Generalized self-calibrating probe approach for CW tissue oximetry

Functional near-infrared spectroscopy (fNIRS) and oximetry are important noninvasive methods to investigate physiological processes and states in the human brain and muscle tissue. Self-calibrating probe methods combine two light sources with typically two detectors to improve the stability of continuous-wave (CW) devices, but so far are limited to fully symmetric geometries.

We developed a generalized self-calibrating probe approach to improve accuracy and stability in CW oximetry for arbitrary configurations of two light sources and detectors.

We used the diffusion theory of light transport to derive the effective attenuation coefficient from self-calibrating probes with arbitrary asymmetric source-detector configurations in reflection. The theory was validated by measurements on tissue-like reference phantoms and by a vascular occlusion test on the forearm muscle of a healthy subject using a dual-wavelength CW fNIRS imager. Results were compared with conventional analysis by spatially resolved spectroscopy (SRS). Time-domain measurements served as a reference.

The phantom studies with the generalized self-calibrating probe approach provided accurate results for effective attenuation and oxygen saturation for the wide variety of source-detector configurations examined. Results from the SRS method showed a slightly larger spread. When submillimeter shifts of the source positions were neglected in the data analysis, large errors were observed in oxygen saturation for both methods. The vascular occlusion test showed good agreement between the self-calibrating approach and the time-domain reference data. Oxygenation profiles from SRS differed strongly across the source-detector combinations, with significant deviations already for the baseline saturation. When CW measurements were analyzed using literature values for the tissue scattering properties, systematic shifts of the oxygen saturation occured. The shifts remain small when the ratio of the reduced scattering coefficients at the two wavelengths of the CW device is close to the true tissue value.

The proposed generalized self-calibrating approach enables quantification of oxygen saturation in vivo for a wide variety of source-detector configurations whereby a good estimate of the ratio of the tissue scattering properties at the two wavelengths is required. Our single-case study shows the potential of the self-calibrating approach to considerably improve the accuracy and stability of CW oximetry for the large dynamic range of a vascular occlusion compared with conventional measurements using SRS with a single light source.