PROJECT SUMMARY Alzheimer’s disease (AD) is a neurodegenerative disorder that manifests as progressive loss of memory and the ability in thinking and action. Despite thirty years accumulation of our knowledge on the pathological mechanisms, over 400 clinical trials of drugs targeting the pathological pathways have largely failed to reduce cognitive decline. Recent evidences from epidemiological, neuroimaging, and clinical reports have suggested that vascular contributions are critical in the pathogenesis of AD. A reduction of cerebral blood flow (CBF) has been recognized in preclinical AD population, many years before the onset of symptoms and the observed structural atrophy in the brain. In parallel, vascular pathophysiology is associated with a lower threshold of AD pathology in cognitive decline and dementia. The characteristic of preceding vascular alterations may offer a new opportunity in early-stage AD diagnosis and therapeutical assessment. However, current in vivo neuroimaging tools such as magnetic resonance imaging (MRI) and functional MRI (fMRI) exclusively focus on large vessels, due to their limited resolution and sensitivity, while leaving the microvascular territories largely unexplored. Our biophysical simulation work and other studies have indicated that capillaries, small arterioles and venules could contribute more than 50% of fMRI signals and alterations of microvascular architecture lead to profound functional changes in the human brain. Despite its intriguing insight on neurodegenerative diseases, those models were either based on oversimplified vascular geometry or anatomical networks derived from ~1mm3 of mouse cerebral cortex, which often failed to predict the complex architecture and hemodynamics in the human brain. The goal of the study is to establish a multiscale optical imaging technique to unravel the microvascular architecture network in the human brain from single capillary level to tens of cubic centimeters of tissue blocks. Pivoting on a serial sectioning optical coherence tomography combined with a two-photon microscopy, the multiscale imaging technique leverages a high-throughput and thorough study of vasculature pathophysiology in AD progression. The study will reconstruct volumetric architectural networks in human brain tissues at different stages of AD, seek for important features to characterize vascular pathological alterations, and correlate with quantitative neuropathological assessment to understand the converging path of AD pathology during disease progression. With the foundation of imaging-based microvascular networks in the human brain, the study will further build a computational model to investigate the cerebral blood flow, oxygenation, and fMRI signals during AD progression. This computational framework has been validated using the microvascular anatomy and dynamics from small animal models, and here we extend it for the first time to the human cortex. Completion of this project will significan...