Investigative Ophthalmology & Visual Science, Vol. 29, No. 4, April 1988 Copyright © Association for Research in Vision and Ophthalmology The Retinal Effects of Copper Vapor Laser Exposure Shimon Gabay.t Israel Kremer,* Isaac Ben-Sira,* Dan Sagie,t Dov Weinberger,* and Gideon Erezf The copper vapor laser is a pulsed gas laser which emits energy in two wavelengths simultaneously: 510.6 nm (green) and 578.2 nm (yellow). Each pulse has a duration of 15 nsec, maximal energy of 3 mJ and a peak power of more than 100 kW. It is a variably high repetition rate laser, in the range between 1 kHz and more than 20 kHz. We studied its interaction with the rabbit retina, while using two different repetition rates, 4 kHz and 18 kHz. The histological analysis of the lesion produced by 4 kHz repetition rate showed undesired retinal effects, similar to those caused by other pulsed lasers. On the other hand, the histological examination of the lesion produced by the 18 kHz repetition rate showed a desired coagulation effect, limited to the outer retinal layers, and comparable to a lesion produced by a continuous wave (CW) laser. Invest Ophthalmol Vis Sci 29:528-533, 1988 Lasers in ophthalmology are used mainly for reti- above 20 kHz, delivering two wavelengths: 510.6 nm nal coagulation, iridectomy and lens capsulotomy. and 578.2 nm.10 The pulse peak power can be in- Argon laser is now widely in use as a retinal photo- creased by decreasing the repetition rate, while the coagulator,12 as it is well-transmitted by the ocular average power has an optimal level at intermediate media3 and well-absorbed by the retinal pigment epi- repetition rate. As irradiance is a function of the laser thelium (RPE) and hemoglobin.4 Since coagulation is focal spot size, the mechanical effects of the CVL can the final result of a thermal effect,5 a continuous wave be increased by decreasing the focal spot size to 50 (CW) laser, such as the argon laser, is a suitable light j^m. Therefore, it is hoped that the CVL will be suit- source for coagulation. Capsulotomy and membrane able for iridectomy as well as capsulotomy, a fact cutting can be done only by a pulsed laser character- which will have to be proven in future experiments. ized by a high peak power, inducing mechanical ef- By increasing the repetition rate and the spot size, the fects.6"8 pulse characteristics are changed toward CW mode, The question is whether one laser system can per- which is suitable for retinal coagulation without any form all the above mentioned ocular treatments suc- undesired mechanical damage. cessfully. A CW laser which is effective for retinal Retinal coagulation is caused by a thermal effect,5 coagulation cannot be suitable for capsulotomy, which depends on the temperature rise in the tissue. where there is a need for very high irradiance. On the The temperature rise depends not only on the energy other hand, a pulsed laser which fits the capsulotomy absorbed by the RPE but also on the exposure time. requirements may cause undesired retinal damage by The CVL pulse width is only 15 nsec; therefore, in mechanical effects. The answer should therefore be a such a short time, the volume of absorbing tissue can variably high repetition rate pulsed laser, which in- be regarded as an isolated volume from which the herently has the characteristics of a pulsed laser, but heat diffusion is negligible. Hence, exposure of the can at the same time act as a quasi-CW laser, de- retina to the CVL beam can instantaneously elevate pending on the pulse energy, the time interval be- the tissue temperature to a higher degree than can a tween the pulses and the reaction time constant. Such CW laser, while delivering the same energy.1011 a laser system is the copper vapor laser (CVL). This Moreover, since the retina was exposed to a burst of laser is a high repetition rate pulsed laser9 which can several hundreds of pulses, the net result depends on operate at a repetition rate range between 1 kHz and the combination between the single pulse energy and the total exposing energy."12 This total energy can be equally delivered either by a high pulse energy com- bined with a low repetition rate or by a low pulse From the *Department of Ophthalmology and Laboratory of Ophthalmic Pathology, Beilinson Medical Center, Petah Tiqva, Tel energy combined with a high repetition rate. Aviv University Sackler School of Medicine, and the fNudear As the CVL is inherently a pulsed laser with a vari- Research Center, Beer Sheva, Israel. ably high peak power, we decided to study its retinal Presented at the annual ARVO Meeting, Sarasota, Florida, May coagulation ability first in order to look for any un- 4-8, 1987. desired mechanical side effects. In this work, we ex- Submitted for publication: June 15, 1987; accepted October 27, 1987. amined the interaction between the copper vapor Reprint requests: I. Kremer, MD, Department of Ophthalmol- laser beam and the rabbit retina, in order to learn the ogy, Beilinson Medical Center, Petah Tiqva 49 100, Israel. safest range of peak power, pulse repetition rate and 528 Downloaded from iovs.arvojournals.org on 09/26/2021 No. 4 RETINAL EFFECTS OF COPPER VAPOR LASER / Goboy er ol. 529 Fig. 1. A diagram illustrating the system components which control the laser power, exposure time and beam spot size. total exposure energy which will be suitable for reti- exposure time and beam spot size. The laser beam is nal coagulation. delivered from the laser cavity (1-3) towards the rab- bit eye, passing through an attenuator (4), which Materials and Methods controls the output power, then meets a narrowing telescope (5, 6) and falls upon a shutter (7). The shut- Copper Vapor Laser ter reflects more than 99% of the light to a power The copper vapor laser used in our experiments is meter (8), which sends a signal to a microprocessor an air-cooled laser which was designed and con- that presents the laser power measured at the power structed by Laser-On (affiliate of Rotem Industries, meter (or the calibrated power at the cornea). The Nuclear Research Center, Beer Sheva, Israel). This is remaining power is used as an aiming beam, which a self-terminating pulsed laser, the emission of which passes through dual polarizing sheets to control the is simultaneously composed of two wavelengths, aiming beam intensity. Opening the shutter for the 510.6 nm (green) and 578.2 nm (yellow). This laser desired exposure time enables full power delivery to emits up to 8 W average power at 4 kHz repetition the retina. The beam then passes through an articu- rate, at a ratio of 2:1 between green and yellow, re- lated arm (9) connected to a Haag Streit (Bern, Swit- spectively. The laser pulse width is about 15 nsec and zerland) Universal Slit Lamp, model BM 900(12). A the beam diameter is 30 mm. An unstable resonator movable focussing lens (10) is placed in such a way was constructed to decrease the beam divergence in that in its backward position the laser beam focusing order to achieve a small focused spot size. The pulse plane coincides with the slit lamp illumination focus- repetition rate.can be varied between 2 kHz and 20 ing plane. In this position, the laser spot on retina is kHz. The two laser wavelengths are well-transmitted minimal. Increasing the laser spot on the retina can by the ocular media and well-absorbed by the RPE be achieved by shifting the lens toward the slit lamp. and hemoglobin, but not absorbed by the xanto- The beam is then reflected by a prism (11) along the phyll.13 Therefore, this laser can be safely used at the slit lamp axis, then passes through the slit lamp out- macular region. Moreover, the yellow wavelength put lens (13) and finally is reflected again by a mirror which is suggested to be the ideal wavelength for reti- (14) towards the retina. The stereo biomicroscope 1415 nal photocoagulation gives this laser an advantage (15) is used to fix the slit lamp focal plane on the over other lasers currently in use. Finally, a compact retina. The filter (16) is positioned in front of the copper vapor laser of about 8 W average power has biomicroscope, prior to full power laser exposure, in low installation requirements of about 2.5 kW elec- order to prevent damage to the physician's eyes by a trical power (single phase) and 2 liter per minute tap scattered laser light. water cooling or even air cooling. The latter charac- All the optical components of the system are teristics make it convenient for installation even in achromates, in order to focus the two wavelengths small eye clinics. (green and yellow) to one small focal point on the retina. The laser control system consists of a micro- Delivery System processor which automatically controls the laser operation, governs the treatment parameters and The diagram illustrated in Figure 1 presents all the takes care of the safety requirements during the ex- system components which govern the laser power, posure process. Downloaded from iovs.arvojournals.org on 09/26/2021 530 INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / April 1988 Vol. 29 ^.v^V • • • • y * *~ % ^w.^.-1 Fig. 2. Complete disruption of photoreceptor outer segments, Fig. 4. Exudative choroidal detachment (arrow) accompanied by severe vacuolar degeneration of RPE cells and a small choroidal subretinal and intraretinal exudation. Severe disruption of the pho- hemorrhage (arrow). Methylene blue, original magnification X400.