Optical probing depth in a wearable device: a Monte Carlo study

Abstract

Currently, wearable devices with optical probing system are widely employed for regular measurement of human physiological parameters. Among most frequently measured parameters are heart rate and blood oxygenation. Heart rate measurements are based on the principles on photoplethysmography based on detection of variation of tissue reflectance as a result of variation of blood content during pulsations. The measurement of oxygenation is based on the difference in the absorption spectra of oxy- and deoxyhemoglobin in visible and NIR bands, which allows for reconstruction of their ratio based on dual-wavelength measurements. Typical wavelength employed in wearable devices for heart rate measurements is 530 nm, while for oximetry a pair of wavelengths of 655 and 940 nm is often used.
The accuracy of the measurements is determined by the probing volume of the employed optical system; however, experimental estimation of the probing depth is a complex problem. In this connection, numerical modeling of light propagation in tissue offers a convenient solution for probing depth analysis.
In this study, we employed Monte Carlo modeling of tissue reflectance for probing wavelengths of 530, 655, 940 nm. This approach allows to simultaneously obtain the dependence of tissue reflectance on source-detector distance and corresponding distribution of maximal probing depths that are reached by individual photon trajectories in the medium.
The source detector distances providing the probing depths in the range of 0.3-1.5 mm were analyzed, since this range corresponds to the typical depth of blood-containing dermis layer, which ensures the most efficient measurements of heart rate and blood oxygen saturation.
For the wavelength of 530 nm the range of the probing depth of 0.3-0.4 mm are achieved for the source-detector distances of 1-2 mm. Since blood absorption in the red and NIR ranges is smaller, the wavelengths from these ranges provide deeper probing. For the wavelength of 655 nm, the probing depth range of 0.3-1 mm is achieved from the source-detector distance of 1-4 mm, and the maximum proximity of the probing depth to the center of the dermis layer (0.8-1 mm) is achieved with a source-detector distance in the range: 2.5-4 mm.

For the wavelength of 940 nm the probing depth range of 0.6-1.4 mm is achieved from the source-detector distance of 2-5 mm, with the maximum proximity of the probing depth to the center of the dermis layer (0.8-1 mm) for the source-detector distance in the range of 2.5-3.5 mm.

It is also shown that for oxygenation measurements in wearable devices at wavelengths of 655 and 940 nm, the optimal source-detector distance is 2.5 mm for both wavelengths, providing the probing depth of 0.8 mm for wavelengths.

Speaker

Egor Esipenok
N.I. Lobachevsky State University of Nizhny Novgorod, Nizhny Novgorod, Russia
Russia

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