The mechanisms of optical contrast in biological tissues and the limits of contrast are discussed. Some cancers develop as cellular masses which displace normal fibrous tissue or stroma, yielding a net drop in scattering properties. Some cancer may be more fibrous than normal tissues yielding a net increase in scattering.
Cancers sometimes develop increased vascularity which yields a net increase in absorption.
Another measure is fluorescence, either autofluorescence or dye-enhanced fluorescence. This paper considers the limits of resolution when optically characterizing a small spherical object within a turbid tissue. The ability to discriminate a small object with a strongly increased absorption vs a larger object with a slight increase in absorption is tested, using frequency-domain light transport with 3 GHz modulation of the light source.