O19 - Measurement of Temperature-Dependent Changes in Optical Properties of Porcine Dermis for Laser-Tissue Interaction Modeling

Introduction: : Laser technology has enabled improvements in modern medicine to include precise diagnostics and innovative treatments. As laser applications advance, it's critical to further characterize laser-tissue interactions—particularly those related to heat-induced damage. Accurately modeling these effects is necessary for predicting tissue responses to laser exposure.
While Monte Carlo simulations excel at optical modeling, they lack direct integration with heat transfer and tissue damage kinetics. In contrast, finite element multiphysics simulations offer a comprehensive approach by combining heat transfer and damage accumulation—thereby providing a holistic view of laser-tissue interactions.
DeLisi et al. developed a computational model focusing on NIR laser photothermal response in lightly pigmented skin. Their study demonstrated accurate temperature history matches with experimental data across a broad exposure time range. They highlighted the importance of incorporating NIR optical properties consistent with the expected distribution of chromophores in skin tissue. While DeLisi's work successfully modeled temperature responses in skin tissue, our investigation extends this foundation by specifically addressing the dynamic changes in optical and thermal properties that occur during both the exposure and cooling phases—thereby providing insights into laser-induced thermal damage across various tissue types.
Given these dynamic tissue changes during thermal damage, the objective was to further characterize laser-tissue interactions and thermal therapy effects. By examining optical and thermal properties during exposure and cooling phases, we sought to interpret mechanisms underlying laser-induced thermal damage. These insights are crucial for optimizing laser-based medical applications, particularly laser-induced thermal therapy (LITT), where precise and minimally invasive treatments show significant promise.

Materials and

Methods: : The experimental setup utilized a dual integrating sphere system with a broadband light source and dual-channel spectrometer to measure tissue optical properties. Porcine dermis samples (10 mm diameter, 1.19 ± 0.29 mm thickness) were harvested from a female Yucatan miniature pig following Institutional Animal Care and Use Committee (IACUC) approved protocols. The protocol allowed for the post-mortem sharing of tissues from pigs that were part of other pre-approved research studies. Samples were stored in saline-moistened gauze at 4 °C until baseline optical property testing.
Following baseline optical property measurements, a thermal treatment protocol was implemented. Samples were transferred to test tubes containing 3 mL of normal saline solution. Two calibrated thermistors were used to capture temperature profiles during tissue treatment. Samples were exposed to various temperatures—22°C, 37°C, 43°C, 50°C, 60°C, and 70°C—in a temperature-controlled water bath.
Optical measurements were taken before and after thermal exposure to quantify changes in tissue optical properties. Post-treatment optical properties were measured and processed using Prahl's inverse adding-doubling (IAD) algorithm to determine the absorption (μa) and reduced scattering (μs') coefficients of each sample. The IAD method allows for the comprehensive determination of optical properties, which is particularly useful when samples undergo changes due to external stimuli such as heating or chemical changes.

Results, Conclusions, and Discussions::

Results:
The optical properties of porcine dermis exhibited temperature-dependent changes across the range of 22°C to 70°C. At 37°C, both the absorption coefficient (μa) and reduced scattering coefficient (μs') of dermis samples showed slight changes compared to those at 22°C; values remained between 0.05-0.075 mm⁻¹ for μa and 2.5-7.5 mm⁻¹ for μ's. As temperatures rose from 37°C to 60°C, μa remained relatively stable, while μ's demonstrated more pronounced differences between pre- and post-treatment measurements. Such changes were particularly evident in the 60°C dataset, where post-treatment values exceeded pre-treatment levels across the wavelength spectrum. At 70°C a dramatic shift in optical properties was observed, with μs' following an inverse pattern where pre-treatment values exceeded post-treatment values at shorter wavelengths. All measurements demonstrated distinctive pre- and post-treatment differences. These differences became more pronounced at elevated temperatures. The wavelength-dependent response remained consistent across the temperature range, though the magnitude of the response varied with temperature—particularly in the higher temperature regimes.
Discussion and

Conclusion:
The comparison of pre- versus post-treatment optical properties in porcine dermis reveals temperature-dependent changes in tissue characteristics. The absorption coefficient showed only subtle differences between pre- and post-treatment states as temperatures increased to 60°C. However, the reduced scattering coefficient demonstrated more pronounced changes, with post-treatment values consistently higher than pre-treatment measurements across the wavelength spectrum. This pattern notably shifted at 70°C, where pre-treatment scattering values exceeded post-treatment values at shorter wavelengths (~500nm)—thus indicating a significant alteration in tissue structure at this elevated temperature. The wavelength-dependent behavior remained consistent in both pre- and post-treatment states across all temperatures, with both μa and μs' showing higher values at shorter wavelengths (~500nm) and decreasing towards 1000nm. However, the magnitude of this difference varied with temperature. The most distinct pre- versus post-treatment differences were observed in the higher temperature regimes (60°C and 70°C); these results suggest that these temperatures represent critical thresholds for irreversible changes in tissue optical properties. Overall, these observations provide insights into how thermal exposure modifies the optical characteristics of dermal tissue; such modifications have potential implications for various therapeutic applications in which heat-induced tissue modifications occur.

Acknowledgements and/or References (Optional)::