Experimental study of migration depth for the photons measured at sample surface

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Abstract
Optical imaging of in vivo tissue and noninvasive optical assessment of deep tissue requires knowledge concerning photon migration paths in the medium. We used intralipid emulsion as a phantom medium to study the distribution of the migration paths. An incident and receiving optical fibers were placed on the medium surface. Light at 760 nm was used. An absorber was placed in the medium at different locations to intersect different photon paths. The study shows that the incident photons migrate to the detector through the paths distributed in a region shaped a 'banana', with its two end connecting the source and detector and it mid portion reaching deepest. This region has a core connecting the source and detector through which the photons have maximum probability to take. The path distribution in depth across the mid portion of the 'banana' and through the core can be described by a random walk model, with its maximum probability at a certain depth below the surface. In this study, this maximum probability depth ranged from 3 millimeters to about 7 or 8 millimeters. The path distribution across the mid portion of the 'banana' horizontally and through the core can be described by a normal probability function. These distributions are affected by the optical properties of the medium and the source-detector separation. This study indicated the capability of using surface optical measurement to image the optical property distribution of in vivo tissue and to assess deep tissue optical properties.

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