Tissue with fibrillar architecture, such as collagen or muscle fiber, exhibits birefringence. In addition to the scalar amount of birefringence, the orientation of the birefringence axis, i.e. the fast optic axis, provides important information on the physical orientation of the fibrillar tissue components. We have previously demonstrated local optic axis mapping using bench-top fiber-based polarization-sensitive optical coherence tomography (PS-OCT), by compensating for the transmission through fiber and system elements, imperfect system alignment, and preceding tissue layers. Using depth-multiplexed PS-OCT, the compensation considers both retardation and diattenuation and is applied in the wavenumber domain, preserving the full axial resolution of the system. Here, we extend our approach to catheter-based imaging. Analyzing a reflection signal from the distal tip of the optical probe, we decompose the recovered system transmission into a static component and a varying catheter transmission to accurately correct for the rotation-dependent transmission through the catheter. Catheter-based local optic axis mapping is validated with a custom-made birefringence phantom. Imaging ex-vivo human bronchus demonstrates the utility of reconstructing the local optic axis orientation to assess airway smooth muscle (ASM), which is oriented approximately orthogonal from the surrounding tissue, offering strong optic axis orientation contrast. Thickening and contraction of the ASM is considered a primary cause of breathing difficulties, and the capacity to clearly image the ASM could lead to an improved understanding of diseases such as asthma.
Non-Gaussian beams can provide extended depth of focus (DOF) at constant and potentially uncompromised transverse resolution, as well as a degree of self-reconstruction after beam shadowing, which may be present in tissue imaging. Hence such beams are being developed for imaging systems throughout many disciplines, including endoscopic imaging, where they hold great potential. General possibilities include up to more than 20-fold extension of DOF, tunable working distance, imaging around obstacles and integrated all-fiber designs.
In all-fiber based optical imaging systems; however, these advantages are limited by system design considerations. Trade-offs between miniaturization, extended DOF, SNR, and fiber availability arise, and estimating the effects of design modifications can be difficult and time consuming.
We model zero-order quasi-Bessel illumination and detection for a range of common probe and sample materials based on an analytic solution of the Fresnel diffraction integral and compare the results to Gaussian beams. We show that these beams, on scales that match optical fiber dimensions, generally have an upper limit for the spot size above which their distinct advantages over Gaussian beams fade. Similarly, we show the existence of a lower limit of practical performance of quasi-Bessel beams, where the imaging SNR penalty compared to a Gaussian beam becomes significant. Additionally to general theoretic considerations we discuss designs, modeling and characterization of all-fiber imaging probes.
This work provides an accessible overview for researchers to estimate what potential benefit non-Gaussian beams can introduce into their optical imaging system.
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