Microvascular Oxygen Tension and Flow Measurements in Rodent Cerebral Cortex during Baseline Conditions and Functional Activation

Measuring cerebral oxygen delivery and metabolism microscopically is important for interpreting macroscopic functional magnetic resonance imaging (fMRI) data and identifying pathological changes associated with stroke, Alzheimer's disease, and brain injury. Here, we present simultaneous, microscopic measurements of cerebral blood flow (CBF) and oxygen partial pressure (pO₂) in cortical microvessels of anesthetized rats under baseline conditions and during somatosensory stimulation. Using a custom-built imaging system, we measured CBF with Fourier-domain optical coherence tomography (OCT), and vascular pO₂ with confocal phosphorescence lifetime microscopy. Cerebral blood flow and pO₂ measurements displayed heterogeneity over distances irresolvable with fMRI and positron emission tomography. Baseline measurements indicate O₂ extraction from pial arterioles and homogeneity of ascending venule pO₂ despite large variation in microvessel flows. Oxygen extraction is linearly related to flow in ascending venules, suggesting that flow in ascending venules closely matches oxygen demand of the drained territory. Oxygen partial pressure and relative CBF transients during somatosensory stimulation further indicate arteriolar O₂ extraction and suggest that arterioles contribute to the fMRI blood oxygen level dependent response. Understanding O₂ supply on a microscopic level will yield better insight into brain function and the underlying mechanisms of various neuropathologies.