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UBC Theses and Dissertations

Multimodal two and three-photon endomicroscopy for 3D tissue imaging Liu, Qihao


Multiphoton microscopy (MPM) is a promising multimodal imaging technique that can potentially provide fast, label-free examination of cells, extracellular matrices, and lipids similar to histopathology but without the need for biopsy. Each modality provides a complementary view of biological tissue. For example, two-photon microscopy can detect second harmonic generation from collagen and myosin, whereas two-photon excitation fluorescence can detect intrinsic fluorophores such as NADH from cells. Meanwhile, three-photon microscopy can detect third-harmonic generation from lipid-water and other interfaces. On top of deeper penetration and lower background noise when compared to single-photon microscopy, MPM also has inherent optical sectioning capability. However, the adoption of this non-invasive diagnostic technology by hospitals has been slow because most MPM are bulky, expensive, and complex benchtop systems with solid-state lasers that are not suitable for in vivo operations. To translate the technology to clinical applications, a miniature MPM probe or endoscope is required. Our group has previously developed a fiber-based MPM endoscope with a low-cost femtosecond fiber laser. My contribution was to improve and optimize the performance of this system. A key feature missing in past designs was an axial scanning module to enable automatic acquisition of Z-stacks for volumetric imaging. This was addressed by implementing a shape memory alloy autofocus actuator with accurate scanning over 300 microns. Other areas of improvement that were investigated included resolution, excitation power, and collection efficiency for multimodal imaging. Several objective lenses were compared and lateral resolutions below 1 micron and axial resolutions below 10 microns were achieved for field of view between 100 to 200 microns. Different anti-reflection coatings were explored for the relay lenses to increase the transmission power, most notably for third-harmonic generation. Lastly, the position of the multi-mode fiber was simulated and tested to collect a maximum amount of signal for all three contrasts. Ex-vivo Z-stack imaging experiments on leaf, murine, and piscine samples were conducted to demonstrate multimodality and depth scanning. All these additions and enhancements will serve as a stepping-stone for a prototype that will be clinically tested in the future.

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