Porcine skin visible lesion thresholds for near-infrared lasers including modeling at two pulse durations and spot sizes.

With the advent of such systems as the airborne laser and advanced tactical laser, high-energy lasers that use 1315-nm wavelengths in the near-infrared band will soon present a new laser safety challenge to armed forces and civilian populations. Experiments in nonhuman primates using this wavelength have demonstrated a range of ocular injuries, including corneal, lenticular, and retinal lesions as a function of pulse duration. American National Standards Institute (ANSI) laser safety standards have traditionally been based on experimental data, and there is scant data for this wavelength. We are reporting minimum visible lesion (MVL) threshold measurements using a porcine skin model for two different pulse durations and spot sizes for this wavelength. We also compare our measurements to results from our model based on the heat transfer equation and rate process equation, together with actual temperature measurements on the skin surface using a high-speed infrared camera. Our MVL-ED50 thresholds for long pulses (350 micros) at 24-h postexposure are measured to be 99 and 83 J cm(-2) for spot sizes of 0.7 and 1.3 mm diam, respectively. Q-switched laser pulses of 50 ns have a lower threshold of 11 J cm(-2) for a 5-mm-diam top-hat laser pulse.

Study of corneal lesions induced by 1,318-nm laser radiation pulses in Dutch belted rabbits (Oryctolagus cuniculus).

BACKGROUND AND PURPOSE: Use of high-energy near-infrared lasers is becoming more prevalent in today's industries, such as technology, medicine, and military operations. Despite wide-range use of these lasers, threshold, median effective dose (ED50), and the mechanism of laser-tissue interaction are not well defined at the 1,318-nm wavelength for human corneal exposures. The goals of the study reported here were to establish the ED50 for single-pulse, 1,318-nm laser exposures on the Dutch Belted rabbit cornea and to characterize microscopic changes. Results of this study were then compared with those of previous corneal studies. METHODS: A neodymium:yttrium aluminum garnet (Nd:YAG) laser was used to deliver single 1,318-nm wavelength pulses to the corneas of 10 female Dutch Belted rabbits (Oryctolagus cuniculus). Single pulses of 0.5-ms duration and radiant energy ranging from 116 to 2,250 J/cm2 irradiated the exposure sites. Sites were clinically evaluated for presence of a lesion at one hour and 24-h after exposure. Results of the 24-h evaluation were used to determine the (ED50). Corneas were subsequently collected at the 24-h endpoint for microscopic evaluation. RESULTS: The ED50 for 1,318-nm exposures to the rabbit cornea was determined to be 382 J/cm2, as measured at the 1/e2 (0.865 times that of the peak power per unit area). At each exposure site, there was a small (< 1 mm in diameter), white, circular, well demarcated corneal lesion characterized histologically by a band of stromal coagulative necrosis and endothelial necrosis, with sparing of the anterior epithelium. In addition, there appeared some potential for damage to Descemet's membrane at the highest energy level tested. CONCLUSIONS: Findings indicate that the rabbit corneais subject to injury at the 1,318-nm wavelength with the established ED50.

Comparison of two porcine (Sus scrofa domestica) skin models for in vivo near-infrared laser exposure.

BACKGROUND AND PURPOSE: The current safety standards for lasers operating in the 1,400- to 2,000-nanometer (nm) wavelength region are based on only a few observations at specific wavelengths. On the basis of experimental results conducted with Yorkshire pigs (Sus scrofa domestica), these standards may not accurately reflect the potential for laser injury when humans are exposed to these wavelengths. It is our belief that one of the damage mechanisms involved in these laser injuries results from energy absorption by skin pigmentation (melanin), and a more highly pigmented animal model, the Yucatan hairless minipig, may be a more suitable subject for laser exposure studies. METHODS: Skin specimens were collected from Yorkshire pigs and Yucatan minipigs for histologic examination, and the thickness of the epidermis was measured. Epidermal thickness of human skin also was determined, and a qualitative assessment of the melanin content in the epidermal layers was conducted. RESULTS: Mean +/- SD thicknesses of the Yucatan minipig flank and dorsal neck epidermis were 68 +/- 34 and 68 +/- 25 microm, respectively. Thicknesses of the Yucatan minipig skin were closely comparable to the thicknesses of human epidermis from the face (68 +/- 26 microm), neck (65 +/- 24 microm) and arms (68 +/- 21 microm). The Yorkshire pig lacks substantial melanin in the epidermis, whereas the skin of the Yucatan minipig is more similar to that of humans. CONCLUSION: On the basis of epidermal skin thickness measurements and melanin assessment, the flank and dorsal neck of the Yucatan minipig are better suited to laser injury studies than are the Yorkshire pig models of human skin.

Median effective dose determination and histologic characterization of porcine (Sus scrofa domestica) dermal lesions induced by 1540-nm laser radiation pulses.

BACKGROUND AND PURPOSE: Light amplification by stimulated emission of radiation (laser) systems operating in the so-called "eye safe" region are gaining widespread use in industry, medicine, and military applications. This research effort was geared to study the effects of laser tissue interaction on human skin by using in vivo porcine skin as an animal model. The goals of the study were to determine the median effective dose (ED50) for 1540-nm laser exposures, to evaluate the Yorkshire pig and the Yucatan mini-pig as animal models for laser exposure, and to characterize laser-induced skin lesions histologically. METHODS: A 1540-nm wavelength laser was used to expose multiple sites on the flanks of 10 pigs, using 0.8-ms pulses, ranging from 7 to 96 joules (J)/cm2. Single pulses were delivered to the flank of Yorkshire and Yucatan pigs in a grid pattern. Exposure sites were evaluated immediately after exposure and at 1 hour and 24 hours for presence of gross lesions. Representative biopsy specimens were collected from lesion sites for histologic evaluation at the 24-hour endpoint. RESULTS: The ED50 for the two breeds differed in the amount of energy required to induce dermal lesions. Grossly, lesions in each breed were well demarcated and pale gray to brightly erythematous. Microscopically, lesions had epidermal layer damage as cellular swelling and nuclear pyknosis, loss of cellular detail, and coagulation necrosis at the dermal layer. CONCLUSIONS: Findings suggest the presence of a different mechanism of laser-tissue damage in these two breeds. Photo-thermal mechanism appears to induce the skin lesions in the Yorkshire pig, whereas photo-thermal and photochemical mechanisms appear to be involved in lesion formation in the Yucatan mini-pig. All data obtained in this study will become part of database used by the American National Standards Institute (ANSI) to recommend laser safety standards for the occupational health and safety programs (OHSP), which will be used by industry and the military to base and update their current OHSP.

Ultrashort laser pulse bioeffects and safety.

Recent studies of retinal damage due to ultrashort laser pulses have shown that less energy is required for retinal damage for pulses shorter than 1 ns than that for longer pulses. It has also been shown that more energy is required for near-infrared (NIR) wavelengths than in the visible because the light focuses behind the retina, requiring more energy to produce a damaging fluence on the retina. We review the progress made in determining the trends in retinal damage from laser pulses of 1 ns to 100 fs in the visible and NIR wavelength regimes. We have determined the most likely damage mechanism(s) operative in this pulse width regime.

Proposed maximum permissible exposure limits for ultrashort laser pulses.

Recent studies have provided considerable ED50 data for both visible and near infrared wavelengths from single laser pulses below one nanosecond of exposure. The current ANSI Z136.1 standard does not offer an approved maximum permissible exposure limit for subnanosecond single laser pulses and the current suggested maximum permissible exposure limit may be overly conservative. Lacking an approved standard industrial, medical, educational, and military uses of these types of laser systems may be limited or prohibited. A new set of laser maximum permissible exposure limits for subnanosecond visible and near infrared single laser pulses is recommended, along with the steps taken to develop the proposed standard.

Comparison of optical coherence tomography imaging of cataracts with histopathology.

This paper presents a comparison of in vivo optical coherence tomography (OCT) captured cataract images to subsequent histopathological examination of the lenticular opacities. OCT imaging was performed on anesthetized Rhesus monkeys, known as the delayed effects colony (DEC), with documented cataracts. These monkeys were exposed to several types of radiation during the mid and late 1960s. The radiation and age related cataracts in these animals were closely monitored using a unique grading system developed specifically for the DEC. In addition to this system, a modified version of a common cataract grading scheme for use in humans was applied. Of the original 18 monkeys imaged, lenses were collected at necropsy from seven of these animals, processed, and compared to OCT images. Results showed a direct correlation between the vertical OCT images and the cataractous lesions seen on corresponding histopathologic sections of the lenses. Based on the images obtained and their corresponding documented comparison to histopathology, OCT showed tremendous potential to aid identification and characterization of cataracts. There can be artifactual problems with the images related to movement and shadows produced by opacities. However, with the advent of increased speed in imaging and multiplanar imaging, these disadvantages may easily be overcome. © 1999 Society of Photo-Optical Instrumentation Engineers.

A comparison of retinal morphology viewed by optical coherence tomography and by light microscopy.

OBJECTIVE: To compare the cross-sectional images of primate retinal morphology obtained by optical coherence tomography (OCT) with light microscopy to determine the retinal components represented in OCT images. METHODS: Laser pulses were delivered to the retina to create small marker lesions in a Macaca mulatta. These lesions were used to align in vivo OCT scans and ex vivum histologic cross sections for image comparison. RESULTS: The OCT images demonstrated reproducible patterns of retinal morphology that corresponded to the location of retinal layers seen on light microscopic overlays. Layers of relative high reflectivity corresponded to horizontally aligned retinal components such as the nerve fiber layer and plexiform layers, as well as to the retinal pigment epithelium and choroid. In contrast, the nuclear layers and the photoreceptor inner and outer segments demonstrated relative low reflectivity by OCT. CONCLUSIONS: Retinal morphology and macular OCT imaging correlate well, with alignment of areas of high and low reflectivity to specific retinal and choroidal elements. Resolution of retinal structures by OCT depends on the contrast in relative reflectivity of adjacent structures. Use of this tool will enable expanded study of retinal morphology, both normal and pathologic, as it evolves in vivo.

Pathology of macular lesions from subnanosecond pulses of visible laser energy.

PURPOSE: To demonstrate how current theories regarding ultrashort laser pulse effects may apply to ocular tissue, a prospective clinicopathologic study of macular lesions from ultrashort laser pulses compared the pathologic effects with the clinical and fluorescein angiographic appearance of the laser lesions. METHODS: Ninety-femtosecond, 3-picosecond, and 60-picosecond laser pulses, throughout a range of energies, were delivered to the retina of Macaca mulatta. Clinical examination and fluorescein angiography were performed at 1 hour in all eyes and 24 hours after exposure in selected eyes. Eyes were enucleated at 1 or 24 hours after lesion placement. The structure and extent of retinal lesions were scored for comparison with the clinical findings. RESULTS: Focal retinal pathologic appearance correlated well with a clinically visible lesion observed 24 hours after laser delivery. Retinal lesions were small foci of retinal pigment epithelium (RPE) and retinal disruption, without choriocapillaris involvement. Lesions that contained focal RPE vacuoles or lifting of the RPE also demonstrated leakage, in fluorescein angiographic studies. Suprathreshold laser delivery frequently caused focal columns of retinal injury and intraretinal hemorrhages from retinal vessel bleeding, with no rupture of choroidal blood vessels. CONCLUSIONS: The retinal response to ultrashort laser pulses at moderate energy followed a pattern of focal damage from laser-induced breakdown without significant thermal spread.

Intraocular laser surgical probe for membrane disruption by laser-induced breakdown.

A fiber probe has been designed as a surgical aid to cut intraocular membranes with laser-induced breakdown as the mechanism. The design of the intraocular laser surgical probe is discussed. A preliminary retinal damage distance has been calculated with breakdown threshold, spot size, and shielding measurements. Collateral mechanical-damage effects caused by shock wave and cavitation are discussed.

Argon laser retinal lesions evaluated in vivo by optical coherence tomography.

PURPOSE: To assess the in vivo evolution of argon laser retinal lesions by correlating the cross-sectional structure from sequential optical coherence tomography with histopathologic sectioning. METHODS: Argon laser lesions were created in the retinas of Macaca mulatta and evaluated by cross-section optical coherence tomography, which was compared at selected time points with corresponding histopathology. RESULTS: Argon laser lesions induced an optical coherence tomography pattern of early outer retinal relative high reflectivity with subsequent surrounding relative low reflectivity that correlated well with histopathologic findings. The in vivo optical coherence tomography images of macular laser lesions clearly demonstrated differences in pathologic response by retinal layer over time. CONCLUSION: The novel sequential imaging of rapidly evolving macular lesions with optical coherence tomography provides new insight into the patterns of acute tissue response by cross-sectional layer. This sequential imaging technique will aid in our understanding of the rapid evolution of retinal pathology and response to treatment in the research and clinical setting.

Retinal damage and laser-induced breakdown produced by ultrashort-pulse lasers.

BACKGROUND: In vivo retinal injury studies using ultra-short-pulse lasers at visible wavelengths for both rabbit and primate eyes have shown that the degree of injury to the retina is not proportional to the pulse energy, especially at suprathreshold levels. In this paper we present results of calculations and measurements for laser-induced breakdown (LIB), bubble generation, and self-focusing within the eye. METHODS: We recorded on video and measured the first in vivo LIB and bubble generation thresholds within the vitreous in rabbit and primate eyes, using external optics and femtosecond pulses. These thresholds were then compared with calculations from our LIB model, and calculations were made for self-focusing effects within the vitreous for the high peak power pulses. RESULTS: Results of our nonlinear modeling and calculations for self-focusing and LIB within the eye were compared with experimental results. The LIB ED50 bubble threshold for the monkey eye was measured and found to be 0.56 microJ at 120 fs, compared with the minimum visible lesion (MVL) threshold of 0.43 microJ at 90 fs. Self-focusing effects were found to be possible for pulsewidths below 1 ps and are probably a contributing factor in femtosecond-pulse LIB in the eye. CONCLUSIONS: Based on our measurements for the MVL thresholds and LIB bubble generation thresholds in the monkey eye, we conclude that in the femtosecond pulsewidth regime for visible laser pulses, LIB and self-focusing are contributing factors in the lesion thresholds measured. Our results may also explain why it is so difficult to produce hemorrhagic lesions in either the rabbit or primate eye with visible 100-fs laser pulses even at 100 microJ of energy.

Retinal effects of ultrashort laser pulses in the rabbit eye.

PURPOSE: To evaluate the effects of ultrashort laser pulses from femtoseconds to nanoseconds on the retinas of live rabbit eyes and to determine the energy requirements for visible lesion development. METHODS: The retinal effects of laser exposures were examined for laser exposures with pulsewidths ranging from 4 ns to 90 fs, with visible wavelengths of 532 nm for durations > 5 ps and 580 nm for durations < 5 ps. The authors examined and scored all laser impact sites in the retina ophthalmoscopically--with fundus photography and with fluorescein angiography--to identify evidence of visible laser effects. RESULTS: The laser energy required for retinal minimal visible lesions was found to be slightly less for pulsewidths < 5 ps and varied from 5 microJ at 4 ns to 1.1 microJ at 90 fs for the 1-hour ophthalmoscopic reading. Lesions from higher energy pulses (7 to 120 microJ) were examined at all pulsewidths. For 90-fs high-energy pulse delivery, an increased intensity of retinal lesions and the development of several subretinal hemorrhages were demonstrated at peak energies of 30 microJ. Fluorescein angiography was found to be much more sensitive as an indicator of retinal damage for both femtosecond pulsewidths. CONCLUSIONS: The low energies required for visible lesion production in live rabbit eyes raise new questions surrounding ultrashort pulse propagation in ocular media, energy deposition at the retina, and mechanisms limiting retinal damage from ultrashort laser pulses.

Visible retinal lesions from ultrashort laser pulses in the primate eye.

PURPOSE: To evaluate the effects of ultrashort laser pulses of visible wavelengths on the retinas of rhesus monkey eyes and to perform threshold measurements for minimum visible lesions (MVLs) at pulsewidths from nanoseconds to femtoseconds. METHODS: Single laser pulses at visible wavelengths were placed within the macular area of live rhesus monkey eyes at varying pulse energies at five pulsewidths (4 ns, 60 ps, 3 ps, 600 fs, and 90 fs). The number of visible lesions was determined after 1 hour and 24 hours postexposure, and a probit analysis was performed for the dosage, causing 50% probability for damage (ED50) as well as the 95% fiducial intervals for ED50. Fluorescein angiography (FA) was performed, and hemorrhagic lesions were recorded as they became visible. RESULTS: The ED50 threshold doses at the 1-hour reading, calculated from the measured data, decreased from 1.5 microJ at 4 ns to 0.60 microJ at 600 fs, but it increased to 1.18 microJ at 90 fs. At the 24-hour reading, the ED50 calculated doses decreased from 0.90 microJ at 4 ns down to 0.26 microJ at 600 fs, but it increased to 0.43 microJ at 90 fs. Fluorescein angiography visible lesion ED50 values were all higher than MVL values, showing that FA was not as sensitive in determining damage levels. CONCLUSIONS: Laser pulses for pulsewidths between 4 ns and 90 fs are capable of producing visible lesions in monkey eyes with energies less than 1 microJ. Fluorescein angiography is not as sensitive in determining threshold levels as visually observing the retina through a fundus camera.

Ultrashort laser pulse effects in ocular and related media.

Relatively little experimental and theoretical data exist on the retinal hazards of ultrashort laser pulses operating in the visible and near infrared spectral regions. Because of potential nonlinear effects that can occur from high-peak irradiance, ultrashort laser pulses propagate from the cornea to the retina, we have developed four projects within our Ultrashort Pulse Effects program. First, we discuss preliminary ED50 threshold values for nanosecond (ns), picosecond (ps), and femtosecond (fs) single pulses for in-vivo ocular exposures in Dutch Belted Rabbits using pulses in the visible spectral region. Then we examine two experiments that study nonlinear absorption using water tubes and measure the nonlinear refractive index of ocular tissue using the Z-Scan technique. Finally, we determine laser-induced breakdown thresholds in ultrahigh purity water. These studies give reasonable estimates of the damage thresholds and insight into the biophysics of how ultrashort pulses interact with ocular media.

Nonlinear refraction in vitreous humor.

We extend the application of the z-scan technique to determine the nonlinear refractive index (n(2)) for human and rabbit vitreous humor, water, and physiological saline. In these measurements there were nonlinear contributions to the measured signal from the aqueous samples and the quartz cell that held the sample. Measurements were made with 60-ps pulses at 532 nm. To our knowledge, this is the first measurement of the nonlinear refractive properties of biological material.