Photon migration

The propagation of light inside biological tissues can be described in terms of a transport process for photons (the quanta of light). In this perspective, the light source injects a certain number of photons per unit time, per unit volume, per unit solid angle into a specific tissue location and these photons travel inside the tissue along certain trajectories that are determined by the absorption and scattering properties of tissue. The collective motion of photons along these trajectories in tissue is called photon migration and can be described with transport theory and its high-scattering limit of diffusion theory. Absorption The main absorbers of near-infrared (NIR) light in blood-perfused tissues are oxy-hemoglobin, deoxy-hemoglobin, and water. Their absorption spectra between 300 and 1300 nm are shown in Fig. 1, which is obtained from compiled absorption data for water (Hale and Querry, 1973) and hemoglobin (Prahl). In Fig. 1, the concentrations of oxy-hemoglobin and deoxy-hemoglobin are assumed to be 50 μM, which is a typical value in blood-perfused tissues. The so-called “medical spectral window ” extends approximately from about 700 to 900 nm, where the absorption of light shows a minimum (see Fig. 1). As a result, light in this spectral window penetrates deeply into tissues, thus allowing for non-invasive investigations. The optical penetration depth into tissues is limited, at shorter wavelengths, by the hemoglobin absorption, and at longer wavelengths by the water absorption. Other NIR absorbers in tissues that may be important in particular cases include myoglobin, cytochrome oxidase, melanin, and bilirubin. The absorption properties of tissue are described by the absorption coefficient (μa), which is defined as the inverse of the average distance traveled by a photon before being absorbed. In the NIR, typical values of μa in tissues range from 0.02 to 0.30 cm-1. The photon mean free path for absorption thus ranges between about 3 and 50 cm