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Over the last few years there has been a vast improvement in our knowledge of light transport in turbid media. Although the basic physics of the transport process was previously well developed, the particular application of the theory to different relevant measurement protocols was not well understood. In this section the basic equations describing light transport in turbid media, using the frequency domain protocol, are reviewed (Sevik et al.). The proposal of using frequency domain methods in the context of photon density waves is expanded and examples of measurements are reported. Two complementary aspects of the frequency domain methods are presented: 1) the capability of the frequency domain technique to detect localized differences of the scattering and absorption coefficients, and 2) the possibility to resolve absorbing objects of different shape. In the photon density wave framework it is shown that objects of different shape scatter the photon density waves with different modalities, raising the possibility of distinguishing of the shape of objects immersed in the turbid medium from their characteristic diffraction pattern. The important issue of the additional advantage of using high-frequency methods to localize absorbing and scattering objects is also discussed. It is shown that the diffraction pattern is not very sharp, even at high frequency (Patterson et al.), but that the particular shape of the wavefront of the scattered photon density wave carries information on the size and shape of the scattering object (Gratton et al.). Finally, the issue of boundary conditions in medically relevant cases is discussed (Chance et al.). It is suggested that by using the substitution method, the escape of light from physical boundaries can be minimized. The consequences of this approach, when applied to the conventional finger oximeter, is discussed.
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