Chapter 7 – Principles of Lasers
NEAL H. ATEBARA
EDMOND H. THALL
• “Laser” is an acronym for light amplification by stimulated emission of radiation.
• Lasers have had a greater impact on ophthalmology than on any other medical specialty, largely because the transparent nature of the ocular tissues allows laser light to reach many parts of the eye noninvasively.
• In modern ophthalmology, lasers are used to treat a wide range of ocular conditions, and their uses are some of the most commonly performed procedures in medicine.
• Lasers are also important in a growing number of diagnostic studies that promise to significantly enhance our understanding and clinical management of many disease processes in the eye.
The word laser is an acronym for light amplification by stimulated emission of radiation, a term coined by Gordon Gould while a graduate student at Columbia University. Stimulated emission of a photon of electromagnetic radiation is the basic physical principle that makes lasers possible. This process was first predicted theoretically by Albert Einstein in 1917, but for many years it was believed that putting this theory into practice was not possible.
In the 1950s, Charles Townes first produced microwaves (radiowaves) using stimulated emission. His work proved that it was possible to produce electromagnetic radiation using stimulated emission. Once this had been established, several groups sought ways to produce shorter wavelengths of electromagnetic radiation (e.g., visible light) by stimulated emission, and in 1960 the first laser was built by Theodore Maiman using a ruby crystal medium.
The first laser system used in ophthalmology utilized a pulsed ruby laser coupled with a monocular direct ophthalmoscopic delivery system, first reported in 1961. It was used to treat retinal breaks and proliferative diabetic retinopathy. In 1968, L’Esperance developed the argon laser, which was technically superior to the ruby laser. The argon laser continues to be one of the types of lasers most frequently used in ophthalmology today.
In order to understand how lasers work, one must first review some basic physical principles, including the nature of photons, the nature of atoms, and how photons and atoms interact. Light may be viewed as being comprised of individual “wave packets” called photons. Each photon has a characteristic frequency, and its energy is proportional to its frequency. Thus, a photon of blue light carries more energy than a photon of red light.
Figure 7-1 Simple model of a helium atom at a single point in time. Electrons orbit a nucleus of protons and neutrons. Each orbit has a unique energy. The two electrons in this case occupy two different orbits. By gaining or losing energy the electrons can move to other, currently empty, orbits.
Although an atom may superficially resemble a miniature solar system, with negatively charged electrons that orbit a positively charged nucleus, there are significant differences. In our solar system, each planet stays in one stable orbit, whereas in an atom the electrons are capable of jumping between different orbits. Further, in our solar system, a planet may theoretically have any energy, but electron orbits are strictly constrained to discrete energy levels.
Figure 7-1 simplistically depicts a helium atom at one particular instant showing the two electrons that orbit the nucleus and several other empty orbits that electrons could occupy. Each orbit has a unique energy, and to jump from one orbit to another an electron must either gain or lose energy. The amount of energy gained or lost by an electron when it changes orbits equals the energy difference between the two orbits. An atom is extremely dynamic, with electrons constantly absorbing and emitting photons and changing orbits.
The three basic ways for photons and atoms to interact include: (1) absorption, (2) spontaneous emission, and (3) stimulated emission. An electron can absorb a passing photon and jump into a higher energy orbit. Absorption occurs only if the photon’s energy exactly matches the difference in energy between the two electron orbits. Absorption begins with a photon and a low-energy electron and yields a higher energy electron with the elimination of the photon.
In spontaneous emission, an electron in a high-energy state spontaneously drops into a lower energy state and, in the process, creates a photon. The photon created has an energy equal to the difference in energies between the two electron orbits. Spontaneous emission begins with a high-energy electron and yields a photon and a low-energy electron.
Spontaneous emission is a random process. At any moment, an electron in a high-energy state may drop into a lower state with the emission of a photon. Generally, electrons spend only a few nanoseconds in the high-energy state before this emission
Figure 7-2 Three basic interactions between light (photons) and electrons. An electron absorbs a photon, which forces it to move to a higher energy orbit. In spontaneous emission a photon is produced by the electron, which then “falls” to a lower energy orbit. In stimulation, a photon passes by an electron and stimulates it to “fall” into a lower energy orbit and produce a second photon, coherent with the first figure.
occurs. Some high-energy states, however, are metastable. Electrons linger in these states for a lengthy few milliseconds before spontaneous emission occurs.
In stimulated emission, a photon passes in the vicinity of a high-energy electron. The photon stimulates the electron to emit a photon and drop into a lower state. The stimulating photon must have an energy equal to the energy difference between the two electron orbits. Stimulated emission begins with a photon and a high-energy electron and yields two photons and a low-energy electron.
Stimulated emission is not a random process. The electron drops at the moment a passing photon stimulates the electron to drop and emit a photon. Importantly, the stimulating photon and the emitted photon will be identical in frequency and phase. In other words, the two photons are coherent. Many ways exist to produce light, but stimulated emission is the only method known that produces coherent light, a property with many practical applications. Figure 7-2 illustrates these three processes—absorption, spontaneous emission, and stimulated emission.
HOW LASERS WORK
Gas lasers are the most widely used lasers in clinical ophthalmology. Atoms of the working gas, such as argon or krypton, are enclosed in a cylindrical tube, called the laser cavity ( Fig. 7-3 ). Under natural conditions there are more electrons in lower energy orbits than higher energy orbits. Eventually one of the high-energy electrons undergoes spontaneous emission, generating a photon. If this photon first encounters a low-energy electron (which is much more common at this point), it is merely absorbed. However, in the event that it encounters another high-energy electron, stimulated emission occurs.
To sustain a large number of stimulated emissions, there must be more electrons in high-energy states than low-energy states, a condition called population inversion. To produce a population inversion in the gas laser, the gas is pumped by a powerful light source or by an electric discharge that forces electrons to go into high-energy states.
Merely achieving a population inversion is not sufficient; it must be maintained, because most high-energy states decay in a few nanoseconds by spontaneous emission. However, when electrons are pumped into a metastable state the population inversion may be maintained for a longer period of time. With the majority of electrons in a high-energy metastable state, a photon generated by spontaneous emission is now more likely to produce a stimulated emission instead of merely being absorbed. The two coherent photons generated by a stimulated emission go on to produce more stimulated emissions, and a chain reaction begins.
In order to maintain the chain reaction of stimulated emissions, mirrors are placed at each end of the cavity, an arrangement called a resonator.  One mirror reflects totally and the other partially (see Fig. 7-3 ). Most of the coherent light generated is reflected back into the cavity to produce more stimulated emissions.
Figure 7-3 Gas laser. A typical design consists of a gas-filled cavity, external optical pumping lights, and a resonator that comprises partially and totally reflecting mirrors. Without optical pumping, most of the gas atoms are in lower energy states and incapable of undergoing either spontaneous or stimulated emission. With optical pumping, photons from the external lights are absorbed by the gas atoms, which raises the energy of the atoms and makes them capable of undergoing spontaneous or stimulated emission. Ultimately, the majority of atoms are in excited states—a population inversion. One of the higher energy atoms spontaneously emits a photon that produces stimulated emissions as it passes by other high-energy atoms. By reflecting the photons back and forth across the cavity multiple times, a chain reaction of stimulated emissions is produced.
The relatively small amount of light that is allowed to pass through the partially reflecting mirror produces the actual laser beam.
CONTINUOUS AND PULSED LASERS
Lasers emit light either continuously or in pulses. Although a pulsed laser produces only modest amounts of energy, the energy is concentrated into very brief periods, and so each pulse has a relatively high power (power is energy per unit time). Neodymium:yttrium–aluminum–garnet (Nd:YAG) and excited dimer (excimer) lasers are examples of pulsed lasers.
A continuous laser modality delivers more overall energy to a target tissue, but it does so over a relatively long time; thus the power is lower. Because clinical applications do not generally require high power (usually less than one watt), most ophthalmic lasers operate continuously with a shutter to control the specific exposure time and thereby allow more control over the energy delivered to the target tissue. Argon lasers, krypton lasers, diode lasers, and dye lasers are all examples of continuous laser modalities.
WHAT COLOR IS YOUR LASER?
The number of optical wavelengths that can be produced by lasers is rather limited, dependent on the particular metastable state of the working material. For instance, in the krypton ion, when an electron drops from its metastable state to a lower energy level, it produces light with a wavelength of 647?nm (which corresponds to red light). Using different nonmetastable states, the krypton ion can produce several other wavelengths, but only at significantly lower powers. For practical reasons, only krypton lasers that operate at the 647?nm wavelength are available commercially. The argon ion has two metastable states, and it therefore produces two prominent wavelengths of light at 488?nm and 514?nm, which correspond to blue–green and green, respectively. Most commercial argon lasers allow the clinician to select either the green 514?nm light or a mixture of blue–green 488?nm and green 514?nm light.
Some laser procedures demand peak wavelengths that do not correspond to the metastable state of any conventional working material. For instance, in the treatment of macular choroidal neovascularization using photocoagulation, xanthophyll pigment in the macula absorbs a significant amount of laser light, thereby increasing the risk of damage to the neurosensory retina and decreasing the amount of energy delivered to the abnormal blood vessels below. Xanthophyll pigment transmits light best at 577?nm, but it is difficult to generate this wavelength with lasers.
Two ways exist to increase the number of available wavelengths: harmonic generation and employing organic dyes. In harmonic generation, laser light is passed through an optically nonlinear crystal, which doubles its wave frequency. When light traverses any medium, a small amount of the light is absorbed. Typically, the absorption is linear in the sense that doubling the light intensity doubles the amount of energy absorbed. In a nonlinear medium, doubling the intensity does not double absorption; it increases it, by perhaps fourfold or more. Laser light causes such nonlinear crystals to vibrate, not only at the laser’s frequency, but also at exact multiples of the laser’s frequency, called harmonics. For instance, the middle A note on a piano has a frequency of 440 cycles per second; its harmonics include 880, 1320, 1760 cycles per second, and so on. Generally, this method of creating new wave frequencies is quite inefficient, and the harmonics generated have very low power. However, a nonlinear crystal has been found that efficiently doubles the frequency of the 1064?nm output of an Nd:YAG laser, producing a 532?nm wavelength, which is relatively close to the transmission window of xanthophyll (577?nm).
Another method used to produce more wavelengths employs organic dyes. As a result of their complex chemical structure, organic dyes provide a large number of metastable orbits that differ little in energy, so a variety of different wavelengths are available. Dye lasers may be tuned to the desired wavelength, allowing
Figure 7-4 Wavelengths of light produced by the more commonly used ophthalmic lasers and where these wavelengths lie on the electromagnetic spectrum.
clinicians to select the optimum wavelength for each procedure. For example, the organic dye laser based on rhodamine 6?G can be tuned continuously from 570?nm to 630?nm. The drawbacks of dye lasers are that they are the least efficient producers of laser energy and most expensive to manufacture. In fact, current tunable dye lasers utilize an argon laser to pump energy into the fluorescent dye. This complex system of two lasers increases the manufacturing cost and the likelihood of mechanical failure.
Figure 7-4 shows the different wavelengths of light produced by the more commonly used ophthalmic lasers and where these wavelengths lie on the electromagnetic spectrum.
CLINICAL USE OF LASERS
Notwithstanding the planet-destroying capabilities of the lasers depicted in Hollywood movies, real-life lasers are not death rays. Although lasers have been used to target or guide military weapons, no laser has become an effective weapon in its own right despite years of research. In fact, lasers are not particularly powerful (a penlight produces more light than any clinical laser) or efficient (it may require thousands of watts of power to produce a mere 1–2?W of laser light). But despite low power and inefficiency, lasers are very useful, because they produce such a highly concentrated focus of coherent light.
Effective clinical use of lasers requires an understanding of the three basic light–tissue interactions as follows: (1) photocoagulation, (2) photodisruption, and (3) photoablation. In photocoagulation, laser light is absorbed by the target tissue or by neighboring tissue, generating heat that denatures proteins (i.e., coagulation). Clinical examples of photocoagulation include panretinal photocoagulation, argon laser trabeculoplasty, peripheral iridectomy, and thermal destruction of choroidal neovascular membranes. Types of lasers which produce photocoagulation include the argon green laser (514?nm), the argon blue–green laser (488?nm), the krypton red laser (647?nm), the ruby red laser (694?nm), the diode laser (810?nm near infrared), the rhodamine 6?G organic tunable dye laser (570–630?nm yellow to red), and the frequency-doubled Nd:YAG laser (532?nm green).
Photodisruption is largely a mechanical effect. Highly focused laser light produces an optical breakdown, which is basically a miniature lightning bolt. Vapor formed by the lightning bolt expands, quickly collapses, and produces a miniature thunder clap. Acoustic shock waves from the thunder clap cause most of the tissue damage. The principal example of photodisruption is the posterior lens capsulectomy produced by the Nd:YAG laser (1064?nm infrared).
Photoablation breaks the chemical bonds that hold tissue together—essentially vaporizing the tissue. Photorefractive keratectomy, using the argon fluoride (ArF) excimer laser (193?nm ultraviolet), is an example. In photoablation, chemical bonds are broken by the absorption of photons, without any external physical pressure. Because of this, the laser is able to remove tissue with more precision and with much less damage to surrounding tissue than even the sharpest surgical scalpel.
Although exceptions to the rule exist, the wavelength produced by a laser generally determines which of the three types of light–tissue interaction will occur. Visible wavelengths produce photocoagulation, ultraviolet yields photoablation, and infrared is used in photodisruption or photocoagulation.
CLINICAL USE OF LASER PHOTOCOAGULATION
The most commonly performed laser procedure in ophthalmology is photocoagulation. In the posterior segment, photocoagulation is used to treat numerous conditions such as proliferative diabetic retinopathy, diabetic macular edema, choroidal neovascularization secondary to age-related macular degeneration, retinal breaks, and retinal detachments. In the anterior segment, photocoagulation is used to perform iridoplasty, iridectomy, trabeculoplasty, and cyclophotocoagulation.
In photocoagulation the surgeon controls the exposure time, the power, and the spot size. It is important that the surgeon has a clear understanding of how these parameters affect the lesions produced. An increase in exposure time (while all other parameters are maintained constant) modestly increases the lesion’s diameter. A tenfold increase in exposure time roughly doubles lesion diameter. Longer exposure time also extends the damage deeper into the target tissue. Very brief exposure times, of the order 0.01–0.05 seconds, allow little time for heat to dissipate from the burn. A small area of intense damage may be produced, resulting in the perforation of delicate ocular structures such as Bruch’s membrane or the neural retina.
An increase in power has a strong influence on lesion diameter. In fact, doubling the laser power almost doubles the size of lesion created. Such increases in laser power create more damage and can be painful to the patient. This can be avoided in some cases by increasing the exposure time rather than laser power.
Careful control of laser spot size is important in order to achieve the desired therapeutic effect. When working in sensitive areas such as the macula, a small spot size (such as 100?µm) is preferred so as to minimize unnecessary damage to adjacent retinal tissue. In contrast, treatment of broader areas of tissue is facilitated by a larger spot, such as a 200–500?µm spot size for panretinal photocoagulation.
Laser contact lenses may also affect the spot size. Whereas the Goldmann three-mirror lens will increase spot size only by a factor of 1.08, the Panfundoscope lens has a multiplication factor of 1.41. The Mainster wide-angle lens multiplies spot size by a factor of 1.47, and the QuadrAspheric lens by a factor of 1.92. If the spot size is increased, power needs to be increased, as well. However, because energy is concentrated in the center of the beam, it is best to raise power only modestly (no more than twofold at a time) and to use test burns to refine the power setting.
Figure 7-5 Energy distribution in a typical clinical laser. Notice that the energy is concentrated in the center of the beam.
COMPLICATIONS FROM LASER PHOTOCOAGULATION
Laser Beam Profile
It is important for the laser surgeon to be aware of the laser beam profile, the way light energy is distributed over the beam’s cross-section. In most photocoagulating lasers, the energy is concentrated in the center of the beam, with less energy at the edges ( Fig. 7-5 ). Therefore, if excessive power is used during laser treatment of the retina, the center of the laser beam may cause water vaporization, possibly resulting in an inadvertent retinal hole. Also, the lower energy at the periphery of the laser beam may produce permanent tissue damage even though it does not produce a visible reaction. It is therefore important to realize that the area of laser damage may extend beyond the area of immediately visible reaction.
Effect of Off-Axis Astigmatism on the Laser Beam
When the peripheral retina is being treated, the aiming beam takes on an elliptical shape as a result of off-axis astigmatism. An elliptical laser beam profile has a higher power density, and this may result in complications such as inadvertent retinal holes. Tilting the lens will counteract this effect, producing a round laser beam and a more even laser beam profile.
Treatment of the Peripheral Retina
The retina thins in the periphery and, consequently, less power is required to treat this area. Laser photocoagulation of the peripheral retina is often more painful for the patient, especially in the horizontal meridians where the ciliary nerves enter the globe. Longer exposure times, lower laser power, or the administration of a local anesthetic may help minimize patient discomfort.
Photocoagulation in the Retinal Posterior Pole
Brief exposure times minimize the risk of inadvertent damage to the fovea that may arise from patient motion. Even when retrobulbar anesthesia is administered in order to produce ocular akinesia, a short exposure time is important because the patient is still capable of moving his or her head, thereby jeopardizing the procedure.
Photocoagulation Treatment of Macular Edema
Where the retina is thickened from edema more power is necessary to achieve a burn, because the retina is farther from the light-absorbing pigment in the retinal pigment epithelium (RPE) and choroid. The clinician must recognize the subtler, deeper burn that is the end point for focal treatment in patients who have macular edema.
CLINICAL USE OF THE YTTRIUM–ALUMINUM–GARNET LASER
The Nd:YAG laser works on the principle of photodisruption. Light is a type of electromagnetic field, and such fields produce forces on charged particles, including electrons. Typically, light energy causes electrons to oscillate as they travel around their nuclei. Extremely high electromagnetic field strengths, from a laser for example, can actually strip electrons from their nuclei, producing an entirely different physical state of matter called plasma.
Where the plasma forms, the chemical nature of the material is destroyed. The orderly array of molecules is fractured into a random mixture of electrons and protons in a process called optical breakdown. A similar effect occurs when a powerful electric field turns air into a plasma, forming a lightning bolt. After the high-intensity laser light passes, electrons and nuclei reunite, and the plasma collapses. An acoustic shock wave, analogous to a thunder clap, is produced. This acoustic shock wave is responsible for most of the physical damage to the ocular tissue produced by the Nd:YAG laser.
Optical breakdown requires electromagnetic fields so powerful they can be produced only by concentrating laser energy into very brief periods, thereby giving each pulse an extremely high power level. There are two ways of pulsing an Nd:YAG laser: Q-switching and mode locking. In Q-switching, a shutter in front of one of the mirrors in the laser cavity blocks laser light emission until a large population inversion has been established. The shutter is opened quickly, and the stored energy bursts forward in the form of a brief pulse that lasts about one millionth of a second. Q-switching is comparatively inexpensive and reliable but cannot produce pulses as short or powerful as mode locking.
In mode locking, electromagnetic energy in the laser cavity exists in various modes that depend on the length of the laser cavity and the construction of the resonator mirrors. In mode locking an optical element (Fabry–Perot interferometer) inside the cavity synchronizes the modes so all the light is emitted in extremely brief pulses. Most clinical Nd:YAG lasers today are Q-switched, because mode-locked lasers are more expensive and difficult to maintain.
The light produced by the Nd:YAG laser is infrared (1064?nm), invisible to the clinician, so an ancillary aiming system, typically a red helium–neon (HeNe) laser, is necessary. Just as a prism causes blue light to bend more than red light, the optics of the patient’s eye cause the red aiming beam to bend more than the Nd:YAG’s infrared light. Consequently, the focus of the Nd:YAG laser rarely coincides precisely with the focus of the aiming beam ( Fig. 7-6 ). Some lasers have an adjustment to compensate for this source of error, called chromatic aberration.
Performing an Nd:YAG laser capsulectomy in an eye with a silicone intraocular lens implant poses a particular challenge, because optical breakdown occurs in silicone at relatively low power. Therefore, damage to the intraocular lens may occur even when the laser is focused posterior to the implant. And if the capsule is in intimate contact with the implant, creating the capsulotomy is even more difficult. A corneal contact lens designed for laser capsulotomy causes the Nd:YAG laser beam to converge at a steeper angle, resulting in a more sharply focused beam of
Figure 7-6 The focus of the infrared YAG light usually does not coincide with the aiming beam’s visible red light.
light. This makes optical breakdown at sites outside the focal point less likely. Intraocular lenses made of polymethylmethacrylate (PMMA) and acrylic are less susceptible to optical breakdown than silicone lenses.
CLINICAL USE OF LASER PHOTOABLATION
Photoablation is the most recent light–tissue interaction to be exploited clinically. It is used to treat corneal pathology such as ulcers and scars, and its use in keratorefractive surgery has become a rapidly evolving field. The argon–fluoride (ArF) excimer laser produces electromagnetic energy with a wavelength of 193?nm, in the extreme ultraviolet. With each pulse of the excimer laser, a large area of the cornea is ablated. With such a large area of tissue being treated at a time, it is important to have a uniform beam profile. When the excimer beam initially emerges from the laser cavity it has a Gaussian profile, with energy concentrated more heavily in the center of the beam. Beam-shaping optics are then used to create an even beam profile. However, these beam-shaping optics are, themselves, ablated slowly by the ultraviolet laser beam, and they must be replaced periodically.
Although the excimer laser removes approximately 0.1?mm of corneal tissue on average with each pulse, irregularities in the corneal stroma and the presence of keratocytes in the stroma may cause uneven ablation, even when the beam profile is uniform. Further, corneal collagen density varies with a number of factors, including altitude, atmospheric pressure, relative humidity, patient age, and duration of the procedure. Fortunately, the microscopic irregularities that may be produced due to patient-specific corneal factors do not appear to affect final visual acuity to a great extent.
In older techniques of corneal ablation, the entire anterior surface of the cornea—including the epithelium and Bowman’s membrane—would be ablated. In laser-assisted in situ keratomileusis (LASIK), a partial lamellar incision through the anterior corneal stroma is performed using a keratome. The anterior corneal surface is then temporarily displaced, exposing the corneal stroma to excimer laser treatment ( Chapter 20 ). In this manner, the corneal surface contour may be reshaped by controlled removal of stromal tissue by the excimer laser, while keeping the corneal epithelium and Bowman’s membrane intact.
Photodynamic therapy (PDT) is a new laser technique for treatment of choroidal neovascularization and various tumors in the eye. In this technique, a special laser-activated dye is used to cause damage selectively to abnormal blood vessels, while minimizing damage to the nearby retinal tissue.
A photosensitizing dye such as verteporfin is first injected intravenously into the patient. The dye preferentially accumulates in neovascular and neoplastic tissue, possibly because the dye binds with low-density lipoproteins (LDL). Cells with high mitotic activity, such as neovascular and neoplastic tissue, have high expression of LDL receptors, and the LDL-bound dye may thereby be taken in the cells via receptor-mediated endocytosis.
The dye in and of itself causes no significant damage to the neovascular tissue, but upon activation with wavelength-matched laser energy, PDT-mediated tissue destruction occurs by several mechanisms. Direct destruction to cell membranes occurs via lipid peroxidation and protein damage through oxygen and free radical intermediates. Vascular endothelial damage causes platelet aggregation and vessel thrombosis. Direct damage to nuclear components, as well as apoptotic mechanisms, have also been reported. In tumors, PDT may produce increased levels of cytokines, enhancing killing activity of cytotoxic T-lymphocytes.
Verteporfin was approved recently by the Food and Drug Administration for treatment of choroidal neovascularization secondary to age-related macular degeneration and myopic degeneration. In these conditions, verteporfin at a dosage of 6?mg/m2 of patient body surface area is infused intravenously over 10 minutes. After allowing 5 minutes for the dye to accumulate in the neovascular tissue, laser light at a wavelength of 689?nm is then applied for 83 seconds at an intensity of 600?mW/cm2 , to give a total light dose of 50?J/cm2 .
The treatment of age-related macular degeneration with photodynamic therapy (TAP) investigation determined that verteporfin PDT reduces the risk of moderate vision loss, defined as a loss of at least three lines of visual acuity, compared with a placebo sham treatment, with 2 years of follow-up data.  However, patients often require repeat treatments after 3–4 months due to a recurrence of leakage from the choroidal neovascularization, as determined by fluorescein angiography. Patients must also avoid direct sunlight exposure for up to 5 days after injection with verteporfin, because sunlight can activate the dye, possibly resulting in PDT-mediated damage to the skin and retina. With more clinical experience in the use of PDT, our indications for its use in other ocular conditions will undoubtedly expand.
DIAGNOSTIC USE OF LASERS
Lasers also have several important diagnostic applications, including scanning laser ophthalmoscopy and optical coherence tomography.
Scanning Laser Ophthalmoscopy
In the scanning laser ophthalmoscope (SLO), developed by Robert Webb and George Timberlake, a narrow laser beam illuminates the retina one spot at a time, and the amount of reflected light at each point is measured. As with natural light, the amount of light reflected back to the observer depends on the physical properties of the tissue which, in turn, define its reflective, refractive, and absorptive properties. Media opacities, such as retinal hemorrhage, vitreous hemorrhage, and cataract, also affect the amount of light transmitted back to the observer. Because the SLO uses laser light, which has coherent properties, the retinal images produced have a much higher image resolution than conventional fundus photography. Also, because only a single point on the retina is illuminated at any given time, the patient is not affected by a bright illumination source or flash. This allows for high-resolution, real-time motion images of the macula without patient discomfort.
Current SLOs use a single wavelength of laser light, so the images produced are monochromatic. SLO angiography can be performed after intravenous injection of fluorescein or indocyanine green in order to study retinal and choroidal blood flow. By varying the brightness of the scanning laser beam, the scanning
Figure 7-7 A, Cross-sectional image of the macula using optical coherence tomography. There is sufficient resolution on this scan to demonstrate the various retinal layers, as depicted in the correlative illustration (B).
laser ophthalmoscope also may be used to perform microperimetry, an extremely accurate mapping of the macula’s visual field.
Optical Coherence Tomography
Optical coherence tomography (OCT) uses diode laser light in the near-infrared spectrum (810?nm) to produce high-resolution cross-sectional images of the retina using coherence interferometry. In coherence interferometry, a partially reflective mirror is used to split a single laser beam into two, the measuring beam and the reference beam. The measuring beam is directed into the patient’s eye and onto the retina. Because many of the retinal layers are transparent to near-infrared light, the laser beam passes through the neurosensory retina to the RPE and the choriocapillaris. At each optical interface, some of the laser light is reflected back to the OCT’s photodetector.
The reference beam, on the other hand, is reflected off of a reference mirror at a known distance from the beam splitter, back to the photodetector. The position of the reference mirror can be adjusted to make the path traversed by the reference beam equal to the distance traversed by the measuring beam to the retinal surface. When this occurs, the wave patterns of the measuring and reference beams are in precise synchronization, resulting in constructive interference. This appears as a bright area on the resulting cross-sectional image. However, some of the light from the measuring beam will pass through the retinal surface and will be reflected off deeper layers in the retina. This light will have traversed a longer distance than the reference beam, and when the two beams are brought back together to be measured by the photodetector, some degree of destructive interference will occur, depending on how much further the measuring beam has traveled. The amount of destructive interference at each point measured by the OCT is translated into a measurement of retinal depth and graphically displayed as the retinal cross-section.
OCT images are displayed in false color to enhance differentiation of retinal structures. Bright colors (red to white) correspond to tissues with high reflectivity, whereas darker colors (blue to black) correspond to areas of minimal or no reflectivity. The OCT can differentiate structures with a spatial resolution of only 10?µm ( Fig. 7-7 ).
In a relatively brief period, lasers have evolved from an obscure research novelty to an invaluable clinical instrument. The continual refinement of existing laser types, as well as the introduction of new laser technology, mark this area of ophthalmology as one of its most energetic and dynamic fields. The role of lasers in clinical ophthalmology has expanded continually, and this trend will doubtless continue.
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