Pressure-induced near infrared spectra response as a valuable source of information for soft tissue classification

Acquiring near infrared spectra in vivo usually requires a fiber-optic probe to be pressed against the tissue. The applied pressure can significantly affect the optical properties of the underlying tissue, and thereby the acquired spectra. The existing studies consider these effects to be distortions. In contrast, we hypothesize that the pressure-induced spectral response is site- and tissue-specific, providing additional information for the tissue classification. For the purpose of this study, a custom system was designed for dynamic pressure control and rapid acquisition of spectra. The pressure-induced spectral response was studied at three proximate skin sites of the human hand. The diffuse reflectance and scattering were found to decrease with the applied contact pressure. In contrast, the concentrations of chromophores, and consequently the absorption, increased with the applied contact pressure. The pressure-induced changes in the tissue optical properties were found to be site-specific and were modeled as a polynomial function of the applied contact pressure. A quadratic discriminant analysis classification of the tissue spectra acquired at the three proximate skin sites, based on the proposed pressure-induced spectral response model, resulted in a high (90%) average classification sensitivity and specificity, clearly supporting the working hypothesis.