Abstract
Purpose :
To determine biomechanical change throughout the whole depth in the superior cornea using polarization-sensitive Brillouin microscopy, explore the limit of imaging depth without phase compensation in this region, and compare biomechanics at mirrored positions in the superior and inferior cornea in healthy controls, post-LASIK, and keratoconus.
Methods :
Depth imaging was conducted repeatedly at a same spot 2.5 mm vertically away from the central cornea with different phase compensation by rotating a quarter wave plate within the Brillouin microscope. With the change of phase, Brillouin signal at different depth was optimized. By truncating these enhanced Brillouin signals and concatenating them according to their corresponding depth, the Brillouin profile along the whole depth was reconstructed. Then, the depth profile without phase compensation was compared with the reconstructed one to determine the effective imaging depth. By averaging Brillouin shifts within the effective imaging depth, biomechanical differences at mirrored positions in the superior and inferior cornea were compared among healthy controls, post-LASIK, and keratoconus.
Results :
There were 10 eyes from 10 patients imaged per group. Thickness at the measured point (0, 2.5) mm was 692 μm. Comparing reconstructed Brillouin profile and the profile achieved without phase compensation, it showed that the first 200 μm shared the same Brillouin shift. The profile without phase compensation became noisy at deeper corneal levels. By averaging Brillouin shifts within the first 200 μm at mirrored positions (0, 2.5) mm and (0, -2.5) mm, the healthy controls showed a difference of 48.4±8.1 MHz. In comparison, the difference of post-LASIK was 62.7±14.3 MHz (p=0.06), and that of keratoconus was 66.0±8.5 MHz (p<0.005). No significant difference was observed between the post-LASIK and keratoconus groups (p=0.5).
Conclusions :
Biomechanics of the deep stroma in the superior cornea can be detected by introducing phase compensation to Brillouin microscopy. As the anterior cornea is mainly impacted by laser vision correction or keratoconus, when focusing on the biomechanics within the first 200 μm, phase compensation is not required. Comparing to the biomechanical difference between mirrored positions in the superior and the inferior in the normal corneas, post-LASIK and keratoconus show larger differences between these two regions.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.