Ex vivo evaluation of an eye-adapted beamforming for axial B-scans using a 20 MHz linear array

In ophthalmic ultrasonography the crystalline lens is known to be the main source of phase aberration, as ultrasounds (US) propagate about 10% faster than in the surrounding intra-ocular medium. Consequently, it impairs significantly both spatial and contrast resolution of axial B-scans, and in addition causes important distortion, especially on the ocular fundus. In this context, we developed in a previous work [1] an adapted beamforming (BF) free from crystalline lens aberration and implemented it on a custom research scanner, the ECODERM, that works with a 20 MHz linear array. The adapted BF lies on a two point ray tracing approach to compute focusing delays that take into account crystalline lens aberrations including refraction at the interface. Previous in vitro experiments consisting in imaging a wire phantom through a synthetic gelatin lens immersed in a liquid reproducing the in vivo velocity ratio between crystalline lens and surrounding humors have demonstrated the efficiency of this adapted BF when the lens is assumed to be homogeneous. As US velocity inside the lens is known to be non uniform, we have extended our adapted BF to take into account the lens heterogeneity. Interest of this consideration is investigated and demonstrated through an ex vivo experiment on an isolated macaque lens in a similar setup to in vitro experiment.