Reproducibility of a time-resolved NIRS, multidistance diffuse correlation spectroscopy system: towards noninvasive...

Longitudinal monitoring of cerebral blood flow (CBF), oxygenation ( ${\mathrm{StO}}_2$ ), and cerebral metabolic rate of oxygen ( ${\mathrm{CMRO}}_2$ ) could allow for early detection of secondary brain injury before clinical signs manifest.

Reproducibility and reliability of an in-house time-resolved near-infrared spectroscopy (trNIRS)/multidistance diffuse correlation spectroscopy (mdDCS) system were assessed to determine the feasibility of daily monitoring. In addition, carotid compression was performed to demonstrate longitudinal monitoring.

Time-resolved NIRS/mdDCS measurements were acquired daily on volunteers across one week. Reproducibility was quantified by the coefficient of variation (CV) and reliability by the intraclass correlation coefficient (ICC) at three levels: within-acquisition, between-acquisition, and between-day. A subset of participants returned on day 21+, and carotid compression was performed to reduce CBF. Signal changes during compression were compared to baseline measurements from that day and previous days.

All optical measurements showed good-to-excellent within-acquisition, between-acquisition, and between-day reproducibility ( $\mathrm{CV}<19\%$ ) and reliability (ICC between 0.67 and 0.99), except short-distance DCS, which was sensitive to probe repositioning. Reductions in CBF, ${\mathrm{StO}}_2$ , and ${\mathrm{CMRO}}_2$ caused by carotid compression were similar in magnitude when using same-day baseline to baseline values up to three weeks prior.

This study demonstrates the feasibility of daily monitoring using a hybrid trNIRS/mdDCS system.