Computational physics methods are described for the evaluation of the role of propagation with regard to laser
damage to tissues. Regions of the optical spectrum, where linear and non-linear propagation affects irradiance
distributions within tissues, are examined. Effects described include group-velocity dispersion, aberrations,
thermal lensing, and self-focusing. Implications to exposure limits within safety standards, incorporating these
irradiance-altering effects, are addressed such that inherent trends agree over wide temporal and spectral ranges,
with damage thresholds measured experimentally. We present current regions of interest to the standard-setting
community and recent works showing how propagation effects may be playing a key role in assessing damage
thresholds.
The Air Force Research Lab has developed a configurable, two-dimensional, thermal model to predict laser-tissue interactions, and to aid in predictive studies for safe exposure limits. The model employs a finite-difference, time-dependent method to solve the two-dimensional cylindrical heat equation (radial and axial) in a biological system construct. Tissues are represented as multi-layer structures, with optical and thermal properties defined for each layer, are homogeneous throughout the layer. Multiple methods for computing the source term for the heat equation have been implemented,
including simple linear absorption definitions and full beam propagation through finite-difference methods.
The model predicts the occurrence of thermal damage sustained by the tissue, and can also determine damage thresholds for total optical power delivered to the tissue. Currently, the surface boundary conditions incorporate energy loss through free convection, surface radiation, and evaporative cooling. Implementing these boundary conditions is critical for correctly calculating the surface temperature of the tissue, and, therefore, damage thresholds. We present an analysis of the interplay between surface boundary conditions, ambient conditions, and blood perfusion within tissues.
KEYWORDS: Luminescence, In vivo imaging, Laser damage threshold, Gaussian beams, In vitro testing, Microscopes, Amplifiers, Sensors, Eye, Argon ion lasers
We performed measurements to validate damage threshold trends in minimum visible lesion (MVL) studies as a function of spot size for nanosecond laser pulses. At threshold levels, nanosecond pulses produce microcavitation bubbles that expand and collapse around individual melanosomes. This microcavitation process damages the membranes of retinal pigment epithelium (RPE) cells. A spot size study on retinal explants found cell damage fluence (energy/area) thresholds were independent of spot size when microcavitation caused the damage, contradicting past in vivo retinal spot size experiments. The explant study (ex vivo) used a top-hat beam profile, whereas the in vivo studies used Gaussian beams. The difference in spot size trends for damage in vivo versus ex vivo may be attributed to the optics of the eye but this has not been validated. In this study, we exposed artificially pigmented human RPE cells (hTERT-RPE1)-in vitro-to 7 ns pulsed irradiation from a Ti:Sa TSA-02 regenerative amplifier (1055 nm) with beam diameters of 44, 86, and 273 μm (Gaussian beam profiles). We detected the microcavitation event with strobe illumination and time-resolved imaging. We used the fluorescent indicator dye calcein-AM, with excitation by an Argon laser (488 nm), to assess cell damage. Our current results follow trends found in the in vivo studies.
The measurements on the nonlinear absorption coefficient for the whole retina and separated molecular components have been determined using open-aperture z-scan. Our recent retinal damage studies have shown that the threshold for retinal damage decreases below one nanosecond exposure. Laser-induced breakdown has been implicated in the threshold-level mechanism for damage, and the threshold is reduced below 100 fs where LIB is the damage mechanism. Our hypothesis for this experiment is that non-linear optical properties of the constituents of the retina will affect the absorption coefficient of the retina for ultrashort pulse laser exposure and lower the retinal damage threshold for these exposures. This suggests that nonlinear absorption effects should be considered in the analysis of any data that relate energy deposition rates from laser exposures in tissue to thermal or photomechanical damage mechanisms that explain cell death. We describe the impact of these measurements on retinal damage thresholds and damage mechanisms for various pulse regimes.
The measured optical density of various laser eye protection samples is presented as a function of irradiance using femtosecond laser pulses. We show that the protective quality of some eyewear degrades as irradiance increases. In previous studies this problem has been demonstrated for samples irradiated by nanosecond pulses, but the current study shows that some modern laser eye protection seems to be robust except for the irradiance level possible with ultrashort laser pulse exposure. We discuss the most likely saturation mechanisms in this pulse duration regime and its relevance to laser safety.
Recent theoretical and experimental findings have produced wide ranges of absorption coefficients for melanin in the visible and near-infrared wavelength ranges. These values have been reported for a variety of laser pulse durations, from less than one nanosecond to CW exposures of several seconds. Here we present the findings of non-linear absorption studies in melanin-doped samples in an effort to reconcile bioeffects experiments and theory across a wide variety of irradiance levels. These studies are of particular interest to laser bioeffects in the retina, for which melanin plays a key role in the absorption of laser energy.
We have measured the optical density of various laser eye protection samples as a function of increasing irradiance. We show that the protective quality of some eyewear degrades as irradiance increases. In previous studies this problem has been demonstrated in samples irradiated by nanosecond pulses, but the current study shows that the modern laser eye protection seems to be robust except for the irradiance possible with ultrashort laser pulse exposure. We discuss the most likely saturation mechanisms in this pulse duration regime and discuss relevance to laser safety.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.