5.4.1　Principles of RCM

The RCM system is mainly composed of diode laser，beam splitter，focusing lens and detectors（Figure 5-7）.The light passes through a beam splitter，a scanning and focusing optical lens and a skin contact device.It penetrates the skin and illuminates a small tissue spot.Light reflected from the focal point reflects back through the lens，which focuses it into a small pinhole and forms an image on a photodetector.The pinhole only allows light from a focal point to pass through（i.e.，confocal），and prevents light from another tissue point or out-of-focus plane from getting through.When the beam is moved in one direction（scanning），a line of reflected signals is generated（X-axis）and when the beam is moved in the other direction（Y-axis），a complete area can be scanned leading to an en face image of the tissue comparable to an“optical slice”.By moving that plane into or out of the tissue（Zaxis，parallel to the beam direction），a stack of images can be generated representing the optical image of a tissue volume，enabling the confocal microscope to visualize at a slice in the sample，similar to CT.Therefore，RCM is also called skin CT.The obtained images are three-dimensionally reconstructed by computer，which is beneficial for the multi-angle observation of the three-dimensional section or overall structure of skin tissue.RCM images are based on the difference in refractive index of organelles and skin tissues to achieve high resolution；melanin，keratin and fibrous tissue in skin tissue have high refractive index，serving as a natural control.The tissue with high refractive index is bright，and the one with low refractive index is dark.

The device uses a diode laser at 830 nm with a power lower than 35 mW at tissue level.The maximum imaging range of RCM is 500 μm×500μm，with the maximum imaging depth of 500μm.The magnification is up to 100 times，with the lateral resolution of 0.5 and the minimum optical section thickness is 1.7μm.A 30×objective lens with a numerical aperture of 0.9 is used with either water（refractive index，1.33）or gel（refractive index，1.3335）as an immersion medium.It captures images with a spatial resolution of 0.5-1.0 mm in the lateral dimension and 4-5 mm in the axial dimension.The advantages of RCM are as follows：①Non-invasive，real-time and dynamic monitor of cell/tissue morphology and physiological functions in the same skin lesion，which is favorable for follow-up observation of its development，changes，and improvement after treatment.②Dynamic changes of blood flow inside skin blood vessels can be observed.③When it is difficult to determine the location for routine histopathological examinations，multiple suspicious lesions can be simply observed with RCM measurement without complicated and tedious processing.④Imaging is rapid，and the data is easy to store and output.(https://www.daowen.com)