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We describe a novel scanning laser ophthalmoscope (SLO)-based on a video-rate second harmonic generation imaging microscope. A titanium:sapphire femtosecond laser was coupled to a modified SLO. The laser beam was scanned over the sample, and the light produced by second harmonic generation (SHG) was collected for imaging at video-speed. The device was used for imaging the lamina cribrosa (LC) of enucleated pig eyes. A resolution comparable to that of commercial multiphoton microscopes was reached. The SHG images were used for determining the average pore size of the LC determined from the images; the pressure dependence of the pore size was studied by the artificial increasing of the hydrostatic pressure in the eye. A pressure increase of 44.3 mmHg enlarged the average pore size of 62 analyzed pores by a statistically significant amount. The relative pore growth was measured at four different pressure levels in 25 pores. The pressure was increased in 15 mmHg steps. A general tendency for monothonic growth was observed, although single pores grew by no means linearly.

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BACKGROUND: The outcome of glaucoma surgery is limited by scar formation. As an alternative to current techniques, the opening of the trabecular meshwork ab interno may lead to increased outflow. This study examined the Nd:YLF picosecond laser for ab interno ablation of the trabecular meshwork without opening of the eye. PATIENTS AND METHODS: A Nd:YLF picosecond laser was used which allows tissue to be ablated even through fluids. We performed ablation with different spot sizes and energies in postmortem eyes. Afterwards the effect of the laser treatment was examined by electron microscopy. RESULTS: The ablation threshold for the cornea was found to be 20 J/cm2 for a pulse width of 30 ps. At a lower level of energy (10 J/cm2) ablation of the trabecular meshwork was possible. The size of the focus which was limited by the optics of the slit-lamp was about 50 microns. CONCLUSION: These initial results with a Nd:YLF picosecond laser confirm the possibility of ablating tissue in the trabecular meshwork ab interno without the need to open the eye.

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Measurements of the wavefront of light reflected from the retina of the human eye can be used to determine optical aberrations of the human eye for large pupils. An instrument based on the Hartmann-Shack principle was developed. The wavefront is refracted by a microlens array and detected by a CCD camera. In first clinical studies human volunteer eyes and preoperative and postoperative refractive surgical patient eyes have been examined. An adaptive optical closed loop system has been devised for preoperative simulation of refractive outcomes of aberration free refractive surgical procedures.

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.The application of multiple fibres for the conformal irradiation of tumours by laser-induced interstitial thermotherapy (LITT) has been investigated. A study was performed to evaluate the coagulated zones produced in porcine muscle tissue in vitro. For delivering specified powers into the tissue, a multifibre system was developed which allows the simultaneous use of up to four fibres. A new quantitative method of magnetic resonance imaging (MRI) has been applied for real-time thermometry. It is based on the temperature dependence of the T1 relaxation time and the equilibrium magnetisation. The MR results were compared with the measurements of fibreoptic thermometers. Since the acquisition time of the selected MR sequence takes only 3 s per slice and the calculation of the temperature measurement could be realised within a few seconds, the temperature mapping works closely to real time. The accuracy of the temperature measurements in muscle tissue was 1.5°C. Whereas single-fibre applications induced convex-shaped isotherms, concave structures were generated by a multifibre LITT.

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A computer-guided laser probe has been developed for the application of ultrashort-pulsed lasers in neurosurgery. It is part of a novel operation concept for the treatment of deep-seated brain tumours. The system combines the positioning accuracy of stereotactic or neuronavigated instruments with the precise and therefore gentle characteristics of surgical lasers. The probe has an outer diameter of 5.5 mm and is directly inserted into the target volume. By a synchronized movement of three coaxial tubes, which guide the embedded optics, the laser radiation is focused at any time onto the current tissue surface. Since every single laser pulse has only a small effective volume of about 8 x 10(5) microm3, the application of a large number of succeeding pulses can be adapted to required geometries. Tissue fragments are removed from the growing operation cavity by continuous irrigation and suction through the laser probe. Blood vessels are detected by a confocal laser-scanning microscope, which is integrated into the probe, and can be closed by an additional coagulating laser. In this paper, the design and technical properties of the laser probe as well as its use in ablation and coagulation experiments are presented. A description of the overall operation system is given.

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PURPOSE: To investigate the variation in anterior and posterior straining under intraocular pressure changes for the central cornea of normally hydrated and swollen rabbit eyes. METHODS: A new method of measuring regional corneal strains, by imaging a specific tissue location at various intraocular pressures, was developed. Sixteen freshly enucleated, New Zealand White rabbit eyes were investigated either in their normal hydration state or after swelling of the deepithelialized cornea. The eyes were mounted on a specially designed eye fixture, and laser-scanning confocal microscopic images of a selected region in the anterior stroma or endothelium were taken at intraocular pressures of 5, 12.5, 20, 35, and 65 mm Hg. The positions of individual keratocytes or endothelial cells were used to calculate the nonhomogeneous two-dimensional strain field over the image. Corneal thickness was measured at the lowest and highest intraocular pressures (5 mm Hg and 65 mm Hg). RESULTS: All pressure strain curves were highly nonlinear for intraocular pressures between 5 mm Hg and 65 mm Hg; the maximal posterior strains (normally hydrated, 2.1 +/- 0.1%; swollen, 4.8 +/- 0.8%) were larger than the maximal anterior strains (normally hydrated, 1.8 +/- 0.1%; swollen, 1.5 +/- 0.2%). Swelling significantly decreased the anterior strain response but increased the posterior one. The corneal thickness decreased 7.4 +/- 0.4% for the normally hydrated and 6.3 +/- 0.5% for the swollen corneas for an intraocular pressure step from 5 mm Hg to 65 mm Hg. CONCLUSIONS: Bending was found to play a significant role in central corneal deformation of swollen eyes but not in the normal hydration state. Microscopic strain measurements of the cornea, using a laser-scanning confocal microscope, are a valuable tool for the assessment of regional nonhomogeneous strains in various depths and locations of the cornea.

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A Hartmann-Shack wave-front sensor is used to measure the wave aberrations of the human eye by sensing the wave front emerging from the eye produced by the retinal reflection of a focused light spot on the fovea. Since the test involves the measurements of the local slopes of the wave front, the actual wave front is reconstructed by the use of wave-front estimation with Zernike polynomials. From the estimated Zernike coefficients of the tested wave front the aberrations of the eye are evaluated. It is shown that with this method, using a Hartmann-Shack wave-front sensor, one can obtain a fast, precise, and objective measurement of the aberrations of the eye.

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Plasma-mediated ablations were performed on human donor corneas with a short pulsed Nd:YLF laser system at 1053 nm. The pulses were 60 psec in duration at a repetition rate of 1.0 kHz. The laser beam was oriented perpendicular to the cornea surface. The threshold energy densities for ablation of epithelium, Bowman's membrane and stroma were measured. They were 6.1 +/- 1.8 J/cm2, 21.0 +/- 5.1 J/cm2 and 10.4 +/- 1.8 J/cm2, respectively. The mean rate of tissue removal at the stromal energy density threshold was about 1 micron per pulse. The walls of the laser excisions were smooth with distortions of less than 1 micron. A new quantitative model of plasma-mediated ablation is introduced and found to closely predict the observed results. Based on the promising nature of the experimental data further investigations are planned in the use of a mode locked Nd:YLF laser as an alternative to excimer lasers for refractive corneal surgery.

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To study the retinal surface in the human eye in normal and diseased states we used laser scanning tomography. The confocal arrangement of the laser tomographic scanner permits examination of retinal topography in the axis perpendicular to the retinal surface. The eyes examined with the laser tomographic scanner included normal eyes, eyes with macular holes, impending macular holes, radiation retinopathy, macular edema, photocoagulation scars, subfoveal scars, and serous detachment of the fovea associated with subretinal neovascularization. The laser tomographic scanner is a new method that allows measurements of the topography of the internal limiting membrane in the macular area and may improve our understanding of the pathophysiologic characteristics and treatment of a variety of disorders of the macula.

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We describe the use of a laser tomographic scanner to objectively measure optic nerve head topography. A laser beam is focused on the optic disc using confocal imaging. Reflected light is detected only if it originates from a small region around the focal place of the special optics. Optic disc parameters are calculated from the measured values of confocal reflected light. In five glaucomatous eyes, maximum cup depth was between 0.33 mm and 0.58 mm. Cup volume was between 0.10 mm3 and 0.65 mm3; it correlated well with ophthalmoscopy. Compared with conventional imaging systems, the laser tomographic scanner has certain advantages: 1) images can be obtained with miotic pupils; 2) clarity of media is not important; 3) low light intensity is employed; and 4) a real time image is obtained.

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Laser scanning tomography can be used to assess retinal nerve fiber layer thickness and optic disc topography of the human eye. A pinhole is located at a plane conjugate to the focal plane of the scanning laser beam. This so-called confocal configuration assures that only the light originating from the illuminated focal plane on the retina passes through the pinhole and is detected by the photomultiplier. Consequently, images with high spatial resolution in all directions are obtained. An active optical system (active mirror) further improves the lateral/depth resolution of the laser tomographic scanner. By partially compensating for the optical aberrations introduced by the cornea and lens, the active optical system allows the illuminating beam to be enlarged to 6 mm, thus improving depth resolution twofold.