Supersonic shear wave imaging to assess arterial anisotropy: Ex-vivo testing of the horse aorta

Supersonic shear wave imaging (SSI) has recently emerged as a reliable technique for soft tissue characterization in bulk tissues (e.g. in the context of breast and liver cancer diagnostics). Another promising application of SSI is arterial stiffness assessment, though challenged by complex shear wave (SW) propagation phenomena in this thin-walled setting such as guided waves, dispersion, reflection and refraction on the arterial walls. Therefore, we investigated the sensitivity of SSI to (i) stretch-induced stiffening and (ii) the arterial fiber organization in a simpler ex-vivo arterial setup based on equine aortic tissue, where the SW propagation is deprived of dispersion and guided-wave effects. For this purpose, we conducted simultaneous dynamic mechanical testing of the tissue along with SSI measurements. The probe was rotated around its axis relative to the tissue to investigate whether SSI is able to determine the dominant collagen fiber direction in the tissue. The cyclic behavior of the SW velocities as a response to the dynamic mechanical testing demonstrated the ability of SSI to detect stretch-induced stiffening, though mainly in the circumferential direction. Furthermore, SW velocities were lower when the probe was positioned away from the circumferential direction of the tissue, which could be explained due to the uni-axial testing, the arterial anisotropy and the progressive recruitment of collagen fibers in the circumferential direction. The elasticity modulus assessed from the SSI measurements and the mechanical testing demonstrated the feasibility of SSI to detect the increase in E-modulus as expected from the measured stress-strain curve (factor 2.1 versus 2.3 increase for SSI and mechanical testing respectively). Future work will include performing histology on the investigated tissue to confirm these findings and clarify the link between SSI measurements and the actual fiber orientation.