Assessment of shear anisotropy using supersonic shear imaging with rotating arrays: In vivo evidence of cornea elastic anisotropy

The cornea is mainly composed of type I collagen fibrils. The organization of those fibers ensures the cornea transparency and determines its biomechanical properties. Understanding the biomechanics of the cornea has become a crucial issue in ophthalmology to predict the cornea response to refractive surgery procedures and to avoid post-treatment complications. In this work, we proposed the Supersonic Shear Imaging (SSI) elastography as a method for the in vivo assessment of the cornea elastic anisotropy. The tissue shear modulus can be retrieved from the speed of a shear wave propagating in this tissue. In the SSI method, the transient shear wave is induced using the ultrasonic radiation force. The resulting shear wave propagates transversally to the ultrasound beam axis. The probe is then switched to an ultrafast imaging mode (30000 frames/sec) to follow the shear wave propagation and thus evaluate its local speed. We implemented SSI with a high-frequency rotating linear array (15 MHz, 128 elements) to estimate the elastic anisotropy of the cornea. We performed 3D scans in vivo and ex vivo on porcine eyes. Elasticity maps of the cornea surface were obtained. Porcine corneas exhibited a significantly higher shear wave speed along the horizontal meridian of the cornea than along the diagonal and vertical directions within a 50°-wide sector. These results are consistent with ex vivo X-ray diffraction measurements reported in the literature [1] that have shown that the collagen fibers are mainly oriented along one preferential direction in porcine corneas within a 45°-wide sector.