Time-domain diffuse correlation spectroscopy: instrument prototype, preliminary measurements, and theoretical modeling

Abstract

Near-infrared spectroscopy (NIRS) is an emerging diffuse optical imaging tool with both clinical and academic applications such as functional brain imaging, breast cancer detection, and cerebral health monitoring. Due to its non-invasiveness, high spatial and temporal resolution, and portability, it has been rapidly growing in popularity over the last 40 years. The technique relies on near-infrared light to measure optical properties { scattering and absorption { which can then be used to infer details of the underlying tissue physiology. Diffuse correlation spectroscopy (DCS) is a complimentary optical technique that relies on long-coherence laser light, also in the near-infrared range, to measure dynamical properties of a medium { in the biomedical context, blood ow. While NIRS and DCS can be used in conjunction to provide even more powerful information, they require separate instrumentation, resulting in reduced portability and difficulty in bedside monitoring. In brain imaging applications, both NIRS and DCS suer from confounds due to layers surrounding the brain, such as the scalp and skull. While this issue has been addressed in NIRS using time-resolved instrumentation known as time-domain (TD) NIRS, it has been largely ignored in the context of DCS. In this work, we demonstrate a novel time-domain diffuse correlation spectroscopy (TD-DCS) technique embodied in a single instrument capable of simultaneously measuring optical and dynamical properties. Along with maintaining portability, the instrument reduces error by directly measuring the absorption and scattering values necessary for precise ow estimation, and removes a major confounding factor by suppressing unwanted signal from superficial layers through time-gating. We describe the construction of the first instrument prototype and demonstrate the depth resolution proof-of-concept with measurements of multi-layer media. We further discuss the theoretical considerations of modeling the light interaction with tissue, necessary for reliable estimates.