Properties of optical waves in turbid media

摘要

Optical measurements represent a valuable tool for in vivo analysis of tissue properties, for example, the average level of oxygenation of perfusing blood. A general problem in turbid medium, such as in the case of most tissues, is distinguishing between phenomena caused by absorption and those due to scattering. These problems can be overcome by using either time- or frequency-domain techniques. Frequency-domain measurements are based on evaluation of the phase and amplitude information of transmitted amplitude-modulated optical beams. These types of measurements might prove to be a valuable tool for in situ evaluation of tissue properties, for example, fluorescence and absorption. Irradiation of turbid media by harmonically-modulated optical beams initiates density waves of diffusely propagating photons. The phase velocity of these waves is quite different from the velocity of light. The velocity, which is strongly dependent on the modulation frequency, can typically vary from the velocity of light as an upper limit down to about 10% of this value in highly scattering, moderately absorbing tissue. This presentation gives a brief discussion of the general properties of these kind of waves in tissues.