Absorption spectroscopy in tissue-simulating materials: a theoretical and experimental study of photon paths

Area of Science:

• Biomedical Optics
• Spectroscopy
• Photonics

Background:

• Noninvasive absorption spectroscopy relies on light interaction with tissues.
• Understanding photon propagation is crucial for accurate signal interpretation.
• The spatial distribution of photon paths influences measurement sensitivity.

Purpose of the Study:

• To model the spatial dependence of a point absorber's signal in noninvasive absorption spectroscopy.
• To determine the photon-visit probability at different locations within tissue.
• To explore methods for controlling photon distribution for improved measurements.

Main Methods:

• Developed a diffusion model for noninvasive absorption spectroscopy.
• Calculated the relative probability of photon visits to specific tissue locations.
• Experimentally validated the model using tissue-simulating materials.
• Derived relationships for steady-state spectroscopy involving mean photon depth, source-detector separation, and optical penetration depth.
• Theoretically investigated the impact of pulsed sources and time-gated detection.

Main Results:

• The diffusion model accurately predicted the point-target response in experimental settings.
• A simple relation was found for steady-state spectroscopy linking photon depth, separation, and penetration.
• Pulsed sources with time-gated detection offer enhanced control over photon spatial distribution.
• The model quantifies the probability of photons visiting specific locations before detection.

Conclusions:

• The diffusion model provides a robust framework for understanding photon transport in noninvasive spectroscopy.
• Experimental validation confirms the model's accuracy in tissue-simulating materials.
• Time-gated detection with pulsed sources offers advanced spatial control for spectroscopic measurements.