilling, and marking, but also to new technologies such as micro-manipulation of parts by laser. In this new approach, bending and shrinkage of the material result due to laser irradiation, allowing fast adjustment and positioning of micro-mechanical and opto-mechanical components with sub-micron accuracy. Furthermore, local annealing of shape memory alloys, leading to stiffness variation in monolithic parts such as micro-grippers, represents a very innovative application in smart materials. On the other hand, the scanner is also a key component in bio-medical and ophthalmologic applications. The first measurements carried out with the scanner for optical tomography of the retina appeared very promising. Furthermore, an optical tweezers set-up has been realized using the scanner, allowing easy and flexible manipulation of laser beam trapped cells and molecules.

EPFL2000

Developing luminescent Brownian probes for near-field investigations of the intracellular environment

Flavio Maurizio Mor

Life sciences have a constantly growing need for novel methodological approaches suitable to investigate the intracellular environment with increased temporal and spatial resolutions. Recently, optical near-field probes, such as laser-irradiated pointed metal, uncoated/metal-coated tapered optical fibers, as well as nano-emitters, such as single molecules or nanoparticles, have attracted increasing attention as key components of high-resolution microscopes. In parallel, super-resolution fluorescence microscopy, operating in far-field regime and overcoming the light diffraction limit, has markedly improved imaging resolution. Although both near- and far-field optical approaches drastically improve image resolution, they still represent numerous drawbacks, such as, technical difficulties concerning probe preparation (near-field) or potential damaging through very high light intensities (far-field). The aforementioned shortcomings of high-resolution optical microscopy can be overcome to a great extent by photonic force microscopy (PFM). PFM employs a strongly focused near-infrared (NIR) laser light to hold a dielectric or metallic particle as a local probe. Such an optical trap enables one to ‘cage’ a mesoscopic particle and track its three-dimensional (3D) thermal fluctuations in the surrounding environment, e.g. a viscous liquid. Therefore, besides offering near-field 3D imaging, PFM reports also on other important quantities concerning local mechanical properties, including force and viscosity as well as dynamical properties of the surrounding medium. This additional information is derived from the careful analysis of the jittery motion (so-called Brownian motion) of the trapped single-particle probe that collides with thermally-activated surrounding molecules. In this thesis, we first study in detail the Brownian motion of a single spherical particle that is confined within the harmonic potential of the optical trap. To this end, we optimize the NIR light path and electronic noise floor of our custom-built PFM set-up for detecting and quantifying resonances in the Brownian motion. These resonances arise from the coupling between the hydrodynamic memory and strong strength of the optical trap. Due to the high sensitivity of the short-time dynamics, the size of the particle can be measured by simultaneous fitting of the velocity autocorrelation function and power spectral density of its thermal fluctuations. In order to choose the best-suitable spherical probe for a given experiment in PFM, computational modeling based on a Matlab toolbox is performed. The generalized Lorenz-Mie theory is computed using the T -matrix method for various experimental conditions, including changes in the size and refractive index of the sphere, as well as different laser polarization states. The axial equilibrium position is examined to predict its location compared with the position of the laser focus. Optical forces acting on the sphere are investigated in 3D to highlight, in particular, the limits of the perfectly harmonic potential assumption. These limits are quantified and discussed. Subsequently, we identify different types of inorganic nano/micro-sized particles that can be trapped by the NIR laser of the PFM and used simultaneously as mechanical and near-field light sources. To this end, we take advantage of the visible light emission from single probes confined in the optical trap and excited by the NIR laser light. The emphasis is on particles revealing nonlinear optical properties. In particular, nonlinear optical field enhancement resulting from excitation of surface plasmon resonance (SPR) on trapped gold particles of 200 nm is demonstrated by measuring the emission of that second-harmonic generation (SHG). Furthermore, we show, under optical trapping conditions, SHG emission from single potassium niobate (KNbO3) nano/micro-sized particles. We also report on the multicolor upconversion luminescence (UCL), at the single particle level, for randomly shaped crystals of sodium yttrium fluoride codoped with ytterbium and erbium, NaYF4:Yb,Er (UCC), when they are held in the optical trap. In parallel, 3D Brownian fluctuations of a single trapped particle are quantified. Moreover, luminescence resonance energy transfer (LRET) between a KNbO3 particle or an UCC and molecules of an organic dye (rose bengal) adsorbed on their surface is highlighted and analyzed. Finally, randomly shaped UCCs and hexagonal nanoplates are characterized in detail at the single particle level. The hexagonal β phase in the polycrystalline and monocrystalline crystals is identified, in both randomly and hexagonally shaped particles by electron microscopy. UCL emission from the trapped crystal is quantified in terms of power and found to be dependent on the laser power density, size and shape of the particle as well as its orientation within the trap. Most importantly, the different colors in the UCL spectrum do not vary with the same trend for different orientations of the randomly or hexagonally shaped crystal upon trapping. This suggests the existence of crystalline anisotropy-dependent UCL. Thereby, we should be able to design the best-suited particle probe for LRET experiments with subwavelength resolution.

EPFL2013

Compact fast-steering tip/tilt laser scanner for high power material processing applications

Moustapha Hafez

EPFL2000

Developing luminescent Brownian probes for near-field investigations of the intracellular environment

Flavio Maurizio Mor

EPFL2013