Two steady-state methods for localizing a fluorescent inhomogeneity in a turbid medium

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Abstract
We present two cw methods for localizing a source of fluorescence buried in a medium with optical properties similar to those of tissue in the near infrared region. The first approach is based on the fact that, for small excitation beam diameters, the absolute intensity at a given depth in the medium depends on the diameter of the incident beam. For a well-chosen pair of beam diameters, the ratio of these intensities in a scattering medium depends uniquely on the depth from the surface of incidence. Thus, the ratio of the fluorescence resulting from sequential excitation using two beam diameters can be used to determine the depth at which the fluorescence originated. The second method is based on spatially resolved surface measurements of the diffuse fluorescence from the buried source. Using a form of the diffusion theory analysis of Farrell et al. (Med. Phys., 1992) for the spatially resolved diffuse reflectance from a pencil beam incident on a scattering medium, it is possible to reconstruct the depth of the source from the shape of the surface fluorescence profile. Preliminary experimental results obtained using a 1.0 cm diameter sphere containing the tumor localizing fluorophore Nile Blue A show that the spatially resolved measurement reports the location of fluorescent sources as deep as 4.0 cm with an accuracy of 0.4 cm or better.

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