Time-resolved reflectance using short source-detector separation

In Optical Time-Resolved Reflectance, a pair of injecting and collecting optical fibers are placed at a fixed source-detector separation from each other typically in the range of 20 to 40mm limited by the detector dynamic range. To increase the sensitivity to higher penetration depth of investigation, the source and the detector separation should be small. We first show with simulation results that short source-detector separations results in the detection of a higher number of photons coming from a greater depth. However, at these shorter distances the number of early arriving photons also increase (mainly coming from the skull and scalp regions in brain imaging) which is a constraint. To reject the early arriving photons we need a gated detector to enable detections at specified time windows. We then confirm these results in an experimental study using a simplified photon detection scheme. The dependency of the photon counts on the gate window and the source-detector separation is analyzed. We conclude that placing the laser source and the detector quite close to each other is an option to consider for the design of optodes so as to improve the image quality in various biomedical appications.