Refractive index estimation in biological tissues by quantitative phase imaging

The refractive index (RI) of biological tissues represents a relevant optical property that correlates with intrinsic factors such as dry mass or water content, providing valuable information about tissue composition. First, we review available methods to measure the RI in biological tissues, making special emphasis on quantitative phase imaging (QPI) and its ability to obtain RI maps as well as tri-dimensional projections of transparent samples at diffraction-limited resolution. Second, we propose a simple, general QPI method to estimate the RI of complex materials without prior knowledge of sample thickness. Here we have tested several samples in various immersion media with different refractive indices (RIs) to use the lineal relation between the optical path difference (OPD) and the RI of the medium. Least squares regression operates to simultaneously determine the RI and the thickness with diffraction-limited resolution and nanometric transversal sensitivity. Our findings categorize a set of immersion media covering a good RI range, identify possible error sources and uncover the applicability of the QPI method, paving the way to its use on heterogeneous biological tissues.

Automated summary

The refractive index (RI) of biological tissues is an important optical property that can provide valuable information about tissue composition. This study reviews available methods to measure RI in biological tissues, with a focus on quantitative phase imaging (QPI) and its ability to obtain RI maps and three-dimensional projections of transparent samples. A simple QPI method is proposed to estimate the RI of complex materials without prior knowledge of sample thickness, using the linear relationship between optical path difference (OPD) and RI of the medium. The findings of this study categorize a set of immersion media, identify error sources, and demonstrate the applicability of the QPI method in heterogeneous biological tissues.