Nonlinear optical microscopy for investigation of gastrointestinal lesions

Abstract

The standard procedure for cancer detection includes rigorous biopsy protocols, which are costly and time consuming; also the accuracy of the current diagnostic procedure relays entirely on the physician’s experience and it is limited by the high probability of miss rates. Therefore new sensitive diagnostic modalities for analysis of biopsy tissue samples or on site, in vivo microscopy tissue examination, are necessary. In this study we present an investigation using nonlinear microscopy techniques for histological sections from biopsy tissue samples analysis. The samples were routinely processed for histological analysis and during the standard sampling the tissue slices were stained with hematoxylin and eosin dyes. The application of nonlinear microscopy techniques, such as two photon excitation fluorescence microscopy and second harmonic generation microscopy in biomedical research for cancer diagnosis has been vastly expanding in the last few years. Two-photon excitation fluorescence microscopy is based on a non-linear optical effect of simultaneously absorption of two photons, thus achieves excited state of the absorbing molecule with energy corresponding to the sum of the energies of two incident photons. This method allows for using an excitation wavelength which is double the typically required one for excitation of diagnostically valuable endogenous fluorophores. This results in more efficient depth penetration of the longer wavelength light in the tissue. The second harmonic generation microscopy is based on the principle of the non-linear susceptibility in noncentrosymmetric structures; such structures in the tissue are formed mainly by the collagen fibers. After excitation with near-infrared photons with wavelength λ of the collagen structures, photons with wavelength ½ λ are emitted — this corresponding to the second harmonic of the excitation beam’s frequency. The applied nonlinear microscopy techniques are suitable for detection and quantification of the morphological changes associated with stroma and epithelial transformation in colon cancer, providing complementary information about the tissue microstructure and displaying distinctive patterns between normal and malignant human colon tissues.