An adapted beamforming free from crystalline lens phase aberration for ocular ultrasonography - In vitro and Ex vivo results with a 20 MHz linear array

Abstract

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 intraocular medium. Consequently, it causes significant decrease in both spatial and contrast resolution together with distortion effects on axial B-scans. An eye-adapted beamforming (BF) has been developed and experimented with a 20 MHz linear array working with a custom US research scanner, the ECODERM. The eye-adapted BF computes focusing delays that compensate for crystalline phase aberration, including refraction effects, by assuming the intraocular medium consists of two homogeneous media (crystalline lens + aqueous and vitreous humors). The proposed BF was tested in vitro by imaging a wire phantom through an eye phantom consisting of a synthetic gelatin lens anatomically set up in an appropriate liquid (turpentine) to approach the in vivo velocity ratio. The synthetic lens shape corresponded to that of an adult human crystalline lens in unaccommodated state. Both image quality and fidelity from the adapted BF were assessed, in relation to that in homogeneous medium and compared with conventional delay-and-sum BF over the aberrating medium. Finally, first ex vivo experimentation on human eyes are presented. In vitro quantitative study showed 2-fold improvement of the lateral resolution, greater sensitivity and 90% reduction of the spatial error (from 758 μm to 76 μm) with adapted BF compared to conventional BF. Compared to optimal results in homogeneous medium (pure turpentine), lateral resolution was only 39% lower with adapted BF. First ex vivo results showed a higher detailed view of the posterior coat and a global restoration of the spatial fidelity promising for biometry and especially phakometry.