Label-free full-thickness imaging of porcine vagus nerve fascicular anatomy by polarization-sensitive optical coherence tomography.

Improving the efficacy of vagus nerve (VN) stimulation therapy requires a detailed understanding of the anatomical and functional organization of nerve fiber bundles and their fascicles. Variousimaging platforms have been optimized for this purpose. However, all existing tools with micrometer resolution require labeling to enhance the fascicle contrast, and this labeling is resource-intensive and time-consuming. Polarization-sensitive optical coherence tomography (PS-OCT) was previously used to perform high-speed, label-free small animal (rat) sciatic nerve imaging but has not been applied for imaging the full-thickness large animal VNs (>1 mm diameter thick) due to tissue-limited imaging depth. We developed a PS-OCT platform that circumvents this problem and demonstrate high-speed label-free imaging of full-depth, multiple centimeters-long mammalian VNs for the first time.We employed a custom-built PS-OCT system with a dual-surface scanning microscope to capture opposite sides of the sample in a single frame. A tailored post-processing algorithm maximized fascicle contrast and merged the two surfaces together. Multi-centimeter-long porcine VNs were imaged.Our approach reconstructed fascicle information throughout the full-thickness of the VN when compressed to a 650m thickness. Moreover, we cross-validated PS-OCT measurements of fascicular organization and retardance to assess myelination against pair histology from the same specimens, showing Spearman's rank correlation coefficient value of 0.69 (-value < 0.001).We demonstrated a label-free optical imaging method for large-volume VN imaging. The time to image a 6.8 cm nerve was 680 s with 0.1 mm slongitudinal sample translation speed, which is more than two orders of magnitude faster than existing modalities that require labeling. With this gain in speed and the possibility of label-free quantification of a fascicle's myelination level, important studies on inter-sample variability in fascicle organization become feasible.