On the application of optical fibers in ophthalmology

Introduction

Optical fibers are widely used today, both as a carrier of information on the one hand and as a carrier of energy on the other. While the transmission of information is associated with low power levels, high power transport is required if substantial amounts of energy have to be made available in short periods of time. The greatest use of optical fibers is currently in telecommunications where information is relayed over large distances, and minimal distortion and absorption, requiring as little power as possible, are a priority. In turn, applications of optical fibers in medicine are usually restricted to local distances, but are related to both low and high power needs, i.e., information transmission and energy intensive applications can likewise be found. Thereby, optical fiber-based diagnostic and therapeutic procedures have been established in a wide range of clinical disciplines.1 Particulary important in this regard are neurosurgery, dermatology, gynecology, and ophthalmology.

A typical information transmission task in clinical medicine consists of remote imaging by way of a fiber bundle, relay lens system, or a GRIN (Gradient Index) rod, i.e., endoscopy. In ophthalmic endoscopy, tips have to be small (diameter of less than about 0.9 mm) such that GRIN-based tips are particularly advantageous (see below). Power requirements of image acquisition, transport, and presentation are per se minimal; in this procedure, aspects relating to image quality, and ease and safety of application are prominent. Another type of physiological or pathophysiological information that can be conveyed by optical fibers consists of various kinds of fluorescent signals. For the generation of such signals, moderate incident power levels are

needed, while the returning signals are in general weak. Other weak optical responses associated with possible fiber applications are, for example, the Doppler shift caused by moving targets, an effect which enables flow measurements in blood vessels or tissues in general, as well as the Raman effect which allows chemical compositions to be determined. Still another use of fibers is made by the transmission of the response of implantable sensor systems, e.g., pressure sensors.

In contrast, the transport of higher amounts of energy is necessary for therapeutic applications such as coagulation or optical breakdown production. Moreover, in photodynamic therapy, the delivery of medium to high optical power is necessary. Low and high power transmission techniques, i.e., imaging and treatment needs, are sometimes combined, in particular when therapeutic procedures are carried out under indirect visual control with the aid of an endoscope.

Furthermore, an important aspect is related to illumination which is needed for many diagnostic and therapeutic procedures in ophthalmology. Thereby, the illumination which is necessary in endoscopy has to be provided at energy levels at which no tissue damage occurs. In ophthalmic endoscopy, the sensitivity of the retina is the limiting factor. (Dangerous levels are in fact not usually reached; in contrast, insufficient light is the main problem in many cases.) A fiber bundle is often applied for carrying the light necessary for illumination. In contrast to imaging bundles, such fibers are not ordered, but, depending on the application, are mixed in order to obtain the desired distribution of light intensity within the target area.

While illumination is typically performed with the aid of a xenon lamp, a popular source of optical

energy in ophthalmology for therapeutic purposes is lasers. Of these, the argon, Nd:YAG and diode lasers are mostly used in connection with fiber-based tools.* The main advantage of laser radiation is the possibility of being able to concentrate the energy in a smaller spot size than can be achieved with ‘normal’ light. Furthermore, optimal wavelengths can be chosen which are adapted to the treatment procedure at hand, and precisely defined pulse lengths and bandwidths are available. This is particularly important in procedures involving fluorescence spectroscopy and photodynamic therapy.

However, optical energy can be provided by other sources than lasers, e.g., a xenon lamp. Even the sun has been tested for this purpose.2

In all applications, an important distinction has to be made with respect to the environment in which a fiber, a fiber bundle, a GRIN rod, or a lens is used: In air, the refractive power of surfaces is usually higher than in biological fluids because the difference in the index of refraction of typical materials used for fibers, fiber tips, or lenses, e.g., silica (index of refraction, n = 1.46), is smaller in case of fluids (n = 1.33 for water) than of air (n = 1.0003, all values given in relation to vacuum). Accordingly, the optical pathways are considerably different when such interfaces are present, and the focal lengths or angles of divergence of light beams can also differ decisively.

For most applications of fiber systems used in ophthalmology, the particular aspects of wave optics can be disregarded and the analysis restricted to geometrical optics. Ray-tracing is the design method of choice for this purpose.

Light sources and optical fibers

Lasers

Lasers used for therapeutic purposes are essentially characterized by their wavelength, power, mode

composition, coherence length/bandwidth, angle of beam divergence, and pulse length. Moreover, from a user’s point of view, installation requirements, size, weight, etc., are of concern. Solid-state lasers have the advantage of minimal maintenance requirements and ease of operation. In addition, diode lasers are small, light weight, simple in terms of instrument complexity, and are easily replaceable in case of malfunction. While the details of the mode composition as such are of minor importance in most ophthalmic applications, pulse length, beam divergence, focal spot size (which, in turn, depends on modal composition and divergence), and energy delivery characteristics are decisive. Depending on the application, lasers are utilized in the continuous wave (cw), free running, and pulsed mode; moreover, Q- switched as well as mode-locking techniques are used for special purposes.

The argon (gas) laser is often utilized mainly for two reasons. Firstly, one of its main wavelengths (514 nm) is well adapted to the absorption characteristics of blood, which makes the instrument especially suitable for coagulation purposes; secondly, it has been widely introduced in the medical community. In turn, the Nd:YAG (solid-state) laser, working at 1064 nm, is characterized by its high power capacity and its capability of producing optical breakdowns when run under Q-switched conditions. Furthermore, by frequency-doubling, a wavelength similar to that of the argon laser can be obtained (KTP laser). Finally, infrared diode lasers are well suited for clinical applications because of their small size, such that they can even be installed on the operator’s spectacles. Since the ophthalmic applications of lasers are treated in depth elsewhere in this book, this subject will not be examined in further detail here.

Lamps used for illumination

The light of the xenon lamp is characterized by a wide spectrum covering the range of ultraviolet to infrared, with a bias towards the blue limit (Fig. 1).

On the one hand, its high blue content is advantageous for imaging3 (CCD and CMOS image sensors have a low sensitivity in the blue range), on the other, UV is potentially cancerogenous, such that the UV content of the illumination has to be carefully controlled. In contrast, halogen lamps are non-problem- atic in this regard, however, they are not suitable for true-color imaging due to their weak blue spectral content. Infrared always has to be filtered because excessive heating has to be prevented and, moreover, sensor response at these wavelengths disturbs the visible (and useful) image information.

Optical fibers, optical transmission systems and exit beam shaping

The characteristic quantities describing the properties of optical fibers are the numerical aperture, wavelength/diameter ratio, transmission characteristics in terms of bandwidth and damping, as well as water solubility.4 The material used is usually silica, but quartz and chalcogenide fibers can also be chosen for UV and long wavelengths, respectively. Typical core diameters vary between 50 and 600 µm, while numerical apertures cover the range of 0.1- 0.4.

A typical fiber transmission system (Fig. 2) for low or high power applications in essence consists of three parts, namely, firstly, an energy control and entrance coupling, where the light is optically coupled into the fiber or fiber bundle, secondly, the fiber or fiber bundle itself, and, thirdly, the exit system. Thereby, the design of the entrance and exit optics has to be adapted to the physical characteristics of

the fiber(s), in particular the aperture, as well as the dimensions and divergence of the laser beam (or intensity distribution of a lamp).

Furthermore, high power transmission by way of optical fibers is critical with respect to material purity (e.g., the presence of OH- ions) and optical coupling.5 Impurities can cause irregularities in energy transport, local energy concentrations, and ‘hot spots’ causing fiber damage. Moreover, hot spots can occur in case of an unfavorable optical design of the transmission system. A careful design of coupling of optical energy into a fiber is necessary, such that the energy is actually coupled to the fiber without damaging the cladding or fiber jacket.

Fiber tips are of particular importance with regard to medical applications. A variety of design strategies can be found, among which, self-focusing tips and beam-shaping microdevices.

By the controlled melting of a fiber tip, a rounded surface at the fiber exit exhibiting focusing properties can be achieved (self-focusing tip).6 Figure 3a shows tips in various stages of melting, while in Figure 3b, light propagation is visualized in fluorescein together with ray-tracing modelling. As mentioned in the introduction, the focusing characteristics in air and water differ considerably. Optical breakdown though an optical fiber (Fig. 4) can be achieved in water, although the focusing characteristics are worse than in air.7 Since optical breakdown production is associated with short pulse/high power transmission, care has to be taken with respect to the entrance coupling and fiber quality.

Side-firing tips are of use in subconjunctival applications or when the eye is being accessed from

Endoscopy

An endoscope is a remote imaging device in which image transmission is a key element. Such instruments are widely used in medicine today; they can also be applied for special procedures in ophthalmology,9 such as imaging in the posterior chamber and the vitreous cavity, particularly in case of opacities of the media. An endoscope can either be made with the aid of a fiber bundle, which is advantageous whenever flexibility is required, for example, for inspection of the lacrimal duct. Imaging quality increases approximately quadratically with the diameter of such an endoscope, because the number of imaging fibers corresponds to the number of pixels in the image. If flexibility is not required, a GRIN rod is preferable, in particular in a small endoscope, due to its better image quality.

Figure 7a shows an ophthalmic GRIN endoscope. When the image quality of fiber bundles and GRIN endoscopes of the same diameter are compared, the superior image quality of the GRIN type becomes apparent (Fig. 8). Illumination is achieved by fibers arranged around the central GRIN rod (Fig. 7b); furthermore, an empty channel is provided in the tip

(not shown in the schematic drawing) in order to insert a fiber for therapeutic applications.

Inspection of the lacrimal duct is best carried out with the aid of a fiber bundle endoscope (Fig. 9), because flexibility is advantageous in this application (although the first results were obtained with a rigid endoscope).10

A further, presently still experimental, application of an imaging fiber bundle in ophthalmology consists of a portable slit lamp. With the aid of a fiber bundle, it is possible to project a slit of acceptable quality into a handheld camera which is brought into contact with the cornea (Fig. 10). However, a relatively expensive fiber bundle containing 100,000 individual fibers has to be used for this purpose, in order to achieve good results.

Selected clinical procedures

Coagulation, controlled destruction of cells, and perforations are typical procedures in which optical fibers can be used. Ab externo and ab interno applications have been proposed in ophthalmology, and contact and non-contact modes have been described.

Fig. 9. Endoscopic image of a lacrimal duct, obtained with a fiber bundle endoscope. (Reproduced by courtesy of Dr F.M. Sens, University Hospital Basel.)

Intraocular procedures

Intraocular treatments where fiber-based irradiation systems are used mainly consist of photocoagulation, sclerostomy,11,12 and trabeculoplasty.13,14 While photocoagulation consists of delivering optical energy such that coagulation of the blood is achieved and a blood vessel becomes permanently blocked, sclerostomy and trabeculoplasty are aimed at the removal of tissue by deposition of optical energy.

Intraocular photocoagulation with the aid of a fiber delivery system, for example, on the retina, is a routine procedure to date. As mentioned earlier, argon or frequency-doubled Nd:YAG (KTP) laser light is mostly used for this purpose because the relation of the absorption characteristics of the blood versus the surrounding tissue is particularly high in this range of wavelengths. Relatively unsophisticated and simple silica fiber systems and routine optics are utilized. In turn, sclerostomy is applied to improve

the outflow of chamber fluid in open-angle glaucoma patients by creating an artificial hole in the chamber angle. As the drainage improves, intraocular pressure decreases to normal levels. The aim of using laser energy for this purpose is the creation of a more stable channel compared to a rapidly healing puncture made mechanically.13,14 A similar aim can be seen in trabeculoplasty, in which the trabecular meshwork in the chamber angle is irradiated.

Nd:YAG laser pulses, typically delivered through a 200-300 µm fiber, are generally used for sclerostomy. An experimental procedure was proposed by Dürr et al.11 whereby a 200-µm silica fiber was inserted into a 20-22 gauge hypodermic needle, and advanced to the desired location (Fig. 11). Typically, up to 10 J were required to achieve a perforation in an excised pig eye (cw or free-running Nd:YAG laser). Moreover, intracanalicular trabeculostomy, with the aim of improving the outflow of chamber fluid using the Er:YAG laser (2940 nm) and a 300µm quartz fiber, has been evaluated experimentally by Kampmeier et al.8 The pulses were applied ab externo with the aid of a side-firing tip. The advantage of Er:YAG radiation is the high absorption in

water, such that minimal energy is required to achieve a therapeutic effect.15

For the same reason, the Er:YAG laser is suitable for cataract removal.16 However, special fibers are needed to transport the long wavelength of the Er: YAG laser with minimal loss. With the aid of zir- conium-fluoride-based and sapphire fibers, satisfactory lens emulsification has been reported.17

The intraocular application of laser light under endoscopic control18,19 is advantageous when treatment is necessary at a location that cannot well be visualized through the pupil, or in case of opaque media. Figure 12 outlines the experimental application of a combined endoscope/fiber system.

Transscleral procedures

The transscleral application of optical energy through optical fibers can be carried out in the contact or non-contact mode. In either case, the transmission characteristics of the sclera are decisive (Fig. 13).20 Furthermore, in the contact mode, transmission of the sclera is dependent upon the pressure with which the fiber tip (or exit optics) is applied.21,22 With

increasing pressure, transmission increases because of the rearrangement of fibers in the sclera, as well as because of water displacement.

Cyclodestruction and cyclophotocoagulation are procedures that are commonly applied in case of refractory glaucoma.23,24 The aim of treatment is to reduce the production of aqueous humor such that the intraocular pressure becomes lower. The Nd:YAG and diode lasers, in combination with fiber delivery systems, are most often used for this purpose. Yet, the successful application of krypton laser irradiation, delivered by way of optical fibers, has been reported as well.25,26

A further treatment where laser light and fiber delivery systems are involved is photodynamic therapy.27 By administration of a photosensitizer, usually photofrin, hematoporphyrin, or phthalocynaine derivatives, cells to be destructed are selectively perfused. Upon irradiation, these cells are deleted as the result of optically-induced oxygen reactions.28-30

Although it is not easy to reach sufficient illumination levels, transscleral illumination has a number of advantages: the corneal region remains free, disturbing reflections from the cornea and intraocular surfaces are absent, and an even distribution of light is achieved. The best results are obtained if a ring of light carrying fibers is applied at the limbal region.

Conclusions and outlook

A number of noteworthy applications of optical fibers in ophthalmology has been presented in this communication. However, this overview is not exhaustive, since other applications are conceivable or have also been made. Yet, the use of optical fibers is now universal and is part of many standard procedures. As, in particular, micro-optics, fibers and fiber bundles, diode lasers and xenon illumination techniques

become less expensive, easier to use, and associated with fewer maintenance needs, fiber-based applications in ophthalmology can be expected to become further developed and more widely used.

However, after having outlined a number of ophthalmic procedures in which the use of optical fibers is advantageous, it should be pointed out that some applications can be critical. For example, problems can arise whenever small optical (wavelength) shifts have to be measured. Doppler measurements are adversely influenced by a moving fiber because an artifactual shift of wavelength arises.31 Similarly, Raman or other types of spectroscopical analyses can become critical when optical fibers are used and motion artefacts cannot be avoided. Moreover, in case of high power transmission, fiber impurities or imprecise optical alignment of coupling optics are a possible source of malfunction, and particular care is required when such applications are being carried out.

In conclusion, research and development in the area of fiber applications in ophthalmology is proceeding and new application techniques can be expected. New types of fibers are being evaluated for their usefulness in medicine: hollow plastic and glass fibers,32 as well as fibers manufactured from special glasses33 (rare earth doped), may open new ways for the delivery of optical energy. And finally, the advent of fiber lasers34 may lead to simpler and less expensive instrumentation.

Acknowledgments

Many of the results presented in this article were obtained and worked out by Dr Pascal O. Rol, University Eye Clinic, University of Zurich, and Adjunct Professor, University of Miami, who lost his life in an aircraft accident near Zurich in January 2000.

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