Physical And Physiological Mechanisms Of Skin Optical Clearing

Optical clearing technique can enhance light penetration in tissues and give advance to optical imaging and therapy by applying high refractive index, hyperosmolarity and biocompatible chemical agents on tissues. And optical clearing for skin is a hot topic in this field. But the mechanism of skin optical clearing is still not clarified, few quantitative results were reported and there is still no research of in vivo mechanism, which limited the development and applications of skin optical clearing technique. In this work, physical and physiological mechanisms of skin optical clearing were investigated with tissue phantom, in vitro skin and in vivo skin:1) Research for physical mechanism of skin optical clearing on tissue phantom. By measuring particle size distribution with electron microscope, the scattering property of Intralipid was predicted by Mie theory. After Mixing Intralipid with six OCAs separately, mixture are all more transparent. PEG200and PEG400induced particles aggregation, while others keep the solution uniform. Reduced scattering coefficients of solutions decreased with agents'refractive indices increasing, which matches well with theory prediction. The results indicate that physical mechanisms of skin optical clearing are increasing of background refractive index and changing of particle sizes.2) A partial least square model was established to measure water content from reflectance spectrum. Porcine skin samples were immersed in six common OCAs separately and their optical properties were measured by an integration sphere with IAD algorithm. The results show a correlation between reductions of water contents and reduced scattering coefficients. But for some OCAs, reduction of scattering coefficient is higher than that of water content. This implies a major mechanism of dehydration during optical clearing process. But some other mechanisms may also influence the process of optical clearing.3) The microstructure and ultrastructure were investigated with HE stained slices, second harmonic generation imaging and electron microscopy after dermal injection of glycerol. The dermal structure of skin did not change much but the thickness decreased; diameter of collagen fibers decreased and got more regular packing. The collagen fibers were not dissociated as in vitro immersion in glycerol. The results indicate in vivo mechanism of optical clearing can be different from in vitro study.4) Exact physiological and optical properties from reflectance spectrum. Monte Carlo simulation was used to get a lookup table between reflectance spectrum and optical properties, and the table was optimized for skin. An approximate model was proposed and combined with least square root fitting, that can obtain melanin volume fraction, blood volume fraction, blood saturation, reduced scattering coefficients and rayleigh scattering fraction from skin reflectance spectrum. The extracted parameters were further tested by multilayer Monte Carlo simulation, commercial melanin probe and blood occlusion experiment. The results show a good accuracy for the method and provide a new in vivo research method for the mechanism of skin optical clearing.5) Monitoring the in vivo optical clearing process, the connection between physiological and optical properties was established and in vivo mechanism was explored. Topical application of OCAs on rat skin was investigated with reflectance spectrum analysis. Reduced scattering coefficients, blood volume fraction and blood saturation was extracted. OCT was used to calculate the image depth improvement. The results show that after15min application of PEG400with thiazone combined with physical massage, reduced scattering coefficients decreased, blood volume fraction increased and blood saturation hardly changed. The reduction of reduced scattering coefficients increased deep signal and increased imaging depth in OCT image.In conclusion, this work investigated the physical and physiological mechanisms with tissue phantom, in vitro skin and in vivo skin. The results show high refractive index of OCAs increased the background refractive index, and improved refractive index matching in skin; the hyperosmolarity induced dehydration of skin and might change the scattering particles; some OCAs might induce reversible change to the particle sizes. This work will contribute to development of high efficacy, biocompatible optical clearing technique for basic researches and clinical applications.