Observation and density estimation of a large number of skin capillaries using wide-field portable video capillaroscopy and semantic segmentation.

Significance: Skin capillaries are non-invasively observable; their structure and blood flow can reflect tissue and systemic conditions. Quantitative analysis of video-capillaroscopy images yields novel diagnostic methods. Because the capillary structure is heterogeneous, analyzing more capillaries can increase the evaluation reliability.

Aim: We developed a system that can observe and quantify numerous capillaries and verified the performance on human skin.

Approach: We developed a portable video-capillaroscope with a spatial resolution higher than and a wide field of view () and a method to evaluate capillary numbers and areas using U-Net. The model was trained and tested with 22 and 11 cropped images () obtained from 11 participants, respectively. They were then applied to the images from four participants. Segmentation results were compared to ground-truth at the pixel level and capillary-region level.

Results: Over 1000 capillaries were simultaneously observed using the proposed system. Although pixel-level segmentation performance was low [intersection over union (IoU) = 24.5%], the number and area could be estimated. These values differed among four participants and seven sites, and they changed after skin barrier destruction.

Conclusions: The proposed system allows for observing and quantifying numerous skin capillaries simultaneously, suggesting its potential for evaluating tissue and systemic conditions.