Quantitative Imaging of Water Diffusion in Brain Parenchyma at Cellular Resolution Using Stimulated Raman Scattering...

The diffusion dynamics of water within the cerebral cortex regulate solute transport and are fundamental to overall brain function. However, cellular-level microscopic water dynamics within the brain parenchyma remain poorly understood due to a lack of direct, high-resolution measurement techniques. Here, we present an optical imaging framework to characterize water dynamics at cellular resolution by quantitatively tracking the diffusion patterns of deuterated water using stimulated Raman scattering (SRS) microscopy. High-resolution imaging of mouse cortical slices reveals heterogeneous water dynamics: free isotropic diffusion within the parenchyma and restricted diffusion around blood vessels. Detailed analysis enabled the simultaneous determination of apparent diffusion coefficients (ADCs) for water and Alexa Fluor 488 (AF488) within the parenchyma, revealing that the water ADC is ≈4.5-fold larger than that of AF488. Using an ischemic slice model, we demonstrate that the water ADC is reduced by 40%, concomitant with AF488. Furthermore, we directly resolve differences in water dynamics inside and outside the cells, both of which are retarded during acute phase of ischemia. These quantitative findings facilitate a fundamental understanding of water and solute dynamics in the brain under both physiological and pathological conditions, and provide a microscopic basis for interpreting MRI signals.