Depth-Sensitive Cerebral Blood Flow and Low-Frequency Oscillations for Consciousness Assessment Using Time-Domain 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-domain diffuse correlation spectroscopy (TD-DCS) in healthy controls, comatose patients, and subjects in the Minimally Conscious State (MCS). A 1064 nm TD-DCS system equipped with superconducting nanowire single-photon detectors (SNSPDs) was used to collect 10-minute resting-state data from 25 healthy adults, 3 comatose patients with traumatic brain injury (TBI), and 2 patients with MCS in the subacute phase. Photon arrival times were temporally gated to distinguish superficial and cortical tissue contributions. The blood-flow index (BFI) was extracted from gated autocorrelation functions, and LFOs were quantified using power spectral density within the Slow-5 (0.01-0.027 Hz), Slow-4 (0.027-0.073 Hz), and Slow-3 (0.073-0.198 Hz) bands. Resting-state LFO power progressively decreased from healthy individuals to MCS and comatose patients, consistent with impaired neurovascular coupling and diminished autoregulatory function in severe consciousness impairment. An auditory "smile" command was delivered to five healthy subjects, one MCS patient, and one Unresponsive Wakefulness Syndrome (UWS) patient to assess task-evoked hemodynamic responses. During the smile task, healthy participants showed clear hemodynamic responses, whereas the MCS and UWS patients demonstrated smaller and more rapidly decaying responses. Overall, TD-DCS provides a noninvasive, depth-resolved approach for differentiating consciousness states and identifying residual cortical responsiveness, supporting its potential for bedside neurocritical-care monitoring.