In Chapter 9 (Section 9.5), we derived the diffusion equation for spatially uniform turbid media (Eq. (9.38)). If one is interested in measuring an average tissue property (say, the average hemoglobin concentration in muscle tissue, or the average hemoglobin saturation over a certain cerebral cortical volume), then Eq. (9.38) may provide a suitable analytical approach, within the limits of the assumption that the investigated tissue is spatially homogeneous. Of course, strictly speaking, this assumption is not correct, since biological tissues are not spatially homogeneous. At a microscopic level, it is the very presence of molecular species, intracellular organelles, and cellular structures that ultimately accounts for the absorption and scattering of light in tissues. However, these tissue inhomogeneities occur over spatial scales of microns (cellular structures), sub-microns (cellular organelles), and nanometers (structural proteins, organelle ultrastructure, biological macromolecules, etc.) that are not directly relevant to the macroscopic treatment of the absorption coefficient, diffusion coefficient, and optical energy density distribution that are considered by diffusion theory. By contrast, the presence of a macroscopic spatial heterogeneity in tissues, associated, say, with the skin and subcutaneous layers, bones, larger blood vessels, tendons, and multiple tissue types within the optically probed volume, may raise questions about the applicability of a model that assumes the spatial homogeneity of tissue.

In this chapter, we identify the length scale that is relevant in diffusion theory; in other words, the typical diffusion length that one should consider in comparison with the length scale over which the tissue optical properties vary. This chapter also derives expressions for the continuous-wave diffuse reflectance under a variety of conditions, and examines its dependence on the optical properties of the medium. Diffuse optical spectroscopy (principles described in Chapters 10–12 and experimental methods in Chapter 13) places an emphasis on the overall optical properties of the examined biological tissue, from which some relevant physiological or diagnostic parameters can be extracted. Diffuse optical imaging (described in Chapter 14), instead, places an emphasis on the spatial distribution of the optical properties of tissue at a macroscopic scale and is relevant in the detection of localized abnormalities (tumors, hemorrhagic or ischemic lesions, fluid-filled cysts, etc.) or focal events (targeted brain activation, localized hemodynamic responses, etc.). This chapter is devoted to continuous-wave methods for tissue spectroscopy, where the emission of the light source is constant; i.e., it is not time-varying.