Detectability of low-oxygenated regions in human muscle tissue using near-infrared spectroscopy and phantom models

Abstract

The present work aims to describe the detectability limits of hypoxic regions in human muscle under moderate thicknesses of adipose tissue to serve as a groundwork for the development of a wearable device to prevent pressure injuries. The optimal source-detector distances, detection limits, and the spatial resolution of hypoxic volumes in the human muscle are calculated using finite element method-based computer simulations conducted on 3-layer tissue models. Silicone phantoms matching the simulation geometries were manufactured, and their measurement results were compared to the simulations. The simulations showed good agreement with the performed experiments. Our results show detectability of hypoxic volumes under adipose tissue thicknesses of up to 1.5 cm. The maximum tissue depth, at which hypoxic volumes could be detected was 2.8 cm. The smallest detectable hypoxic volume in our study was 1.2 cm³. We thus show the detectability of hypoxic volumes in sizes consistent with those of early-stage pressure injury formation and, consequently, the feasibility of a device to prevent pressure injuries.

Fig. 1.

Absorption spectra for normal and hypoxic muscle tissue model (curves are computed from published spectra of the molar extinction coefficients of [ $O_{2}Hb$ ] and [ $HHb$ ] [26]).

Fig. 2.

Two-layer geometry with hypoxic volume used for computer simulations (ATT stands for adipose tissue thickness and SDD for source-detector distance).

Fig. 3.

Fabricated and measured phantoms.

Fig. 4.

Measurement set-up. The numbers mark angles of ${36}^{\circ }$ .

Fig. 5.

Comparison of measurement with FEM simulations for three phantom configurations. Error bars stand for standard error. The average residual, plotted in the bottom right, is calculated as the mean of the difference of measurement and simulation at all SDD per phantom configuration.

Fig. 6.

Results of detectability of hypoxic volumes according to FEM computations. Red stands for highly detectable ( $\mathrm{\Delta }OD>0.1$ ), orange for detectable ( $\mathrm{\Delta }OD>0.05$ ), yellow for almost detectable ( $\mathrm{\Delta }OD>0.025$ ) and light yellow for not detectable ( $\mathrm{\Delta }OD\le 0.025$ ). If printed in grayscale, red appears darkest and light yellow lightest.

Fig. 7.

Hypoxic volume placed at a depth of 1.2 cm. ATT of 0.1 cm.

Fig. 8.

Hypoxic volume placed at a depth of 1.7 cm. ATT of 0.1 cm.

Fig. 9.

Hypoxic volume placed at a depth of 1.7 cm. SDD of 1.2 cm.

Fig. 10.

Hypoxic volume placed at a depth of 1.7 cm. SDD of 2.8 cm.