Measuring oxygen saturation using Spatially Resolved Spectroscopy (SRS)

There are methods for measuring absolute muscle tissue oxygen saturation (StO₂) that do not require expensive time-domain or frequency-domain systems. One such method is Spatially Resolved Spectroscopy (SRS). Originally developed in the Hamamatsu NIRO systems, SRS is now best known for its implementation in devices such as the Artinis PortaMon.

SRS assumes homogeneous scattering within tissue. To visualize this, imagine shining a torch into a dilute scattering solution such as milk: the light that emerges appears broadly diffused, and its brightness does not change much with small changes in viewing position. This is because scattering redirects light in many directions, creating a relatively uniform glow. If you add a chromophore to the milk, the brightness decreases with distance because of absorption.

In an SRS measurement, a single light source is combined with multiple detectors placed close together relative to their distance from the source (or vice versa). The slope of the change in optical density with source–detector separation can then be used to estimate absorption by the chromophores without having to measure the absolute scattering coefficient.

This gives values proportional to:

K × [oxy-heme]

K × [deoxy-heme]

K × [total-heme]

To calculate StO₂, you simply divide K × [oxy-heme] by K × [total-heme] and multiply by 100 to express it as a percentage. The constant K cancels out in the ratio, allowing estimation of tissue oxygen saturation.

The original reference for this method is:

Suzuki S, et al. “Tissue oxygenation monitor using NIR spatially resolved spectroscopy.” Proceedings of SPIE 3597 (1999): 582–592.

An interesting technical discussion of the SRS method and its limitations is provided in this open-access article:

Amendola C, et al. “Robustness of tissue oxygenation estimates by continuous wave space-resolved near infrared spectroscopy.” Journal of Biomedical Optics 28 (2023): 075002.