Measuring oxygen saturation using other Multi-Distance Methods

Methods other than SRS use multiple source–detector distances to estimate StO₂. While the methods vary, a shared principle is that shorter distances sample more superficial tissues, while longer distances sample deeper regions — such as the cerebral cortex or muscle tissue. These methods are sometimes loosely referred to as spatial resolution or self-calibrating approaches and, confusingly, the term spatially resolved spectroscopy is also occasionally used. Importantly, however, these methods do not use the same analytical approach as the classic SRS algorithms used in systems such as the Hamamatsu NIRO or Artinis PortaMon.

Among commercial devices, the most widely used has probably been the INVOS system, now sold by Medtronic. The traditional adult sensor uses detectors placed 3 cm and 4 cm from the source. The signal from the shorter separation, which is more sensitive to superficial tissue, is used to reduce the superficial contribution to the signal measured at the longer separation. The resulting signal is then used to estimate oxygen saturation in the deeper tissue. Although designed primarily for cerebral monitoring, INVOS has also occasionally been used for measurements in muscle.

An example of the INVOS being used to measure muscle oxygenation, including during exercise and arterial occlusion, is:

Hyttel-Sorensen S, et al. “Tissue oximetry: a comparison of mean values of regional tissue saturation, reproducibility and dynamic range of four NIRS-instruments on the human forearm.” Biomedical Optics Express 2 (2011): 3047–3057.

For mNIRS, the MOXY is probably the best-known example of a wearable NIRS device using a different multi-distance approach. It has source–detector separations of approximately 1.25 and 2.5 cm and uses a model of light transport through tissue to estimate muscle oxygen saturation.

The approach underlying the MOXY algorithm is described in a patent by its inventor, Roger Schmitz. Monte Carlo simulations of light transport through tissue are used to generate pathlength data, from which simulated diffuse reflectance values are calculated for different combinations of tissue optical properties. These are used to generate a lookup table that can be compared with the measured optical signals to estimate tissue oxygenation.

Schmitz R. “Systems and methods for measuring oxygenation.” US Patent No. 8,941,830 B2 (2015).
https://patents.google.com/patent/US8941830B2/en

The MOXY source–detector separations and its performance compared with an SRS instrument are discussed in:

McManus CJ, Collison J, Cooper CE. “Performance comparison of the MOXY and PortaMon near-infrared spectroscopy muscle oximeters at rest and during exercise.” Journal of Biomedical Optics 23 (2018): 015007.