Depth-Sensitive Cerebral Blood Flow and Low-Frequency Oscillations for Consciousness Assessment Using Time-Gated Diffuse Correlation Spectroscopy.

This study evaluates the feasibility of depth-sensitive bedside monitoring of cerebral blood flow (CBF) and low-frequency oscillations (LFOs) using time-gated diffuse correlation spectroscopy (TG-DCS) in healthy controls, a comatose patient with traumatic brain injury (TBI), and a subject with covert consciousness in the subacute phase. A 1064 nm TG-DCS system with superconducting nanowire single-photon detectors collected 10 min resting-state data from 25 healthy subjects, one comatose patient, and one covertly conscious subject. Photon arrival times were gated to separate superficial and deep signals. The blood flow index (BFI) was extracted from autocorrelation functions, and LFOs quantified via power spectral density in Slow 5 (0.01-0.027 Hz), Slow 4 (0.027-0.073 Hz), and Slow 3 (0.073-0.198 Hz) bands. A "smile" auditory paradigm was delivered to five healthy subjects and the covert case to assess functional responses. LFO power rose progressively from healthy to covert to coma, with larger differences in late-gated signals. Late-gated BFI was higher in injured states. During the smile task, healthy late-gate responses showed a canonical hemodynamic profile, whereas the covert case exhibited delayed reactivity. These findings demonstrate TG-DCS as a noninvasive bedside tool for distinguishing brain states and detecting residual cortical reactivity, supporting validation in larger cohorts.