Video-rate quantitative phase and polarimetric imaging (QP2I) for label-free imaging of contracting muscle cells

Abstract

Quantitative phase imaging (QPI) has become a standard approach for label-free microscopy of transparent specimens, mapping the optical path length. However, the isotropic phase is blind to the molecular order underlying optical anisotropy. Implementations that also add quantitative contrast to birefringence, rely on multi-frame illumination or polarization-state scanning, or on dedicated interferometric hardware, that trade away acquisition speed or the simplicity of a standard microscope. We present Quantitative Phase and Polarimetric Imaging (QP2I), a computational imaging framework that converts a standard differential interference contrast (DIC) microscope with a polarization camera into a single-shot, video-rate, dual-contrast imaging system. We developed a new globally regularized phase reconstruction that eliminates artifacts of prior DIC-to-QPI approaches, providing quantitative phase information and used per-pixel Stokes-based reconstruction to obtain quantitative anisotropic retardance. We validate the approach across simulations, static reference targets, fixed muscle tissue and living dynamically moving cells. Thereby, QP2I enables to study contractile dynamics in live cardiomyocytes all-optically and label-free. It provides both structural and functional readouts at frame rates that are sufficient to temporally resolve fast cardiomyocyte contractions. While conventional fluorescent labels bleach out over time, perturb physiology, and preclude long-term monitoring, QP2I provides similar established structural readouts, such as frame-by-frame sarcomere length, as well as previously inaccessible measurements, like quantitative analysis of retardance changes during muscle contraction, as a direct optical signature of muscle photoelasticity. QP2I thus enables long-term, label-free interrogation of dynamic cellular processes at quantitative contrast.