Abstract 2984: Optical-resolution Photoacoustic Microscopy: a Novel Imaging Modality Capable of Noninvasively Measuring Brain Oxygen Metabolism at the Microvascular Level during Focal Ischemia

Abstract

Studying oxygen metabolism during brain ischemia has been limited by imaging modalities that have either low resolution but good tissue penetration, or high resolution requiring invasive preparations (open-skull windows). Here, we report a study of cerebrovascular oxygen metabolism in mouse focal ischemia using a novel minimally invasive, high-resolution technique known as optical-resolution photoacoustic microscopy (OR-PAM). Using OR-PAM, we serially imaged cortical vessels through the intact skull of Swiss Webster mice (n=3) before, during, and up to 72 hrs after a 1-hr intraluminal transient middle cerebral artery occlusion (MCAO). Vessels were imaged at 2 different wavelengths (532 & 563 nm) to quantify oxy-, deoxy-hemoglobin and oxygen saturation (sO 2 , Fig , A-C). Oxygen extraction fraction (OEF) was calculated from venous sO 2 values (assuming a constant arterial sO 2 of 95%). Seventy-two hrs after ischemia, animals were sacrificed and whole brains were stained with TTC to delineate infarct at the cortical surface (D). Arterioles were easily distinguished from venules in baseline images (A, mean arterial sO 2 , 87%; mean venous sO 2 , 76%). During MCAO, mean arteriolar sO 2 values did not change; however, venular sO 2 decreased precipitously (B, E), greater in regions that went on to infarct (45±2%) compared to surrounding regions (52±2%, p<0.001). Three and 7 hrs after reperfusion, venous sO 2 slowly recovered but not back to baseline levels. Twenty-four (C) and 72 hrs after reperfusion, venous sO 2 in regions of infarction increased above baseline (83%), approaching arterial sO 2 values of 90%, while venous sO2 in peri-infarct regions (71%) returned near baseline levels of 76% (E). OEF increased from 18% to 55% in regions of eventual infarction, during MCAO; following reperfusion, OEF progressively dropped below baseline levels (12%). In surrounding surviving tissue, OEF returned close to baseline levels (24%) following reperfusion (F). OR-PAM is capable of measuring oxygen metabolism at high resolution during focal ischemia. Consistent with previous PET studies in human stroke, we found OEF increased during acute ischemia; however, OEF remained elevated (above baseline values) after reperfusion, suggesting that injured tissue may have increased oxygen metabolism. OEF gradually decreased below baseline values in infarcted tissue, but returned to near-baseline values in viable peri-infarct regions.