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Investigation of signal dependence on tissue thickness in near infrared spectral imaging.

The signal intensity in near infrared autofluorescence and polarization sensitive light scattering imaging is explored as a function of tissue thickness using homogeneous porcine cardiac tissue samples as a model system. Eight images are recorded from each tissue sample including two autofluorescence images obtained under 408 nm and 633 nm excitation and six light scattering images acquired with alternating linear polarization orientations (parallel or perpendicular) under 700 nm, 850 nm, and 1000 nm linearly polarized illumination. The mean image intensity of each sample for each imaging method is plotted as a function of tissue thickness. The experimental results indicate a strong dependence of the detected signal on tissue thickness up to approximately 2 mm. Furthermore, the intensity of the spectral ratio images also exhibit thickness-dependent changes up to about 3 mm. The behavior of the light scattering experimental data was reproduced using a mathematical model based on a modified version of the random walk theory of photon migration.