Optical coherence tomography.

A technique called optical coherence tomography (OCT) has been developed for noninvasive cross-sectional imaging in biological systems. OCT uses low-coherence interferometry to produce a two-dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. OCT has longitudinal and lateral spatial resolutions of a few micrometers and can detect reflected signals as small as approximately 10(-10) of the incident optical power. Tomographic imaging is demonstrated in vitro in the peripapillary area of the retina and in the coronary artery, two clinically relevant examples that are representative of transparent and turbid media, respectively.

Unscheduled DNA synthesis following excimer laser ablation of the cornea in vivo.

The amount of unscheduled DNA synthesis (UDS) produced in the cells adjacent to excimer laser ablations in the cornea was compared for 193-nm and 248-nm laser wavelengths. UDS is interpreted to indicate the process of excision repair of pyrimidine dimers formed in DNA. 193-nm laser ablation did not produce a statistically significant difference in the amount of UDS as compared to a negative control (diamond-knife corneal incision). However, 248-nm laser ablation did produce a highly statistically significant difference in the amount of UDS as compared to both the negative control (P < 0.001) and the 193-nm laser irradiation (P < 0.001). Other forms of DNA damage (single-strand DNA chain breaks and DNA-protein crosslinks) are not measurable by UDS and need to be investigated in the evaluation of the oncogenic potential of 193-nm laser ablation.

High-speed photography of Er: YAG laser ablation in fluid. Implication for laser vitreous surgery.

The mechanism of Er:YAG laser-induced long-range damage in intraocular surgery was investigated using high-speed photography. A short pulse of 2.94-microns radiation delivered by an optical fiber into an aqueous medium causes rapid localized heating and vaporization and creates a bubble at the tip of the fiber. The size of the bubble depends on the pulse energy and is about 1 mm at 1 mJ. The shape of the bubble has multiple lobes, which can be attributed to the spiky output of the laser pulse. The expanding bubble can cause thermal and mechanical damage to tissues. In addition, laser spikes propagating through the bubble can strike and damage tissue on the distal side of the bubble. In both mechanisms the damage zone approximates the bubble size and can be greater than 1 mm, ie, 1000 times the steady-state absorption length of water at 2.94 microns. The authors discuss ways to reduce the damage zone by bubble confinement.

Femtosecond optical ranging of corneal incision depth.

Excimer laser ablation has been proposed as a technique for keratorefractive surgery. Clinical acceptance of linear-incision laser keratectomy may depend on the availability of a method for accurately and noninvasively monitoring incision depth during the ablation process. We have developed a femto-second optical ranging technique for measurement of corneal incision depth. This technique uses nonlinear optical cross-correlation to determine the time-of-flight of an ultrashort laser pulse between the anterior corneal surface and the bottom of the keratectomy incision. Longitudinal and transverse resolution are estimated to be 5 micron and 10 micron, respectively.

Interferometric technique for investigation of laser thermal retinal damage.

We describe a new technique for investigating laser-tissue interactions based on the use of an interferometric laser exposure pattern. A Michelson interferometer is used to generate a sinusoidal fringe exposure pattern. The periodicity of the fringe pattern may be adjusted from macroscopic dimensions to a scale of microns without the need for an imaging plane. Since fringe pattern periodicity is more adjustable and directly measureable than laser spot size, this technique offers significant advantages for studying the effects of thermal damage and diffusion in the irradiated tissue. In addition, the comparison of tissue response with theoretical models is simplified since the sinusoidal fringe pattern is itself an eigenfunction of the thermal diffusion equation. This technique is demonstrated for argon laser photocoagulation in the rabbit retina. Exposures at durations comparable to the thermal relaxation time produced spatially confined lesions, while those at much longer durations resulted in significant diffusion of the thermal damage beyond the primary targeted regions. The role of thermal diffusion can thus be assessed directly from the ophthalmoscopic and histologic appearances of the lesions. This technique can be employed to study thermal diffusion and other transport phenomena occurring in laser-tissue interactions for a variety of laser sources and tissue targets.

Time-resolved studies of Nd:YAG laser-induced breakdown. Plasma formation, acoustic wave generation, and cavitation.

The use of high intensity ultrashort pulsed laser radiation to produce optical breakdown is an important approach for the surgical treatment of intraocular structures. We have investigated the transient properties of Nd:YAG laser induced breakdown in a saline model using time-resolved spectroscopic techniques. Spatially resolved pump and probe techniques are applied to study the dynamic behavior of the plasma formation, acoustic wave generation, and cavitation processes which accompany the optical breakdown. Measurements of plasma shielding and luminescence indicate that the laser induced plasma forms on a subnanosecond time scale and has a lifetime of several nanoseconds. An acoustic transient is generated at the breakdown site and propagates spherically outward with an initial hypersonic velocity, then loses energy and propagates at sound velocity. Transient heating following the plasma formation produces a liquid-gas phase change and gives rise to cavitation or gas bubble formation. This gas bubble expands rapidly for several microseconds, then slows to reach its maximum size and finally collapses.

A new model of experimental choroidal neovascularization in the rat.

Choroidal neovascularization in rat eyes was induced by krypton laser photocoagulation. Lesions were studied weekly by ophthalmoscopy, fundus photography, and fluorescein angiography. Morphologic correlation was provided by serial sectioning of lesions for light and transmission electron microscopy. In addition, vascular casts were prepared for scanning electron microscopy. Choroidal neovascularization occurred in 25 (60%) of 42 lesions, as evidenced by growth of capillaries through breaks in Bruch's membrane. In addition, 24 (28%) of 86 lesions studied by fluorescein angiography demonstrated leakage. This study provides the most complete angiographic, histologic, and ultrastructural documentation of experimental choroidal neovascularization in the rat. This model may be useful for in vivo studies of choroidal angiogenesis and its modulation via drug therapy.

A comparative study of masking fluids for excimer laser phototherapeutic keratectomy.

Several fluids of different viscosity were used to mask deeper tissues while exposing protruding irregularities during therapeutic keratectomy of an irregular anterior corneal surface with the 193-nm argon fluoride excimer laser. A model of an irregular anterior corneal surface was developed in deepithelialized calf eyes using grade 8-0 sandpaper. Therapeutic keratectomy was then performed on 28 eyes at a fluence of 180 mJ/cm2, a repetition rate of 10 Hz, and 500 pulses per eye. Solutions of 0.3% hydroxypropylmethylcellulose 2910 and a 0.1% dextran 70 solution, 1% carboxymethylcellulose sodium solution, or 0.9% saline solution were applied to the corneal surface of 21 eyes. Seven control eyes underwent ablation without the addition of fluid. Scanning electron microscopy demonstrated that corneas treated with dextran 70 had the least surface irregularity, and those treated with carboxymethylcellulose or saline solution had intermediate surface irregularity. Corneas that were ablated without additional fluid had the greatest surface irregularity. The application of a moderately viscous solution during therapeutic excimer laser keratectomy enhances the smoothing effect of surface ablation.

Erbium-YAG laser surgery on experimental vitreous membranes.

We used a pulsed erbium-YAG laser emitting in the midinfrared region (2.94 microns) to cut experimental vitreous membranes in rabbits via an intraocular fiberoptic delivery system. Thirty-four membranes were cut at distances of 500 to 3600 microns from the retina. All 34 of the membranes treated were effectively cut. Sixteen cases had no evidence of retinal injury, including one in which the membrane was only 800 microns from the retina. Retinal injuries in the remaining cases consisted of small (less than 300 microns) retinal burns and hemorrhages. In 94% of the cases with retinal injury, the fiberoptic was within 2000 microns of the retina. We conclude that a pulsed erbium-YAG laser allows near tractionless cutting of vitreous membranes and may have potential for clinical use if further studies show it to be efficacious and safe near the retina.

High-speed photography of excimer laser ablation of the cornea.

We have used laser-based high-speed photography to investigate excimer laser ablation of the cornea. Photographs of the ablation plume were obtained 500 ns to 150 microseconds after incidence of a 193- or 248-nm excimer laser pulse on the surface of the cornea. Ejection of material from the cornea begins on a time scale of nanoseconds and continues for 5 to 15 microseconds following the excimer pulse. At 193 nm the ablation plume resembles a burst of smoke, and individual particles are too small to be optically resolved with our apparatus. At 248 nm the plume resembles a spray of larger, discrete droplets. Material is ejected from the cornea at supersonic velocity but decelerates rapidly; the velocity for the first 500 ns following the excimer pulse averages 400 m/s at 193 nm. Plume size and velocity increase with increasing fluence.

Semiconductor laser endophotocoagulation of the retina.

Laser endophotocoagulation is an important technique in vitreoretinal surgery. We performed successful retinal endophotocoagulation in the eyes of Dutch-belted rabbits, using high-power phased-array semiconductor lasers, emitting at 808 and 817 nm. The laser itself measured 25 X 30 X 21 mm, was air cooled, and was portable. At a treatment power of 100 mW, uniformly white photocoagulation lesions were seen at exposure durations of 0.2 to 1.0 s. Lesions were similar in appearance to argon laser photocoagulation lesions, as determined by ophthalmoscopy and fluorescein angiography. Thin-section histologic examination of acute lesions revealed injury localized to the outer retina; chorioretinal scar formation was noted ten days after photocoagulation. To our knowledge, this study is the first in which therapeutically useful lesions were produced using a diode laser and demonstrates the feasibility of using these highly efficient and compact laser sources for ophthalmic photocoagulation.

One-year results of excimer laser photorefractive keratectomy for low to moderate myopia.

OBJECTIVE: Excimer photorefractive keratectomy is a promising method to reduce myopia. We evaluated the effectiveness, predictability, stability, and safety of photorefractive keratectomy performed by two surgeons at a single site with a 1-year follow-up. METHODS: An excimer laser (Summit Excimed UV200, Summit Technology, Waltham, Mass) was used to treat 47 eyes. Preoperative uncorrected visual acuity averaged 20/277 and ranged from 20/60 to 20/500. Spherical equivalent averaged -3.9 diopters and ranged from -1.5 to -6.1 D. RESULTS: One year postoperatively, best corrected visual acuity averaged 20/13 and ranged from 20/10 to 20/20. No patients lost any of best corrected visual acuity. The uncorrected visual acuity at 1 year averaged 20/20 and ranged from 20/10 to 20/200. Ninety-four percent of eyes had an uncorrected visual acuity of 20/40 or better; 83%, 20/25 or better; and 74%, 20/20 or better. An initial hyperopic overcorrection is followed by gradual regression to emmetropia during a 12-month period. Refractive spherical equivalent averaged 0.44 D and ranged from -3 to +2.6 D at 1 year. The proportion of eyes within 1 D of emmetropia was 80% at 1 year. The most significant shifts in refraction occurred between the first and second months, but small shifts occurred up to 12 months. Subgrouping the results according to age revealed a significantly greater effect with larger residual hyperopia in those older than 40 years. Patient satisfaction with the procedure was high. CONCLUSION: During a 1-year period, excimer photorefractive keratectomy appears to be an effective, predictable, stable, and safe method to reduce myopia. Age may play a role in the effectiveness of the procedure, with greater hyperopia resulting in those older than 40 years.

Experimental gypsy moth (Lymantria dispar) ophthalmia nodosa.

Ophthalmia nodosa is the nodular granulomatous inflammatory response of ocular tissue to caterpillar hairs. We experimentally simulated this condition by surgically implanting gypsy moth hairs in the rabbit cornea. We were not, however, able to produce ophthalmia nodosa solely by placing cilia in the rabbit cul-de-sac. The experimentally induced inflammation closely resembled human ophthalmia nodosa in both its clinical and histopathologic features.

Q-switched neodymium-YAG laser trabeculopuncture in monkeys.

In nine cynomolgus monkeys, trabeculopuncture was performed with a Q-switched neodymium-YAG laser, using a pulse energy of 5 to 7 millijoules and an exposure time of 14 nanoseconds. A penetration into Schlemm's canal was successfully achieved with two to four pulses; this penetration was accompanied by intraocular pressure (IOP) reduction and blood reflux into the anterior chamber. However, after eight days, IOP returned to baseline level, while white tissue was observed gonioscopically to fill in the puncture sites. Histologically, one hour after laser treatment, a blasting effect on the trabecular meshwork was observed with no signs of necrosis. Ater eight days, a hypertrophic scar formed, with the corneal endothelium extending over the scarred surface. At eight weeks and at six months after laser treatment, further shrinkage of the scar and the formation of a membrane over it was evident. Attempts to control scar formation by preventing blood reflux or injecting fluorouracil subconjunctivally for two weeks were unsuccessful. Scar formation at the trabecular puncture site severely limits the applicability of this potentially simple glaucoma treatment.

Corneal ablation by nanosecond, picosecond, and femtosecond lasers at 532 and 625 nm.

We produced corneal excisions with nanosecond (ns)-, picosecond-, and femtosecond (fs)-pulsed lasers at visible wavelengths. The threshold energy for ablation was proportional to the square root of the pulse duration and varied from 2.5 microjoules (microJ) at 100 fs to 500 microJ at 8 ns. Excisions made with picosecond and femtosecond lasers was ultrastructurally superior to those made with nanosecond lasers and, at pulse energies near threshold, showed almost as little tissue damage as excisions made with excimer lasers at 193 nm. We conclude that ultrashort-pulsed lasers at visible and near-infrared wavelengths are a possible alternative to excimer lasers for corneal surgery and might have advantages over conventional ophthalmic neodymium-YAG lasers for some intraocular applications.

Photodynamic therapy for experimental intraocular melanoma using chloroaluminum sulfonated phthalocyanine.

Chloroaluminum sulfonated phthalocyanine (CASPc), a novel photosensitizing dye, was evaluated for treatment of experimental intraocular melanoma in 33 rabbit eyes. An argon ion pumped dye laser, operating at an emission of 675 nm, was used in a nonthermal mode to irradiate iris tumors in rabbits 24 hours after they received in intravenous dye injection (23 eyes). The effects of laser irradiation alone and dye alone were examined in ten control eyes. A threshold tumoricidal dose was established for photodynamic therapy with CASPc and laser irradiation. Vascular occlusion was produced in a well-circumscribed area corresponding to the boundaries of laser irradiation after CASPc injection. Tumors successfully treated with CASPc and laser irradiation were arrested in growth and exhibited no viable tumor cells on histologic examination. Control tumors continued rapid growth, unaffected by dye or laser. Our data indicate that CASPc demonstrates a strong photosensitizing effect on both tumor and normal tissue. These results suggest that CASPc is a potential photosensitizing compound that may be useful in the treatment of choroidal melanoma.

Immunofluorescence study of corneal wound healing after excimer laser anterior keratectomy in the monkey eye.

We performed anterior keratectomies on six monkey eyes, four by excimer laser large-area ablation at 193 nm and two by mechanical keratectomy. Immunofluorescence was used to study the wound healing response histopathologically. The distribution of fibrinogen, fibronectin, laminin, collagen types III, IV, and VI, and keratan sulfate was determined at postoperative intervals of 24 hours, 6 days, and 1 month. At 24 hours, fibrinogen and fibronectin coated the ablated surface, but corneal epithelial cells had not yet migrated over the wound. By 6 days and persisting at 1 month, an epithelial ingrowth of seven to 10 layers, mild stromal hypercellularity, and new collagen formation were present in the repair region. At 1 month, fibrinogen, fibronectin, laminin, and type III collagen were strongly detected in the repair region. Type VI collagen was present in both normal and healed corneal stroma at all intervals, and type IV collagen was present in Descemet's membrane only. Sulfated keratan sulfate was absent from the newly synthesized collagen stroma at all intervals. Slit-lamp photographs demonstrated corneal haze in the ablation zone in all cases at 24 hours, persisting for 1 month. The fluorescence patterns produced by excimer laser ablation and mechanical keratectomy were qualitatively identical.

Experimental use of semiconductor diode laser in contact transscleral cyclophotocoagulation in rabbits.

Acute and long-term effects of contact transscleral semiconductor diode laser cyclophotocoagulation were studied in rabbits. Immediately following cyclophotocoagulation, light microscopy revealed coagulation necrosis of the ciliary pigmented and nonpigmented epithelia and stroma; destruction increased in relation to power. There was architectural disruption with higher energy levels. The findings with diode and continuous-wave contact neodymium-YAG lasers were similar. In rabbits treated and followed up for 6 weeks there was a decrease in intraocular pressure during the observation period (P less than .05, paired Student's t test). The gross and light microscopic examination revealed focal atrophy and fibrosis of the ciliary processes, with pigment-laden macrophages on the ciliary processes and in the outflow pathways. Higher energy levels caused lens capsule damage and vitritis. The diode laser produced ciliary destruction similar to the continuous-wave contact neodymium-YAG laser in rabbits, with significant intraocular pressure lowering during the 6-week follow-up.

Holmium-YAG laser surgery on experimental vitreous membranes.

We used a pulsed holmium-YAG laser that emitted in the midinfrared (2.12 microns) to cut experimental vitreous membranes in rabbits via an intraocular fiberoptic delivery system. Thirty-five membranes were treated at distances of 0.5 to 3.8 mm from the retina. All of the membranes that were treated were transected or partially cut. In cases that were treated with the optical fiber encased in a retinal-shielding pick, membranes could be readily transected without retinal injury. Laser vitreous membrane cutting may offer the advantage of near-tractionless membrane segmentation. The combination of a moderately strong target tissue absorption and the ready availability of commercial optical fibers for transmission of 2.12-microns laser energy suggests that the holmium-YAG laser merits further investigation for this application.

Neodymium-YAG laser surgery on experimental vitreous membranes.

A Q-switched neodymium-YAG laser was used to cut experimental vitreous membranes in rabbit eyes. We were able to cut vitreous membranes up to 4 mm from the retinal surface using single pulse energies of up to 4 mJ without fluorescein angiographic evidence of retinal injury. When cutting membranes between 1.5 and 3 mm from the retinal surface, injury was documented in five of seven cases. The most frequently observed fundus injury was choroidal hemorrhage. The neodymium-YAG laser may be useful in cutting avascular membranes in the posterior segment of the eye.