Structural characterization of edematous corneas by forward and backward second harmonic generation imaging

The purpose of this study was to image and quantify the structural changes of corneal edema by second harmonic generation (SHG) microscopy. Bovine cornea was used as an experimental model to characterize structural alterations in edematous corneas. Forward SHG and backward SHG signals were simultaneously collected from normal and edematous bovine corneas to reveal the morphological differences between them. In edematous cornea, both an uneven expansion in the lamellar interspacing and an increased lamellar thickness in the posterior stroma (depth > 200 microm) were identified, whereas the anterior stroma, composed of interwoven collagen architecture, remained unaffected. Our findings of heterogeneous structural alteration at the microscopic scale in edematous corneas suggest that the strength of collagen cross-linking is heterogeneous in the corneal stroma. In addition, we found that qualitative backward SHG collagen fiber imaging and depth-dependent signal decay can be used to detect and diagnose corneal edema. Our work demonstrates that SHG imaging can provide morphological information for the investigation of corneal edema biophysics, and may be applied in the evaluation of advancing corneal edema in vivo.