6. Summary

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The mechanisms of contrast in these polarized light images are still being investigated.

• The ability of small structures like nuclei, mitochondria, membranes, and collagen fibers to interact differently with different wavelengths of light can contribute to these colors.
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• Differences in tissue thickness, density, and birefringence can influence the colors.

The key feature of such a difference image, PAR - PER, is that the multiply scattered light is canceled and image contrast is based only on superficially scattered photons. The image is sensitive to the superficial few hundred micrometers of tissue.

Various groups around the world are now attempting to achieve image contrast based on photon scattering. Such contrast avoids the need for introducing exogenous fluorophores or contrast agents into the body. Scattering offers a fingerprint for characterizing the ultrastructure of a tissue.

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