What Remote PPG Oximetry Tells Us about Pulsatile Volume?

Area of Science:

• Biomedical Optics
• Physiological Measurement
• Optical Engineering

Background:

• Remote photoplethysmography (rPPG) is used in health monitoring, but lacks established theoretical underpinnings.
• Current rPPG oximetry relies on empirical methods, similar to traditional pulse oximetry's ratio-of-ratios calculation.
• The analytical relationship between the ratio-of-ratios (R) and tissue oxygenation in rPPG is not well-defined.

Purpose of the Study:

• To establish theoretical foundations for pulse oximetry in the context of rPPG.
• To derive an analytical expression for the ratio-of-ratios (R) used in rPPG oximetry.
• To investigate the relationship between R and arterial blood saturation (SaO2).

Main Methods:

• Applied perturbation approach within diffuse approximation for light propagation in tissues.
• Derived explicit expressions for the AC/DC ratio and the ratio-of-ratios (R) for rPPG signals.
• Simulated the dependence of R on SaO2 (70-100%) using various Red/Infrared (R/IR) light source pairs.

Main Results:

• Obtained explicit analytical expressions for the AC/DC ratio and R in rPPG.
• Demonstrated a highly accurate linear relationship between R and SaO2 (R² = 0.98-0.99) for multiple R/IR wavelengths.
• Successfully extracted information about the pulsatile volume location, estimating its depth at 0.6 mm in forehead rPPG data.

Conclusions:

• The study provides the first analytical derivation of the ratio-of-ratios for rPPG, establishing a theoretical basis.
• The findings confirm a strong, predictable linear relationship between rPPG signal parameters and tissue oxygenation.
• rPPG data can yield insights into microvascular dynamics, suggesting origins in the papillary dermis/subpapillary vascular plexus.