Increased knowledge of laser tissue interactions favors the development of new laser technology specifically designed for medical applications, particularly in ophthalmology where the new laser applications are continuously growing. In this presentation will be presented the clinical use of new lasers in the treatment of different ocular pathologies.
The problem of selective laser coagulation, i.e. optimal laser control over dimensions, shapes, and location of the region of tissue coagulation (thermodenaturation) by an appropriate choice of parameters of the active laser radiation is considered.
Elastic light scattering and Raman light scattering applied to the same eye-lens have been used, respectively, to extract information on the spatial variations in the intensity of scattered light and the protein content. A combination of the results allows one to obtain the distribution of the scattering coefficient, the size of the scattering particles and the molecular weight of the scattering particles. The design of the light scattering set up is such that the results can be compared with those obtained with a `Scheimpflug' camera. The increase in light scattering at the anterior and posterior cortex in young (< 20 - 30 yrs.) eye-lenses is in accordance with the theory for short-range crystalline order in eye-lenses.
The report includes information about the new solid laser ophthalmocoagulator working at two wavelengths (lambda) 1 and (lambda) 2 simultaneously with a controlled ratio of these components intensities. The thermal and biological effects such as mixing in our conditions is equivalent to the effect of dye laser mono-chromatic radiation with the variable wavelength within the range from (lambda) 1 to (lambda) 2.
Human laser accident exposure cases may involve severe macula retinal injury resulting in long term visual acuity and spatial vision deficit. In order to investigate this problem, we have chosen to model the effects of such long term exposure in the monkey retina for parafoveal Q- switched exposure produced at two parafoveal exposure sites. Our results suggest significant loss of retinal function in and adjacent to the scar region. Recovery occurs in regions with less scar formation, the foveal region, although long term changes in foveal receptor mechanisms are apparent even after 1 year post-exposure, they do not correlate completely with recent human investigations of severe accidental exposure nor with brain enzyme analysis in our monkey model.
The effects of the Ho:YAG (2.13 micrometers ) laser on rabbit scleral tissue ablation were determined. Over a period of 12 months the status of the drain hole was monitored. A 3 mm incision was made on 26 rabbits through the conjunctiva and the laser probe was advanced and applied under it. A fluence of 255 J/cm2 was used to perforate the sclera with a pulse duration of 300 microsecond(s) ec full width at half maximum. Results of this study show that this laser thermal sclerostomy procedure provided a patent hole in 6 of 8 (75%) eyes at 3 months, 11 of 15 (73%) eyes at 6 months, and 3 of 3 at 12 months. No postoperative inflammation, hypotony or subsequent scar formation was observed in any of the rabbits.
A new method for the fast measurement of the Z-transferfunction in the Fovea Centralis of the human eye using a confocal laser scanning device is presented. The tested eye is illuminated by a collimated helium-neon laser beam and therefore focuses the light onto the retina. Reflected light is detected using confocal techniques, i.e. a pinhole in the detection unit assures that only light originating from the focal plane is sampled. The focus of the laser beam is scanned in 1-D along the visual axis of the eye through the regina by adding a slide defocus to the incoming beam. The Z-transferfunction can be obtained directly by measuring the intensity of the reflected light for different focal planes. A fixation target is offered to the eye at infinity to stabilize the accommodation as well as the direct viewing. Different pinholes in the detection unit are tested to investigate signal to noise ratio and depth resolution.
The influence of intraocular light scatter on letter identification in the absence and presence of glaring light is studied through computer simulations and simple experiments on healthy test subjects wearing diffusive glasses with known light scattering properties. An explicit expression for the point-spread function of a cataractous eye is used to calculate retinal images. For estimating resolution in the calculated images an assumed neural contrast sensitivity function is needed. The function used is derived from measurements by Campbell and Green. Experimental results are in agreement with theory. The analysis shows that visual acuity is quite insensitive even to substantial diffusive scattering provided that glare sources are not present in the field of view. Comparing measurements of `letter size' with measurements of `letter contrast' the latter method is found to measure intraocular light scattering more sensitively.
Changes in parapapillary retina height may be an early indicator of glaucomatous tissue damage. The subject of this study is the development of an automatic computer analysis for the height variations of the parapapillary retina obtained from simultaneously taken pairs of stereophotographs of optic nerve heads. The exact evaluation of stereo photographs is, so far, processed manually in the USA (Takamoto, Boston) in a timeconsuming procedure. An automatic analysis can be considered as an essential improvement in glaucoma diagnosis.
Glaucomatous diseases are one of the main reasons for blindness in modern industry nations. In its chronical form this disease leads to a reduction of the nerve fiber layer of the retina. Therefore it is a point of interest to measure the thickness of the nerve fiber layer in the human eye in vivo. Because of the birefringent abilities of the nerve fiber layer an ellipsometric method can be used to measure its thickness. The next sections will describe a method based on standard ellipsometry, but using electro-optical devices instead of mechanical ones to measure the nerve fiber layer thickness faster and with less errors then it is done by mechanical ellipsometry.
An interferometric method to measure distance variations between cornea and retina, caused by the periodic filling of choroidal vessels is presented. Measurements in normal subjects and in diabetic retinopathy patients before and after laser treatment are compared.
The important part of ergophthalmology is the problem of diagnosing and treatment of refraction errors, accommodation and visual disorders by means of the special optical systems. The using of our diagnostical approach helps to choose the right treatment strategy. Our therapeutical approach permits to normalize the muscle tonus and working capacity of eye accommodation apparatus and gives the possibility to obtain the stable positive results in treatment of the refraction amblyopia as well.
The paper describes the development of laser Doppler anemometer system for the determination of in vivo retinal blood velocity and to provide information relevant to the flow of blood in the human retina. Due to the large dimension of the laser Doppler measurements probe volume relative to a typical retinal blood vessel, a special correction scheme has to be applied. In the present work the ratio of probe volume length to tube diameter was about 2.0, which unless compensated for could result in inaccuracies larger than 20% when attempting to recover the flow rate within the vessel. In order to investigate the magnitude of the phenomena, we devised a two phase flow (liquid-solid) experimental system. The program was designed to verify the ability of the maximum velocity (at the tube center) to represent the average cross sectional velocity.
We describe a new multi-purpose maskless method of corneal surgery based on the point-by- point scanning of a focused laser beam which allows the application of a low-energy excimer laser. The crucial scanning parameters (beam diameter, step width, overlap...) to achieve a smooth corneal surface have been investigated. A computer program for the simulation and optimization of the point-by-point scanning process has been developed and tested on contact lenses consisting of PMMA. In addition, a method of measuring the eye-movement by means of the computer-assisted interpretation of photographs was proved for its application in an eye- tracking-system.
Rapid development of ArF excimer laser keratectomy discovered new problems connected with incessant eye movements during the operation. The authors report the results of these movements study using TV camera, assisting the operation of myopia correction, and special programs developed to animate the IBM PC compatible PCVISIONplus frame grabber. Three kinds of movements are described: one-point sight fixation, having not and having a preferable direction of movements, and two or even more points of sight fixation. The only radical way to compensate for all these types of movements is automatic laser beam tracking during the operation.
Homogeneous deep intrastromal coagulations are thought to be optical for effective laser thermokeratoplasty. We therefore investigated the thermal effects created by two different flashlamp-pumped IR lasers (Er:glass and Cr:Tm:Ho:YAG) as well as two different application modalities (direct fiber contact and intrastromal focusing). The physical and thermal parameters of corneal tissue and the threshold temperatures for corneal collagen denaturation in different time domains were determined using a water bath and IR laser radiation. The temperature required to coagulate corneal tissue in the millisecond time regime was found to be above 100 degrees C. Based on these results a thermal model was developed to calculate temperature profiles and coagulation zones in space and time for different exposures situations. Model calculations show the possibility to induce deep stromal effects with a spatially nearly homogeneous temperature distribution within the application volume while minimizing adverse effects in the corneal endothelium.
The effect of different wavelengths on corneal tissue ablation has been studied using the 1st, 2nd, and 4th harmonics of a picosecond Nd:YLF laser system. This laser system consists of an actively mode locked Nd:YLF oscillator generating 30 ps pulses and a regenerative amplifier. The laser beam was focussed onto fresh human donor or calf corneas. The corneas could be moved in 3D patterns using computer controlled stepping motors. The amount of the ablated tissue and the quality of the resulting surfaces were judged by light and scanning electron microscopy. The histology showed that the ablation depths of the IR and green laser pulses are very similar at a given energy density. SEMs revealed that UV light causes a denser packaging of the underlying collagen fibers, similar to the pseudo Bowman's membrane generated with excimer lasers. Further studies are planned concerning the investigation of the 5th harmonic at 211 nm.
Low-power helium-neon laser stimulation accelerates squint treatment process and stabilizes the achieved results. Due to laser-stimulation followed by prismatic correction treatment, squint was eliminated and binocular sight restored in 79 percent cases and thus surgical treatment was avoided.
The author gives first a short report about the history of light application as a therapeutic method in medicine and about the xenon- and sunlight-photocoagulators first used. He commemorates Prof. Gerd Meyer-Schwickerath, who was not only the inventor of this new method, but also a leading person in ophthalmology in the post-war years. The first YAG- LASER instrument for ophthalmology in Hungary was set up in 1986 in the Saint Roch Hospital. Up to the end laser year 2762 YAG-LASEr treatments have been performed. The form and the effectiveness of the YAG-LASER treatment will be discussed and evaluated.
Laser sclerostomy is a surgical procedure which allows the formation of fistula associated with minimal tissue disturbance. Properly delivered laser energy can create functional sclerostomy, with minimal surrounding damage. Moreover, the laser is transmitted via fibers that cause minimal conjunctival disturbance while inserted beneath the conjunctiva to reach the intended sclerostomy site. At present, clinical experience using these lasers to create sclerostomies in humans is limited both in terms of number of eyes and extent of follow up, yet the overall success rate of all the lasers is clearly inferior to that typically achieved with trabeculectomy.
Laser sclerostomy is currently being evaluated as an approach which can possibly provide a better means of controlling open angle glaucoma than by conventional filtering surgery. This article will introduce various laser systems and their applications in glaucoma filtering surgery.
The erbium laser could provide an interesting solid state alternative to the excimer laser (193 nm) for photokeratectomy. The very high absorption of the erbium laser radiation (2.94 micrometers ) by water leads to precise excisions in the cornea with limited thermal damage. In this work we have studied the effects of Q-switch erbium pulses on rabbit eyes. An ultra-fast imaging technique, based on an intensified video camera and on a nitrogen pumped dye laser, was used to investigate the dynamic evolution of the plume during the laser interaction. During the interaction with eyes deprived of the epithelium the ratio between vapor and fragments in the plume strongly depends on the hydration of the stroma. Histologic analysis was carried out to estimate the quality of incision and the amount of thermal damage.
This study evaluated the effect of the Ho:YAG laser operating at a wavelength of 2.1 micrometers and a repetition rate of 2 Hz on a human scleral tissue. The effects were assessed in terms of the ablation rate (micrometers /pulse) and the thermal damage (micrometers ) induced. The results were compared to those found from porcine scleral ablation. Data indicate that for the pulsed Ho:YAG laser, the ablation rate of scleral tissue increases linearly with laser fluence. The ablation rates are about 40% lower for the human scleral tissue than for the porcine scleral tissue at the same fluences. Data indicate that the mean Ho:YAG laser induced thermal damage is not significantly affected by varying the fluence.
In the present paper, by using computer simulation investigation of thermal interaction of a laser radiation beam with different wavelengths with intraocular hemorrhages and nonpigmented tumors, which leads to thermal denaturation (coagulation) of blood and tissues is performed.