Investigation of Confounding Factors in Measuring Tissue Saturation with NIRS Spatially Resolved Spectroscopy

Abstract

Performing absolute measurements of tissue saturation of the brain with near-infrared spectroscopy (NIRS) is a clinically desirable brain monitoring tool. Tissue oxygenation index (TOI) is an indicator of absolute tissue mixed arterial and venous oxygen saturation, and can be calculated using a NIRS technique called spatially resolved spectroscopy (SRS). SRS instruments measure the change of light attenuation with distance by using multiple light source-detector distances at two or more wavelengths. The aim of the study is to use broadband NIRS SRS data to investigate the effects on the calculation of TOI of different parameters: wavelength selection, scattering dependence, source-detector distance, and resolving for water. In total, 55 neonates with hypoxic-ischemic encephalopathy were monitored using a broadband multi-distance continuous wave NIRS system; 172 datasets were recorded. Using a “Standard” approach, TOI values between 0 and 100% (“good”) were calculated in 157/172 datasets with a mean TOI of 50%. By changing the wavelength selection, the number of “good” data sets increases to 165/172 with a mean of 60%. Alteration of the dependence of scattering on wavelength acts as a constant which shifts the absolute value of TOI significantly (p < 0.05), demonstrating the importance of having a subject-appropriate estimation of scattering dependence. In general, changing the combination of source-detector distances does not significantly alter the TOI (the mean TOI ranges from 41% to 53%) which suggests that the algorithm is robust to different source-detector combinations. The study shows the broadband NIRS SRS algorithm gives the opportunity to explore the calculation of TOI and could further improve the measurement of tissue saturation in a clinical setting.

Cerebral Near Infrared Spectroscopy Monitoring in Term Infants With Hypoxic Ischemic Encephalopathy—A Systematic Review

“…However, broadband NIRS devices measure brain tissue light attenuation over a wide range of NIR wavelengths, allowing enhanced spectroscopic information and the possibility to resolve multiple chromophores. Broadband NIRS systems are particularly useful for monitoring changes in CCO (53) together with tissue saturation (46,72). Due to the relatively low concentration of CCO in vivo, the selection of the specific wavelengths and number of wavelengths becomes an important factor for monitoring CCO.…”

“…New NIRS monitors that can monitor cerebral oxygenation and blood flow [Babylux: combining DCS and time-resolved reflectance spectroscopy (TRS) (76), Metaox: combining FD-NIRS and DCS (77)], Cyril: monitoring of CCO (BNIRS) and Florence: monitoring of CCO and blood flow together (combining BNIRS and DCS, currently being used by UCL group) are encouraging innovations in this area. There is continued research in the optical community to investigate the validity of existing tissue oximetry algorithms, especially regarding their precision and reproducibility (46,78). New approaches, such as using broadband spectra (72) or novel combinations of spectral and multidistance techniques (79), are being developed to obtain more robust measurements of cerebral oxygenation.…”

Muscle oxygenation maintained during repeated-sprints despite inspiratory muscle loading

“…To assess muscle oxygenation status, we used spatially-resolved spectroscopy to calculate a Tissue Saturation Index (TSI). The Artinis system uses three light sources to calculate a correction factor which is used to determine [O 2 Hb] and [HHb] [30–32]. Then, the ratio of [O 2 Hb] to total haemoglobin is calculated (TSI = [O 2 Hb] / ([O 2 Hb] + [HHb], expressed in %).…”

The Various Oximetric Techniques Used for the Evaluation of Blood Oxygenation

“…StO 2 can also be obtained by the spatially-resolved NIRS by differentiating the diffusion equation with respect to ρ, which provides a relationship between the gradient of R(ρ) and µ a µ s ’ [ 96 , 107 , 108 , 109 , 110 ]. Since µ a = εC µ a µ s ’ = µ s ’εC = ε O StO 2 µ s ’C + ε D (1-StO 2 )µ s ’C, …”

“…See Equation (19). Measuring µ a µ s ’ with two wavelengths and assuming that the difference in C between the two wavelengths is negligible and that µ s ’ decreases slightly and linearly with the wavelength [ 107 , 110 ], StO 2 can be derived from Equation (20) in two wavelengths, using an algorithm that is similar to that for SaO 2 measurement by pulse oximetry, see Equations (7) and (8).…”