In vivo time-domain diffuse correlation spectroscopy at 1 µm

Diffuse correlation spectroscopy (DCS) is an optical technique which, by studying the speckle intensity fluctuations of coherent light diffused in a turbid medium, retrieves information regarding the scatterers motion. In the case of biological tissues, the particles of interest are the red blood cells, from which is possible to measure non-invasively microvascular blood flow (BF). However, being based on a continuous-wave light source, depth discrimination is achievable only by using multiple source-detector separations. On the other hand, time-domain (TD) DCS is a novel approach which exploits a pulsed yet coherent light source to discriminate the intensity fluctuations at different photon time-of-flights. This additional information is beneficial for in vivo applications, due to the physical relationship between photon time-of-flight and mean depth penetration. TD-DCS is typically performed in the spectral range between 700 and 800 nm. Here, we explore TD-DCS in a new spectral range compared to the typical one, moving to the spectral region beyond the water absorption peak (i.e., >970 nm). We performed liquid phantom and in vivo experiments on the human muscle at a wavelength of 1000 nm. Also, the possible advantages in terms of depth sensitivity and signal-to-noise ratio are discussed.