All-optics technique for monitoring absolute cerebral blood flow: validation against magnetic resonance imaging perfusion.

Significance: The ability to monitor cerebral blood flow (CBF) at the bedside is essential to managing critical-care patients with neurological emergencies. Diffuse correlation spectroscopy (DCS) is ideal because it is non-invasive, portable, and inexpensive. We investigated a near-infrared spectroscopy (NIRS) approach for converting DCS measurements into physiological units of blood flow.

Aim: Using magnetic resonance imaging perfusion as a reference, we investigated the accuracy of absolute CBF measurements from a bolus-tracking NIRS method that used transient hypoxia as a flow tracer and hypercapnia-induced increases in CBF measured by DCS.

Approach: Twelve participants (7 female, $28\pm 6$ years) completed a hypercapnia protocol with simultaneous CBF recordings from DCS and arterial spin labeling (ASL). Nine participants completed the transient hypoxia protocol while instrumented with time-resolved NIRS. The estimate of baseline CBF was subsequently used to calibrate hypercapnic DCS data.

Results: Moderately strong correlations at baseline ( $\text{slope}=0.79$ and $R^2=0.59$ ) and during hypercapnia ( $\text{slope}=0.90$ and $R^2=0.58$ ) were found between CBF values from calibrated DCS and ASL (range 34 to $85\mathrm{mL}/100g/\min$ ).

Conclusions: Results demonstrated the feasibility of an all-optics approach that can both quantify CBF and perform continuous perfusion monitoring.