PROJECT SUMMARY/ABSTRACT High-resolution imaging techniques are essential for investigation of the pathogenesis and development of therapeutic and screening strategies for cerebral small vessel disease (SVD), including Alzheimer Disease (AD) and AD-related dementias. Previous studies have demonstrated a link between retinal and cerebral microvascular changes. Moreover, the accessibility of the retinal tissue for noninvasive imaging offers an advantage for identifying microvascular dysfunction due to SVD. However, a significant gap remains in our understanding how changes in the capillary networks in cortex and retina mirror each other during both normal ageing and SVD progression. Filling the gap requires innovative technologies to measure absolute microvascular oxygenation, perfusion, and morphology at subcapillary scale in both retina and cortex, applied to animal models that recapitulate the clinical and histopathological features of microvasculopathy. We propose to develop novel methods to investigate retinal O2 delivery and consumption with unprecedented spatial resolution and to correlate changes that occur in parallel in retinal and cortical microvasculature during normal ageing and AD. Aim 1 will develop and validate a multimodal two-photon fluorescence imaging method for combined measurements of absolute oxygen tension (pO2) and blood flow in major retinal vessels and derive global measurements of O2 metabolic rate. We will then investigate how normal aging affects microvascular structure and function in cortical and retinal tissue. Aim 2 will develop and validate an adaptive optics two-photon microscopy imaging method for pO2 in the retinal capillaries, coupled with imaging of capillary red blood cell (RBC) flux using the previously established RBC passage method. High-resolution retinal tissue pO2 imaging will also be developed, as well as a deep Differential Aberration imaging of pO2 in vasculature and tissue. Aim 3 will test the hypothesis that microvascular hemodynamic and tissue metabolic changes during early stages of AD occur in parallel in retina and cerebral cortex. We will compare retinal and cortical microvascular oxygenation, blood flow, and O2 metabolic rate in experimental mouse model of AD. Development of novel methods to investigate retinal O2 delivery and consumption will advance our understanding of the concurrent microvascular dysfunction in cortex and retina during both normal ageing and SVD progression, provide broad utility in assessing retinal microvascular domains in animal models of eye disorders, and augment knowledge of vascular mechanisms of progressive neurodegeneration and cognitive impairment in common forms of SVD as well as in AD and related dementias.