Design of a single-fiber, wavelength-resolved system for monitoring deep tissue oxygenation.

We propose a single-fiber, zero source-detector separation system with wavelength-resolved detection for measuring oxygen saturation in deep brain structures. The system consists of a white light emitting diode (LED) source, optics to couple light into a 240-μm-diameter fiber, a beam splitter to separate the collected from the delivered photons and a spectrometer for detection. Depth resolution is achieved by inserting the fiber, comparable in size to microelectrodes used for electrophysiology, into the tissue of interest. Since most of the diffuse reflected light travels through a small volume at the tip of the fiber, this arrangement allows efficient collection of signal. Fresnel reflections are minimized using polarizers. Monte Carlo simulations across 400-1000 nm indicate that ~0.5% of the incident light can be collected and effectively interrogate a ~0.02 mm(3) volume at the fiber tip. System design, characterization data and phantom experiments using an absorptive dye in scattering media are presented. The simple nature of the instrumentation can potentially lead to a miniaturized system capable of detecting oxygen saturation in deep brain structures in freely-moving animals.