Depth resolution in coherent hemodynamics spectroscopy

Coherent hemodynamics spectroscopy (CHS) is a novel method based on the frequency-resolved study of induced hemodynamic oscillations in living tissues. Approaches to induce hemodynamic oscillations in human subjects include paced breathing and cyclic thigh cuff inflation. Such induced hemodynamic oscillations result in coherent oscillations of oxy-, deoxy-, and total hemoglobin concentrations in tissue, which can be measured with near-infrared spectroscopy (NIRS). The novel aspect of CHS is to induce hemodynamic oscillations at multiple frequencies in order to obtain frequency-resolved spectra of coherent hemodynamics. A dedicated mathematical model recently developed by our group, can translate the phase and amplitude spectra of these hemodynamic oscillations into physiological parameters such as capillary and venous transit times, and the autoregulation cutoff frequency. A typical method used in near-infrared tissue spectroscopy to measure oscillations of hemoglobin concentrations is based on the modified Beer-Lambert law, which does not allow for the discrimination of hemodynamic oscillations occurring in the scalp from those occurring in the brain cortex. In this work, we show preliminary results obtained by using diffusion theory for a two-layered medium, so that the hemodynamic oscillations obtained for the first and second layer are assigned to hemodynamic oscillations occurring in the scalp/skull and brain cortex tissues, respectively.