Depth-sensitive cerebral blood flow and low-frequency oscillations for consciousness assessment using time-domain diffuse correlation spectroscopy

<b>Significance</b>: Continuous bedside monitoring of cerebral blood flow (CBF) and neurovascular dynamics is important for assessing brain function and detecting secondary injury in patients with traumatic brain injury (TBI) and disorders of consciousness (DOC). However, most neuroimaging modalities lack portability or depth specificity for use in neurocritical care settings. <b>Aim</b>: This study evaluates the feasibility of using time-domain diffuse correlation spectroscopy (TD-DCS) for depth-sensitive monitoring of cerebral blood flow and low-frequency oscillations (LFOs) in healthy individuals and patients with DOC. <b>Approach</b>: A 1064-nm TD-DCS system equipped with superconducting nanowire single-photon detectors (SNSPDs) was used to acquire resting-state measurements from 25 healthy adults and five TBI patients with DOC. Photon arrival times were temporally gated to separate superficial and cortical-weighted signals. Cerebral blood flow index and LFO characteristics were analyzed using power spectral density within the Slow-5, Slow-4, and Slow-3 frequency bands. Task-evoked responses were also assessed using an auditory "smile" command paradigm. <b>Results</b>: Compared with healthy controls, DOC patients exhibited altered resting-state LFO amplitude and spectral distribution, with a relative shift toward slower oscillatory components. Task-evoked measurements demonstrated clear hemodynamic responses in healthy participants, while responses in DOC patients were attenuated and more transient. <b>Conclusion</b>: TD-DCS provides a noninvasive and depth-resolved approach for monitoring cerebral hemodynamics and spontaneous oscillatory activity, supporting its potential as a portable bedside tool for assessing cerebrovascular dynamics and residual cortical responsiveness in neurocritical-care patients.