Imaging 3D tissue fiber organization using optical polarization tractography
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Optical polarization tractography (OPT) is a new imaging technology developed based on an advanced Jones matrix implementation of polarization-sensitive optical coherence tomography (PSOCT). OPT can acquire high-resolution, three-dimensional (3D), depth-resolved images of fiber organization in tissue. To validate OPT's accuracy in measuring fiber orientation, a comprehensive histology comparison study was conducted using heart tissues which are known to have a depth-dependent fiber orientation change. A systematic image processing procedure was developed to register histology images with OPT images so that the pixel-wise difference between the two measurements can be compared in details. The validation results indicated that OPT can reveal the tissue fiber tractography with histology-like resolution. OPT was then applied to image freshly excised heart samples of the mdx mouse model of Duchene muscular dystrophies (DMD). A rotational imaging platform was developed to obtain OPT images of the excised whole mouse heart. The imaging light was repetitively scanned along the long axis of the heart while the heart was rotated continuously on the imaging platform. The acquired 3D image data were then transformed to construct the 3D whole heart image. The "cross-helical" laminar architecture of the myocardial fibers can be clearly visualized. More importantly, the OPT results revealed significant global and microscopic structural remodeling in the heart of the mdx mouse. OPT can also be applied to image fiber organization in other fibrous tissue samples. For example, OPT revealed focal fiber disorganization in the tibialis anterior (TA) muscle of the mdx mouse, which was confirmed in histology as muscle damage. The 3D OPT images of the TA muscle can be quantified by analyzing the randomness of the fiber orientation distribution. A "fiber disarray" index can be computed to automatically identify and visualize all damaged tissues in the 3D TA muscle. Since the OPT only images the projected fiber orientation within a plane perpendicular to the light propagation, a dual-angle imaging procedure was developed to obtain the absolute 3D fiber orientation. The two 3D OPT images of the same tissue acquired at two different view angles were registered to reconstruct the image of the absolute 3D fiber orientation. This new method was validated by imaging a mouse extensor digitorum longus (EDL) muscle placed at various known positions. The capability of this dual-angle OPT method was demonstrated by visualizing the absolute 3D muscle fiber structure in mouse TA muscles and the unique arcade collagen fiber architecture in a piece of articular cartilage. In summary, the results presented in this dissertation study indicated that the newly developed OPT technology can obtain high-resolution 3D image of tissue fiber organization. OPT may provide a practical tool for studying disease related fiber structural changes in many fibrous tissues.
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