The specificity of optical spectral diffusion techniques is discussed in this chapter. As usual for this tutorial, two types of instruments and measuring techniques are presented: spectroscopic, used for tissue local parameters monitoring, and tomographic, used for tissue pathology imaging. Some of them are based on CW light source tissue probing. A few examples of CW measuring and imaging instruments and results of clinical studies are presented. Time-resolved techniques and instruments, which are the most promising for an accurate in vivo measurement, are also analyzed. In accordance with the basic principles discussed in Chapter 1, three types of time-resolved techniques and instruments are considered: the time-domain technique, which uses ultrashort laser pulses; the frequency-domain technique, which exploits an intensity-modulated light and narrowband heterodyne detection; and the phased array technique, which utilizes an interference of photon diffusion waves.
6.1 Continuous wave spectrophotometry
6.1.1 Techniques and instruments for in vivo spectroscopy and imaging of tissues
For the in vivo study of thick tissue (for example, the female breast), the collimated light transmittance can be described by an exponential law such as Eq. (1.1), taking into account that due to multiple scattering, the effective migration path of a photon before it is absorbed should be larger than the thickness of the tissue. For a slab of thickness d, the diffusion equation can be used to calculate a mean path length L of the photons as where μeff is defined by Eq. (1.18).