A review on time domain diffuse optics: principles and applications on human biological tissues

The term diffusive media refers to all the media for which the photon diffusion equation provides an accurate description of light propagation. Indeed, this is the case for a plethora of natural media, such as biological tissues and agricultural products, when illuminated by red and near-infrared light. Diffuse Optics (DO) is the branch of Optics that studies how absorption and scattering phenomena affect light propagation in diffusive media. In this review paper, we present an introduction to time domain (TD) DO, a specific implementation of DO that employs picosecond light pulses, fast and sensitive photodetectors and timing electronics to record the distribution of photon time-of-flight (or photon path lengths) in diffusive media. By interpreting the TD DO signals with the physical model provided by the photon diffusion theory, it is possible to estimate the absorption and scattering properties of the medium that in case of biological tissues can be related to physiological and pathological conditions. We focus on the physical principles of TD DO, the building blocks of TD DO instrumentation, and the applications of TD DO targeting human biological tissues (such as the brain, breast, muscle, and others).