Fibrillar collagen orientation delineates the speed orientation profile of symmetric (S0) mode waves in fibrillar tissues

This study investigates the relationship between fibrillar collagen orientation and wave propagation in fibrillar soft tissues using a multimodal imaging approach. Dissected samples (n=4 per tissue) of porcine flexor tendon, cornea, and sclera samples were evaluated. Guided waves were generated via an air-coupled ultrasonic transducer and measured with a custom-built phase-sensitive swept-source OCT system. The speed of symmetric (S0) and antisymmetric (A0) lamb wave modes were estimated across 4 meridians equally spaced using a phase-derivative algorithm. Collagen orientation was quantified from SHG images using a principal component analysis approach applied to overlapping image windows, enabling local fiber orientation mapping. The angular distribution of collagen orientation was then compared to the wave speed profiles. We observed significant linear correlations (p < 0.05) between the S0 phase speed and collagen alignment in scleral and tendon tissues, consistent with the theoretical dependence of S0 speed on the tensile modulus. These findings highlight the utility of combining ACUS-OCE and SHG microscopy for mapping mechanical properties anisotropy.