Characterization of angle-resolved measurement of diffuse reflected light

Abstract

Techniques for measuring the intensity of diffuse reflected light that propagates inside biological tissues are widely used for characterizing biological activities or functions. The authors proposed, for the first time, the use of angular distribution of diffuse reflected light to discriminate the variations in absorption coefficients caused in two individual layers of highly diffusive media. The capability of the technique was examined in terms of partial optical path length (POPL) as well as spatial sensitivity distribution (SSD) by means of Monte-Carlo simulation. It was found that the characteristic thickness of the top layer, for which the POPL or the sum of the SSD inside the two layers becomes equal, significantly differs with the direction of photodetectors. This fact indicates that the relative sensitivity of the variations in absorption coefficients varies depending on the direction of photodetectors. In order to evaluate the net effect of the angle-resolved measurement, the effective source-detector (S-D) distances were matched for the two oppositely oriented photodetectors. It was seen that the outermost contours of the SSDs still differed significantly for the two photodetectors, and a dipole-like structure, which could not be seen for the conventional (non-angle-resolved) measurement, appeared immediately below the detection area in the difference SSD. These results cannot be simply explained by the difference in effective S-D distance. Thus, the origin of the difference in the SSDs would be attributed to this dipole-like structure. The proposed method will be useful for measuring brain activities more accurately with fewer optical probes.