Dynamic optical property changes: implications for reflectance feedback control of photocoagulation

Area of Science:

• Biomedical Optics
• Laser-Tissue Interaction
• Photocoagulation Modeling

Background:

• Laser treatments cause tissue coagulation, altering optical properties like scattering.
• The formation of white lesions significantly changes light distribution within tissues.
• Understanding these optical changes is crucial for effective laser therapy, particularly in retinal photocoagulation.

Purpose of the Study:

• To simulate and analyze the dynamic effects of white lesion formation on laser light reflection and fluence rate.
• To model light distribution in a layered tissue phantom with varying coagulated layer thicknesses.
• To investigate the relationship between lesion thickness and optical properties, including reflectance and fluence.

Main Methods:

• Simulated a model medium with absorbing and albumin layers, including coagulated (white) and uncoagulated (clear) sections.
• Determined optical properties of each layer.
• Employed one-dimensional adding-doubling and three-dimensional Monte Carlo methods to model light distribution.
• Experimentally measured reflectance using a CCD camera.

Main Results:

• A coagulated layer ≥275 microns thick decreased fluence reaching the absorbing layer.
• Highly scattering coagulated layers caused sub-surface fluence peaks exceeding incident levels for thicknesses <275 microns.
• Reflectance increased with coagulation thickness, plateauing at 9 optical depths (2 mm).
• Central reflectance measurements correlated with computed trends, unlike total reflectance.

Conclusions:

• Coagulation significantly alters light distribution and optical properties during laser treatment.
• Central reflectance imaging offers a viable method for controlling laser lesion thickness.
• These findings provide a theoretical basis for optimizing laser treatment parameters based on real-time reflectance feedback.