Dual-slope near-infrared spectroscopy for selective sensitivity to cerebral hemodynamics

The dual-slope approach was conceived to alleviate superficial contamination of non-invasive near-infrared spectroscopy (NIRS) measurements and is especially advantageous when combined with frequency-domain NIRS (FD-NIRS) measurements of the phase of photon-density waves. In this work, we report analytical simulations using diffusion theory, and experimental results on the human brain to show the advantages, disadvantages, and optimal scenarios for brain measurements with different FD-NIRS data-types (intensity and phase) in single-distance and dual-slope configurations. Simulations show that dual-slope phase (DS𝜙) has a stronger sensitivity to absorption changes in the bottom (cerebral) layer compared to single-distance phase (SD𝜙), dual-slope intensity (DSI), and single-distance intensity (SDI). Our investigation of functional activation across four subjects showed variability in the relative activation amplitude recovered with FD data types for different cortical depths, indicating the dependence of the relative spatial sensitivity of these FD data-types to the details of tissue heterogeneity. Our two-layer analytical simulations using diffusion theory indicate that the two-layer model is unable to recover these relative sensitivities observed in vivo and requires the use of a more sophisticated model to better mimic the impact of tissue inhomogeneity on brain measurements using these FD data-types.