Time-Resolved Diffuse Reflectance Using Small Source-Detector Separation and Fast Single-Photon Gating

We demonstrate the feasibility of time-resolved diffuse reflectance measurements at small source-detector separations using a single-photon avalanche diode operated in time-gated mode. Photon time distributions at an interfiber distance of 2 mm were obtained on a homogeneous tissue phantom with a dynamic range of ${10}^6$ and collecting photons at arrival times up to 4 ns. Moreover, we were able to detect a local inhomogeneity deeply buried within a diffusive medium with better spatial resolution, higher signal intensity, and same contrast of a larger (20 mm) interfiber distance. Finally, the proposed approach proved valuable to detect in vivo a task-related brain activation.

Figure 1

Time-resolved photon distribution at $\rho =2\mathrm{mm}$ obtained combining slices of time-gated reflectance measurements normalized by the injected energy. The recovered curve (gray line) is fitted to the convolution (solid line) of the theoretical model with the IRF (dotted line). Inset: Simplified schematics of the experimental setup.

Figure 2

Contrast for a set of x-scans (top) and z-scans (bottom) for measured data (left) and simulations (right), at $t_g=0.5\mathrm{ns}$ (triangles) and $t_g=1.5\mathrm{ns}$ (diamonds) for $\rho =2\mathrm{mm}$ (filled symbols) and $\rho =20\mathrm{mm}$ (empty symbols). Data for the z-scans refer to a depth $z=8\mathrm{mm}$ (triangles) and $z=18\mathrm{mm}$ (diamonds). Simulated data without convolution with the IRF are also shown as gray lines.

Figure 3

In vivo measurement on the left motor cortex at $\rho =2\mathrm{mm}$ with $t_g=1.5\mathrm{ns}$ for a finger tapping exercise with the right hand (solid line), the left hand (gray line), and at rest (dotted line). Each curve corresponds to the folding average over 5 trials. The vertical lines mark the start and the stop of the exercise.