Events on Tuesday, July 15th, 2025
- A combined theoretical and computational analysis of collagen structure in biological tissues by 3D second harmonic generation excitation and emission tomography
- Time: 10:00 am - 12:00 am
- Place: Engineering Centers Building 1045
- Speaker: Emily Shelton, Physics PhD Graduate Student
- Abstract: Second harmonic generation (SHG) has been used to great extent as an imaging modality that selectively targets fibrillar collagen without the need for exogenous dyes to investigate the collagen architecture in biological tissues, as this structure is frequently altered in many diseases including cancers and fibroses. While SHG metrics have been developed to characterize the fiber, fibril and supramolecular aspects of collagen, there remains a need to better understand the underlying non ideal phase-matching that governs the contrast and the emission directionality, where these arise from the sub-resolution intermediary fibril size, packing, and polarity. This dissertation presents a new combined theoretical and computational treatment based on quasi-phase-matching of the how the three-dimensional SHG spatial emission pattern is determined by the fibril organization. This is used to place bounds on the fibril size and packing parameters as well as explore the effects of heterogeneity in both fibril size and polarity clustering on the emission pattern. This work is then expanded to include fibril organization into fibers and simulate the fibrils in a more realistic fashion in order to explore the effects of fiber rotation on the emission pattern and more accurately model individual tissue types, specifically focusing on normal and cancerous ovarian tissue. This treatment not only provides a more rigorous physical basis for understanding SHG in the non-ideal phase-matching regime, but also as an additional characterization tool of collagen alterations in diseased states.
Although SHG microscopy has intrinsic optical sectioning, it is not a true 3D modality as fibers with axes that lie along the direction of laser propagation are transparent to SHG as the interaction is electric dipole forbidden. As such, current SHG microscopy could miss important information and does not provide a full tomographic image of the collagen structure in biological tissue. Previously a stage-insertable platform was developed that rotates the sample to allow for multiview SHG imaging such that any fibers missing in any single view would be visualized in another. While the initial reconstruction method resulted in a reasonable tomographic view of rat tail tendon, it did not perform as well on other, less aligned tissues. However, evaluating reconstruction methods on experimental data is inherently difficult due to the lack of a ground truth. In this dissertation, a toy model of tomographic SHG imaging was developed through the derivation of the dependence of SHG intensity on the angle between the fiber and laser axes, which is then used to better examine the limitations of our current reconstruction methods. This model could be used to develop and evaluate new reconstruction methods leading to improved tomographic SHG imaging. - Host: Paul Campagnola