Quantitative Phase Imaging *
Quantitative Phase Imaging (QPI) has emerged as a valuable method for investigating cells and tissues. QPI operates on unlabelled specimens and, as such, is complementary to established fluorescence microscopy, exhibiting lower phototoxicity and no photobleaching. QPI is a label-free technique in which various methods (for example off-axis digital holography, wavefront sensing, spatial light interference, ptychography) are used to retrieve the phase information of light passing through the cell. In contrast to traditional qualitative label-free techniques such as phase contrast or DIC, QPI measures the absolute phase delay and is high-contrast. In 2D acquisitions the images represent quantitative maps of optical path length delays introduced by the specimen, which correspond to differences between refractive index of the cellular components and medium, and the length of the optical path within the cell. The phase delay can be directly converted into a dry mass of the cell. In 3D acquisition a spatial distribution of refractive indices is obtained, giving a three-dimensional shape of the cell and its compartments.
QPI provides an objective measure of morphology and dynamics, free of variability due to contrast agents not used. QPI data are suitable for image segmentation, making label-free cell counting and tracking easy.
The interpretation of the phase signal has proven to deliver novel parameters for studying physiological processes in living cells, such as transmembrane fluid flux, dry mass and water content changes, intracellular transport as well as tissue structure and density changes. Protein concentrations, growth and cell motility can be precisely quantified. The morphologies of cells and organelles can be established by phase tomography. Their studies provide information on the biomechanical characteristics of cell structures and membranes. These data are indicative of biomolecular activity, which can be affected by pathology.