Discrimination of collagen in normal and pathological skin dermis through second-order susceptibility microscopy

Polarization-resolved, second harmonic generation (P-SHG) microscopy at single pixel resolution is utilized for medical diagnosis of pathological skin dermis. In analyzing the large area, pixel by pixel, second-order susceptibility of normal and pathological skin dermis, we found that P-SHG can be used to distinguish normal and dermal pathological conditions of keloid, morphea, and dermal elastolysis. Specifically, we found that the second order susceptibility tensor ratio of d(33)/d(31) for normal skins is 1.27+/-0.20, while the corresponding values for keloid, morphea, and dermal elastolysis are respectively 1.67+/-0.29, 1.79+/-0.30, and 1.75+/-0.31. We also found that the histograms of the d(33)/d(31) ratio for the pathological skins contain two peak values and are 1.5 times wider than that of the normal case, suggesting that the pathological dermal collagen fibers tend to be more structurally heterogeneous. Our work demonstrates that pixel-resolved, second-order susceptibility microscopy is effective for detecting heterogeneity in spatial distribution of collagen fibers and maybe used for future clinical diagnosis and in vivo studies of collagen pathological conditions.