With continued advancement of solid-state laser technology, high-energy lasers operating in the near-infrared (NIR) band are being applied in an increasing number of manufacturing techniques and medical treatments. Safety-related investigations of potentially harmful laser interaction with skin are commonplace, consisting of establishing the maximum permissible exposure (MPE) thresholds under various conditions, often utilizing the minimally-visible lesion (MVL) metric as an indication of damage. Likewise, characterization of ablation onset and velocity is of interest for therapeutic and surgical use, and concerns exceptionally high irradiance levels. However, skin injury response between these two exposure ranges is not well understood. This study utilized a 1070-nm Yb-doped, diode-pumped fiber laser to explore the response of excised porcine skin tissue to high-energy exposures within the supra-threshold injury region without inducing ablation. Concurrent high-speed videography was employed to assess the effect on the epidermis, with a dichotomous response determination given for three progressive damage event categories: observable permanent distortion on the surface, formation of an epidermal bubble due to bounded intra-cutaneous water vaporization, and rupture of said bubble during laser exposure. ED50 values were calculated for these categories under various pulse configurations and beam diameters, and logistic regression models predicted injury events with approximately 90% accuracy. The distinction of skin response into categories of increasing degrees of damage expands the current understanding of high-energy laser safety while also underlining the unique biophysical effects during induced water phase change in tissue. These observations could prove useful in augmenting biothermomechanical models of laser exposure in the supra-threshold region.
We have proposed a modified Monte Carlo approach to the solution of the radiative transport equation which has the unique
feature of incorporating refractive index gradients within a multi-layer biological tissue model. In the approach, photon
trajectories are computed using a solution of the Eikonal equation (ray-tracing methods) rather than linear trajectories. The
method can be applied to the specific problem of incorporating thermal lensing and other non-linear effects in turbid media
(biological tissues) by coupling the radiative transport solution into heat-transfer and damage models. In turn, the method
can be applied in the establishment of laser exposure limits for tissue-penetrating wavelengths, as well as a number of
additional applications in imaging, spectroscopy and vision science.
In this paper we report on our combined measurements of the visible lesion thresholds for porcine skin for wavelengths in the infrared from 810 nm at 44 fs to 1318 nm at pulse durations of 50 ns and 350&mgr;s to 1540 nm including pulse durations of 31 ns and 600 &mgr;s. We also measure thresholds for various spot sizes from less than 1 mm to 5 mm in diameter. All three wavelengths and five pulse durations are used extensively in research and the military. We compare these minimum visible lesion thresholds with ANSI standards set for maximum permissible exposures in the infrared wavelengths. We have measured non-linear effects at the laser-tissue interface for pulse durations below 1&mgr;s and determined that damage at these short pulse durations are usually not thermal effects. Damage at the skin surface may include acoustical effects, laser ablation and/or low-density plasma effects, depending on the wavelength and pulse duration. Also the damage effects may be short-lived and disappear within a few days or may last for much longer time periods including permanent discolorations. For femtosecond pulses at 810 nm, damage was almost instant and at 1 hour had an ED50 of 8.2 mJ of pulse energy. After 24 hours, most of the lesions disappeared and the ED50 increased by almost a factor of 3 to 21.3 mJ. There was a similar trend for the 1.318 &mgr; laser for spot sizes of 2 mm and 5 mm where the ED50 was larger after 24 hours. However, for the 1.54 &mgr; laser with a spot size of 5 mm, the ED50 actually decreased by a small amount; from 6.3 Jcm-2 to 6.1 Jcm-2 after 24 hours. Thresholds also decreased for the 1314 nm laser at 350 &mgr;s for spot sizes of 0.7 mm and 1.3 mm diameter after 24 hours. Different results were obtained for the 1540 nm laser at 600 &mgr;s pulse durations where the ED50 decreased for spot sizes 1 mm and below, but increased slightly for the 5 mm diameter spot size from 6.4 Jcm-2 to 7.4 Jcm-2
An assessment of skin damage caused by near-IR laser exposures is reported. The damage from two distinct laser-tissue temporal regimes is compared at two wavelengths (1.3 &mgr;m and 1.5 &mgr;m). Skin damage caused by thermal effects from single laser pulses is compared to damage caused by LIB (laser induced breakdown) using histological examinations. Modeling applications are explored to determine crossover points between thermal and photomechanical damage thresholds.
Er:glass lasers have been in operation with both long pulses (hundreds of microseconds) and Q-switched pulses (50 to 100 ns) for more than 35 yr. The ocular hazards of this laser were reported early, and it was determined that damage to the eye from the 1.54-µm wavelength occurred mainly in the cornea where light from this wavelength is highly absorbed. Research on skin hazards has been reported only in the past few years because of limited pulse energies from these lasers. Currently, however, with pulse energies in the hundreds of joules, these lasers may be hazardous to the skin in addition to being eye hazards. We report our minimum visible lesion (MVL) threshold measurements for two different pulse durations and three different spot sizes for the 1.54-µm wavelength using porcine skin as an in vivo model. We also compare our measurements to results from our model, based on the heat transfer equation and the rate process equation. Our MVL-ED50 thresholds for the long pulse (600 µs) at 24 h postexposure were measured to be 20, 8.1, and 7.4 J cm–2 for spot diameters of 0.7, 1.0, and 5 mm, respectively. Q-switched laser pulses of 31 ns had lower ED50 (estimated dose for a 50% probability of laser-induced damage) thresholds of 6.1 J cm–2 for a 5-mm-diam, top-hat spatial profile laser pulse.
Skin damage thresholds were measured and compared with theoretical predictions using a skin thermal model for near-IR laser pulses at 1318 nm and 1540 nm. For the 1318-nm data, a Q-switched, 50-ns pulse
with a spot size of 5 mm was applied to porcine skin and the damage thresholds were determined at 1 hour and 24 hours postexposure using Probit analysis. The same analysis was conducted for a Q-switched, 30-ns pulse at 1540 nm with a spot size of 5 mm. The Yucatan mini-pig was used as the skin model for human skin due to its similarity to pigmented human skin. The ED50 for these skin exposures at 24 hours postexposure was 10.5 J/cm2 for the 1318-nm exposures, and 6.1 J/cm2 for the 1540-nm exposures. These results were compared to thermal model predictions. We show that the thermal model fails to account for the ED50 values observed. A brief discussion of the possible causes of this discrepancy is presented. These thresholds are also compared with previously published skin minimum visible lesion (MVL) thresholds and with the ANSI Standard's MPE for 1318-nm lasers at 50 ns and 1540-nm lasers at 30 ns.
The reflectance and absorption of the skin plays a vital role in determining how much radiation will be absorbed by human tissue. Any substance covering the skin would change the way radiation is reflected and absorbed and thus the extent of thermal injury. Hairless guinea pigs (cavia porcellus) in vivo were used to evaluate how the minimum visible lesion threshold for single-pulse laser exposure is changed with a topical agent applied to the skin. The ED50 for visible lesions due to an Er: glass laser at 1540-nm with a pulse width of 50-ns was determined, and the results were compared with model predictions using a skin thermal model. The ED50 is compared with the damage threshold of skin coated with a highly absorbing topical cream at 1540 nm to determine its effect on damage pathology and threshold. The ED50 for the guinea pig was then compared to similar studies using Yucatan minipigs and Yorkshire pigs at 1540-nm and nanosecond pulse duration.1,2 The damage threshold at 24-hours of a Yorkshire pig for a 2.5-3.5-mm diameter beam for 100 ns was 3.2 Jcm-2; very similar to our ED50 of 3.00 Jcm-2 for the hairless guinea pigs.
A very high energy Q-switched Er-glass laser is reported. We incorporated a rotating, resonant mirror/Porro-cavity reflector optical arrangement to achieve very high shutter speeds on the cavity Q of a laser designed for energetic, flashlamp-pumped, 600-μs, 1540-nm pulses. Reproducible 3.75-J, 35-ns, 1533-nm laser pulses were obtained at a repetition rate less than 1 minute. Our work shows that reliable, very high energy, Q-switched, Er-glass laser pulses at 1533 nm can be generated mechanically with no apparent damage to laser cavity components. We demonstrate the applications of this "eye safe" wavelength to energetic processes such as LIBS and materials processing. The laser could also serve as a new tool for bioeffects studies and targeting applications.
An optical phantom was designed to physically and optically resemble human tissue, in an effort to provide an alternative for detecting visual damage resulting from inadvertent exposure to infrared lasers. The phantom was exposed to a 1540-nm, Erbium:Glass, Q-switched laser with a beam diameter of 5 mm for 30 ns at varying power levels. Various materials were tested for use in the phantom; including agar, ballistic media, and silicone rubber. The samples were analyzed for damage lesions immediately after exposure and the Minimum Visible Lesion - Estimated Dose 50% (MVL-ED50 ) thresholds were determined from the data. In addition, any visible damage was evaluated for similarity to human tissue damage to determine if the phantom tissue would be a suitable substitute for in vivo exposures.
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