High-speed quantitative optical imaging of absolute metabolism in the rat cortex

Abstract

Significance: Quantitative measures of blood flow and metabolism are essential for improved assessment of brain health and response to ischemic injury. Aim: We demonstrate a multimodal technique for measuring the cerebral metabolic rate of oxygen ( ${\mathrm{CMRO}}_2$ ) in the rodent brain on an absolute scale ( $\mu M{O}_2/\min$ ). Approach: We use laser speckle imaging at 809 nm and spatial frequency domain imaging at 655, 730, and 850 nm to obtain spatiotemporal maps of cerebral blood flow, tissue absorption ( ${\mu }_a$ ), and tissue scattering ( ${{\mu }_s}^{\text{'}}$ ). Knowledge of these three values enables calculation of a characteristic blood flow speed, which in turn is input to a mathematical model with a "zero-flow" boundary condition to calculate absolute ${\mathrm{CMRO}}_2$ . We apply this method to a rat model of cardiac arrest (CA) and cardiopulmonary resuscitation. With this model, the zero-flow condition occurs during entry into CA. Results: The ${\mathrm{CMRO}}_2$ values calculated with our method are in good agreement with those measured with magnetic resonance and positron emission tomography by other groups. Conclusions: Our technique provides a quantitative metric of absolute cerebral metabolism that can potentially be used for comparison between animals and longitudinal monitoring of a single animal over multiple days. Though this report focuses on metabolism in a model of ischemia and reperfusion, this technique can potentially be applied to far broader types of acute brain injury and whole-body pathological occurrences.

Keywords: brain ischemia; brain metabolism; cardiac arrest; cerebral blood flow; cerebral metabolic rate of oxygen; diffuse optical imaging.