Coherence domain path length resolved approaches in optical doppler flowmetry

Laser Doppler flowmetry (LDF) is a non-invasive technique for monitoring blood microcirculation in biological tissues. Skin perfusion measurements using this technique depend not only on the instrumental factors, but also on the extent of interaction of photons with moving red blood cells. Thus for a constant perfusion, the LDF output signal is affected by the variance in individual photon path lengths due to the changes in tissue optical properties and probe geometry. In this article, we first give a concise introduction and an overview on the principles and practice of LDF. Special emphasize is given on the recent coherence gated path length resolved approaches made to overcome the limitation of conventional laser Doppler techniques, where the perfusion signal depends on the optical path length. An outlook to future developments is proposed in the last part of the paper, which will translate the novel methodology into a potential clinical tool that can bring more quantitatively reliable tissue perfusion information. In general, these approaches have potential applications in the fields of fundamental as well as applied research in monitoring the spatial and temporal variations in optical properties in turbid media.