Near infrared and visible spectroscopic measurements to detect changes in light scattering and hemoglobin oxygen saturation from rat spinal cord during peripheral stimulation

In this study, we investigated dynamic changes in light scattering and hemoglobin oxygen saturation (SscO2) on the rat spinal cord due to peripheral electrical stimulation by measuring near infrared (NIR) and visible spectroscopy, respectively. The spectral slope in the wavelength region between 700 and 900 nm is used as an index (SNIR) to quantify light scattering. With a 100-μm (source-detector separation) fiber-optic needle probe, optical reflectance was measured from the left lumbar region, specifically LL5, of the spinal cord surface at a height of 575 μm from the spinal cord surface. Graded electrical stimulations from 20 to 50 V, in increments of 10 V, were given to the plantar surface of the rat left hind paw for a period of 20 s. Changes in both light scattering (SNIR) and SscO2 were determined as a difference between the baseline and the maximum of slope value and hemoglobin oxygen saturation, respectively, during the stimulation period. There were significant differences in both SNIR and SscO2 during stimulation, with the average percentage changes of 10.9% and 15.5%, respectively. We observed that both SNIR and SscO2 measured at the spinal cord are insensitive to the intensity of the electrical stimulus, which is possibly caused by the nonlinear process of neurovascular coupling. Our finding essentially indicates that peripheral electrical stimulation results in significant changes in both light scattering and hemoglobin oxygen saturation on the rat spinal cord, and ignoring light scattering changes could lead to possible negative offsets of hemodynamic parameters (oxy-, deoxy-, and total hemoglobin concentrations) obtained in the functional optical imaging in the brain.