Local axis orientation mapped by polarization sensitive optical coherence tomography provides a unique contrast to identify caries lesions in enamel.

Due to rod-like hydroxyapatite crystal organizations, dental enamel is optically anisotropic, i.e., birefringent. Healthy enamel is known to be intrinsically negatively birefringent. However, when demineralization of enamel occurs, a considerable number of inter-crystallite spaces would be created between the crystallites in the enamel, which could lead to a sign reversion in birefringence of the enamel structure. We propose that this sign reversion can be leveraged in polarization sensitive OCT (PSOCT) imaging to differentiate early caries lesions from healthy enamel. In this study using PSOCT, we first confirm that the change in birefringence sign (negative to positive) can lead to a 90-degree alteration in the local axis orientation because of the switch between the fast and slow optic axes. We then demonstrate, for the first time, that the local axis orientation can be utilized to map and visualize the WSLs from the healthy enamel with a unique contrast. Moreover, the sharp alteration in local axis orientation gives a clear boundary between the WSLs and the healthy enamel, providing an opportunity to automatically segment the three-dimensional WSLs from the healthy enamel, enabling the characterization of their size and depth information in an intuitive way, which may aid clinical decision making and treatment planning.