Polarization birefringence measurements for characterizing the myocardium, including healthy, infarcted, and stem-cell-regenerated tissues

Tissue polarimetry: concepts, challenges, applications, and outlook

Polarimetry has a long and successful history in various forms of clear media. Driven by their biomedical potential, the use of the polarimetric approaches for biological tissue assessment has also recently received considerable attention. Specifically, polarization can be used as an effective tool to discriminate against multiply scattered light (acting as a gating mechanism) in order to enhance contrast and to improve tissue imaging resolution. Moreover, the intrinsic tissue polarimetry characteristics contain a wealth of morphological and functional information of potential biomedical importance. However, in a complex random medium-like tissue, numerous complexities due to multiple scattering and simultaneous occurrences of many scattering and polarization events present formidable challenges both in terms of accurate measurements and in terms of analysis of the tissue polarimetry signal. In order to realize the potential of the polarimetric approaches for tissue imaging and characterization/diagnosis, a number of researchers are thus pursuing innovative solutions to these challenges. In this review paper, we summarize these and other issues pertinent to the polarized light methodologies in tissues. Specifically, we discuss polarized light basics, Stokes-Muller formalism, methods of polarization measurements, polarized light modeling in turbid media, applications to tissue imaging, inverse analysis for polarimetric results quantification, applications to quantitative tissue assessment, etc.

Two-photon microscopy of healthy, infarcted and stem-cell treated regenerating heart

Two-photon excitation autofluorescence (produced in myocytes) and second-harmonic generation (produced mainly by collagen) allow label-free visualization of these two important components of myocardium. Because of their different emission wavelengths, these two signals can be separated spectrally. Here, we examine two-photon microscopy images of healthy, infarcted and stem-cell treated rat hearts. We find that in infarcted heart, regions distant from the site of infarct are similar to healthy tissue in composition (mostly myocytes, very little collagen) and organization (densely packed myocytes), but infarct regions are characterized by sparse myocytes and high collagen content indicative of scar tissue formation. Stem cell treated hearts, in contrast, show regions of intertwined myocytes and collagen throughout the infarct, suggesting reduced tissue damage. Finally, these results offer interesting insights into our ongoing polarized light studies of cardiac tissue anisotropy, and reveal that both tissue composition and tissue micro-organization are reflected in polarization-measured linear retardance values.