Motivation: Traditional methods, such as blood volume fraction and vessel size index, often obscure variations in vessel size distributions within imaging voxels. Goal(s): This study introduces a novel technique for assessing microvascular structure changes through vessel size distribution imaging (VDI) using decay due to diffusion in the internal field (DDIF). Approach: Monte Carlo simulations demonstrate the capability of VDI to differentiate distinct vessel distributions. In vivo experiments validate this approach, showing sensitivity to vascular changes following acetazolamide administration and in ischemic stroke models. Results: These findings underscore VDI's potential for microvascular disease diagnosis and monitoring, warranting further exploration in various pathological contexts. Impact: VDI not only validate the effectiveness of the DDIF approach for assessing microvascular dynamics but also highlight its potential application in early disease diagnosis and monitoring.