Robustness of tissue oxygenation estimates by continuous wave space-resolved near infrared spectroscopy

Abstract

Continuous wave near infrared spectroscopy (CW-NIRS) is widely exploited in clinics to estimate skeletal muscles and brain cortex oxygenation. Spatially resolved spectroscopy (SRS) is generally implemented in commercial devices. However, SRS suffers from two main limitations: the a priori assumption on the spectral dependence of the reduced scattering coefficient [μ 0s ðλÞ] and the modeling of tissue as homogeneous.Aim: We studied the accuracy and robustness of SRS NIRS. We investigated the errors in retrieving hemodynamic parameters, in particular tissue oxygen saturation (S t O 2 ), when μ 0 s ðλÞ was varied from expected values, and when layered tissue was considered.Approach: We simulated hemodynamic variations mimicking real-life scenarios for skeletal muscles. Simulations were performed by exploiting the analytical solutions of the photon diffusion equation in different geometries: (1) semi-infinite homogeneous medium and constant μ 0 s ðλÞ;(2) semi-infinite homogeneous medium and linear changes in μ 0 s ðλÞ;(3) two-layered media with a superficial thickness s 1 ¼ 5, 7.5, 10 mm and constant μ 0 s ðλÞ. All simulated data were obtained at source-detector distances ρ ¼ 35, 40, 45 mm, and analyzed with the SRS approach to derive hemodynamic parameters (concentration of oxygenated and deoxygenated hemoglobin, total hemoglobin concentration, and tissue oxygen saturation, S t O 2 ) and their relative error.Results: Variations in μ 0 s ðλÞ affect the estimated S t O 2 (up to AE10%), especially if changes are different at the two wavelengths. However, the main limitation of the SRS method is the presence of a superficial layer: errors strongly larger than 20% were retrieved for the estimated S t O 2 when the superficial thickness exceeds 5 mm.Conclusions: These results highlight the need for more sophisticated strategies (e.g., the use of multiple short and long distances) to reduce the influence of superficial tissues in retrieving hemodynamic parameters and warn the SRS users to be aware of the intrinsic limitation of this approach, particularly when exploited in the clinical environment.

Muscle Oximetry in Sports Science: An Updated Systematic Review

“…The other CW-NIRS studies, assuming constant path length, potentially lead to errors in estimating of muscle metabolic changes due to incorrect assumptions about tissue scattering. This in turn underestimates actual muscle oxygenation/deoxygenation as compared with measurements obtained by real-time path-length determination by either TD-NIRS or FD-NIRS [ 209 , 210 ]. Therefore, the implementation of TD- and FD-NIRS approaches on dedicated wearable sensors remains to be achieved.…”

Retrieval of chromophore concentration changes in a digital human head model using analytical mean partial pathlengths of photons

“…We have considered two hypothetical situations, which are opposite from each other. This choice is intended to summarize the wide spectrum of possibilities arising from the combination of hemodynamic activities in both the scalp and the cortex when considering real clinical cases; 16 , 18 , 53 – 57 hence, we have assumed that any of these combinations could lie within the situations depicted in this work.…”