Diffuse optical spectroscopy (DOS) is applied to the local measurement of the optical properties of scattering media, whereas diffuse optical imaging (DOI) and diffuse optical tomography (DOT) are used to characterize or image the spatial distribution of the properties. These methods can be converged into powerful spectral imaging measurements that combine information on the wavelength dependence of the optical properties of the scattering medium (which are relevant for the characterization of its composition and microscopic scattering properties) and on their spatial dependence (which provides localization of regions of interest and indications on large-scale organization). A quantitative description of the volume probed by an interrogating optical signal in a scattering medium is significant for both spectroscopy and imaging measurements. In spectroscopy (localized measurements) it specifies the sampled region, whereas in imaging (spatially resolved measurements) it gives information on the spatial resolution. Such a quantitative description, leading to the definition of a region of sensitivity in the scattering medium, is presented in detail in this chapter.

There are numerous methods for diffuse optical imaging, ranging from straightforward backprojection and optical topography (i.e., 2D projections from diffuse reflectance data) to more complex linear and nonlinear image reconstruction schemes. Some of these methods (for example topography and backprojection) directly translate the measured optical data into an image of the investigated tissue. Other methods (such as linear image reconstruction approaches) translate the imaging problem into an algebraic matrix inversion that can be performed with numerical methods. The most powerful tomographic approaches are nonlinear iterative techniques that make use of forward solvers to calculate the optical signals that correspond to a given spatial distribution of the tissue optical properties. An initial estimate of a spatial distribution is iteratively updated until its associated optical signals match, to within a given tolerance level, the measured optical signals and constitute the desired reconstructed optical image.

These imaging methods have their relative strengths and weaknesses, and are subject to different trade-offs among robustness, simplicity, power, and accuracy. Our goal in this chapter is to describe the basic principles and quantitative methods for diffuse optical imaging techniques that are frequently used in the study of biological tissues.