Noninvasive continuous optical monitoring of absolute cerebral blood flow in critically ill adults

Abstract

We investigate a scheme for noninvasive continuous monitoring of absolute cerebral blood flow (CBF) in adult human patients based on a combination of time-resolved dynamic contrast-enhanced near-infrared spectroscopy (DCE-NIRS) and diffuse correlation spectroscopy (DCS) with semi-infinite head model of photon propogation. Continuous CBF is obtained via calibration of the DCS blood flow index (BFI) with absolute CBF obtained by intermittent intravenous injections of the optical contrast agent indocyanine green. A calibration coefficient ( $\gamma$ ) for the CBF is thus determined, permitting conversion of DCS BFI to absolute blood flow units at all other times. A study of patients with acute brain injury ( $N=7$ ) is carried out to ascertain the stability of $\gamma$ . The patient-averaged DCS calibration coefficient across multiple monitoring days and multiple patients was determined, and good agreement between the two calibration coefficients measured at different times during single monitoring days was found. The patient-averaged calibration coefficient of $1.24\times {10}^9(\mathrm{mL}/100g/\min )/({\mathrm{cm}}^2/s)$ was applied to previously measured DCS BFI from similar brain-injured patients; in this case, absolute CBF was underestimated compared with XeCT, an effect we show is primarily due to use of semi-infinite homogeneous models of the head.

Keywords: absolute cerebral blood flow; brain-injured patients; continuous monitoring; diffuse correlation spectroscopy; time-resolved contrast-enhanced near-infrared spectroscopy.