In vivo time-domain diffuse correlation spectroscopy of the human muscle above 1000 nm

Time-domain diffuse correlation spectroscopy (TD-DCS) is an emerging optical technique with the potential to resolve the blood flow (BF) in depth. The first in vivo measurements have been shown recently on humans, however improvements in terms of signal-to-noise ratio (SNR) and depth sensitivity would be beneficial for biological applications. In this contribution, we explore the possibility of in vivo TD-DCS measurements above 1000 nm, and discuss its possible advantages compared to standard wavelengths (i.e. 700-800 nm). In our experimental setup, we exploited a tunable pulsed laser source extended more to the infrared and an InGaAs photomultiplier. Here, we report the results of a cuff occlusion on the forearm of a healthy adult subject at a wavelength of 1000 nm. Compared to the same experiment at standard wavelength (785 nm), the electric-field auto-correlation functions show a slower decay rate during all the experiment (both during and after the occlusion) as expected, suggesting a higher SNR. Even longer wavelengths, for diminishing water absorption, can be obtained through optimization of the laser source and the use of more efficient detectors.