Time-resolved diffuse optical spectroscopy (TRDOS) provides a method for directly and independently determining the optical transport coefficients of turbid media. A multispectral, gated TRDOS system was built using a supercontinuum laser source and a fast single photon avalanche photodiode (SPAD) for detection. Electronic time-gating of the SPAD allowed for detection of time-gated photon distributions, and showed an increase of nearly 80x in dynamic range relative to ungated detection. TRDOS measurements using both ungated and gated detection schemes on two-layer tissue mimicking phantoms were acquired. The distribution of time-of-flight (DTOF) of photons was measured from a two-layered tissue simulating phantom at multiple source-detector separations and wavelengths. Measured DTOFs were matched to predictions from diffusion-theory (DT) for two-layered media after numerical convolution with measured instrument response functions. We show a dependence of the two-layer DT model on the input values of the upper layer thickness and refractive indices. It was found that both the upper layer thickness and refractive index parameters must well-determined for DT predictions to match measurements in two-layer media.
Near-infrared spectroscopy has been widely employed in biophotonics to study and quantify the optical properties of biological tissues. Unlike steady-state approaches, time-resolved spectroscopic techniques enable optical absorption and scattering properties of the medium to be separated, allowing for quantitation of depth-dependent absolute tissue optical properties. However, robust analysis of time-resolved signals requires careful consideration of calibration techniques and computational models. Here, we consider the effect of the time window employed when fitting a diffusion theory model to Monte-Carlo simulations. Next, we describe the impact of the temporal position of the instrument response function (IRF) in recovery of the optical properties. Finally, we discuss a technique to analyze time-resolved measurements without knowledge of the timescale of the IRF or the time-resolved measurement by fitting the relative shape of the photons’ distribution time-of-flight (DTOF).
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