In medical diagnostic procedures for examining the cervix and the tissues of the vagina and vulva, long working-distance (-30 cm) lighted binocular microscopes (colposcope) that provide up to 25x optical magnification are used to create an illuminated magnified view. Speculum dilations can give rise to specular reflections from the tissue surface, causing physicians to overlook possible abnormalities – thus decreasing the quality of a colposcopy.
Researchers at the National Institute of Child Health and Human Development (NICHD) developed a polarimetric accessory that overcomes this limitation and enhances the visibility of subsurface structures of the scattering object. Linearly polarized light is used for cervical illumination and imaging performed through an additional polarizer that separates the specularly reflected light from the diffusely backscattered light originating in deeper tissue layers. This technology provides enhanced imaging of the hidden subsurface tissue structure (texture). The region of interest is illuminated by linearly polarized light, and backscattered light passes through the polarization filter that is detected by a digital camera. A custom optical design preserves the polarization state of the backscattered light in the microscope, without interfering with the standard optical path and operation of the microscope – including its binocular system. Special algorithms to visualize regions of statistical similarity in the image were developed.
Though the diffusely backscattered light presents only a small fraction of the detected light, its analysis – using the customized design and image processing procedures – provides useful information about internal structures of biological tissues. The polarimetric accessory includes a linear polarizer for the illuminating beam, two beam splitters for preserving polarization state, lens system for imaging, polarization analyzer, band-pass optical filter, digital camera, and electronic triggering system.