DryMass: handling and analyzing quantitative phase microscopy images of spherical cell-sized objects

摘要

Quantitative phase imaging (QPI) is a technique frequently used for the determination of the refractive index (RI) and related physical properties, such as protein concentration or dry mass, of biological cells and cell-sized objects [ – ]. The physical quantity that QPI measures is the optical phase retardation introduced by the sample, which depends on its three-dimensional (3D) RI distribution. Until recently, one of the main obstacles in QPI was the extraction of the RI from the measured phase. This problem has been resolved, to a large extent, by optical diffraction tomography (ODT) which takes into account multiple viewing angles to construct a 3D map of the sample’s RI. While ODT yields RI maps with subcellular resolution, it requires elaborate experimental setups and computationally expensive tomographic reconstruction algorithms [ – ]. Furthermore, for objects that are homogeneous and exhibit a regular shape, a two-dimensional (2D) phase analysis can be sufficient. For spherical protein droplets and microgel beads, this approach enables an accurate evaluation of refractive index and size [ ]. For suspended cells, it can approximate the average refractive index and allows the quantification of relative differences [ , ]. Thus, many studies favor 2D QPI as a fast and efficient way to track and quantify changes in the RI.