We introduce a novel method to enhance the sensitivity of near-infrared spectroscopy (NIRS) to deep tissue (i.e. brain cortex, skeletal muscle, etc.) in non-invasive diffuse optical measurements. Our method relies on the collection of the phase of photon-density waves, launched by intensity-modulated light in frequency-domain NIRS, from two paired sets of multi-distance data. The two sets of data are combined into a phase dual-slope, which features a stronger sensitivity to deeper vs. superficial tissue. For typical conditions of functional NIRS, the maximum sensitivity of phase dual-slopes is at a depth of ~11 mm, which approaches the depth of cortical tissue.
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