Reflectance Quantitative Polarized Light Imaging for Microstructural Evaluation of Musculoskeletal Soft Tissues

Musculoskeletal soft tissue injury is common and can lead to debilitating pain, loss of tissue function, and overall decrease in quality of life. Proper biomechanical function of musculoskeletal soft tissues is dictated by their hierarchically organized collagenous extracellular matrices, which are disrupted in periods of disease or injury. To best understand dynamic structure-function relationships of these tissues in normal and pathological conditions, it is imperative to be able to visualize collagen microstructure using a quantitative, high spatiotemporal resolution imaging modality. The innate birefringence associated with collagen fibers permits the use of biomedical polarimetry techniques to probe collagen in a specific and non-contact manner. Historically, we have used a transmission mode based, division-of-focal-plane Stokes polarimetry technique referred to as quantitative polarized light imaging (QPLI) to characterize and correlate tendon and ligament mechanics with their underlying collagen fiber microstructures. However, this technique requires excision and thinning of tissues, limiting its overall physiological relevance. The work described in this dissertation is centered around the development of a reflectance mode imaging configuration for QPLI. By modifying the technique to be based on reflected light, in situ tissues under more complex loading regimes and environmental conditions can be imaged. We hypothesized that reflectance mode QPLI could enable nuanced, real-time analysis of collagen fiber architecture in scenarios where information about the ECM response is currently not well understood. The dissertation aims first to evaluate the fundamental biological underpinnings of signal gathered from reflectance QPLI and compare that to transmission mode and other optical imaging modalities. Polarization sensitive Monte Carlo models of photon transport in tissue were also developed to computationally probe polarized light-tissue interactions across simulated tissues with a range of biologically relevant optical properties. Further, experimental methods were leveraged to assess the validity of noise propagation theory in common QPLI outcomes when imaging biological tissues to aid in translation of the technique to low-light scenarios such as arthroscopy. Finally, we explored a novel application area where reflectance mode QPLI may have high potential for clinical impact: monitoring of progressive, biologically mediated damage in tendon. Outcomes from the studies presented herein further establish reflectance mode QPLI as a powerful tool for monitoring collagen fiber dynamics in musculoskeletal soft tissues, but also serve as principles and context for data interpretation that are applicable to biomedical polarimetry modalities more broadly