ABSTRACTObjective:To determine microvascular diameter and perivascular interstitium thickness at the lung surface in thein situ, in vivolung.Methods:Microscopic images of the lung surface collected through a “pleural window” by a videocamera were digitized with a monochrome frame grabber (512 × 512 pixels, 8 bits per pixels) to be computer analyzed by image processing techniques.Results:We found that the maxima in the distribution of the standard deviations of gray levels in adjacent neighbors 7 × 7 pixels wide identify the edges between the microvessel lumen and the surrounding perivascular interstitium. Furthermore, the maxima in the distribution of the standard deviation of the standard deviations of gray levels identify the edges between the perivascular interstitium and the lung tissue.Conclusions:This technique can be applied to microvessels ranging in diameter from 30 μm to 200 μm and perivascular interstitial thickness of the order of 10–150 μm. Our approach allows for the definition of microvascular geometry even for noisy images and represents an improvement compared to other edge detection methods. The proposed analytical procedure may provide a useful tool to study lung fluid balance and microvascular reactivity in thein situlung in the normal state and in response to a variety of functional