Automated assessment of the remineralization of artificial enamel lesions with polarization-sensitive optical coherence tomography

Abstract

Accurate measurement of the highly mineralized transparent surface layer that forms on caries lesions is important for diagnosis of the lesion activity because chemical intervention can slow or reverse the caries process via remineralization. Previous in-vitro and in-vivo studies have demonstrated that polarization-sensitive optical coherence tomography (PS-OCT) can nondestructively image the subsurface lesion structure and the highly mineralized transparent surface zone of caries lesions. The purpose of this study was to develop an approach to automatically process 3-dimensional PS-OCT images and to accurately assess the remineralization process in simulated enamel lesions. Artificial enamel lesions were prepared on twenty bovine enamel blocks using two models to produce varying degree of demineralization and remineralization. The thickness of the transparent surface layer and the integrated reflectivity of the subsurface lesion were measured using PS-OCT. The automated transparent surface layer detection algorithm was able to successfully detect the transparent surface layers with high sensitivity ( = 0.92) and high specificity ( = 0.97). The estimated thickness of the transparent surface layer showed a strong correlation with polarized light microscopy (PLM) measurements of all regions (R(2) = 0.90). The integrated reflectivity, ΔR, and the integrated mineral loss, ΔZ, showed a moderate correlation (R(2) = 0.32). This study demonstrates that PS-OCT can automatically measure the changes in artificial enamel lesion structure and severity upon exposure to remineralization solutions.

Keywords: (170.0110) Imaging systems; (170.1850) Dentistry; (170.4500) Optical coherence tomography.

Fig. 1

PLM, CP-OCT and TMR images of the samples from the acidic pH remineralization model (A-D) and the neutral pH remineralization model (E-H) for the four time periods of exposure to the remineralizing solution. PLM (A and E) and CP-OCT B-scans (unprocessed B and F; processed C and G) show decreased depth of the lesion body as well as an increase in transparent surface layer thickness over the periods of exposure to remineralizing solution. The transparent surface layer is highlighted in yellow in PS-OCT images. TMR (D and H) shows increased mineralization of the lesion after exposure to remineralizing solution for 12 days.

Fig. 2

A plot of transparent surface layer thickness estimated from the automated algorithm (PS-OCT) vs. thickness determined with histology (PLM). Only the true-positive windows are shown above (n = 44).

Fig. 3

A plot of estimated lesion (body) depth measurements vs. lesion (body) depth determined with histology (PLM). True-positive windows exhibited the transparent surface layer with both PS-OCT and PLM (n = 44). True-negative windows did not exhibit the transparent surface layer with both PS-OCT and PLM (n = 31). The FWHM method was used for lesion (body) depth estimation. The line with matching color represents the best-fit line.

Fig. 4

A plot of integrated reflectivity, ΔR, over the estimated lesion (body) depth (PS-OCT) vs. integrated mineral loss, ΔZ, over 200 µm (TMR). Windows with uneven thickness or fractured surface were excluded from the comparison (n = 52).

Fig. 5

Two-dimensional projection images of the acidic pH remineralization sample. A red dotted line in the visible light reflectance image (A) represents the section shown in Figs. 1(A)-1(C). Two-dimensional OCT surface projection images of the same sample are shown including (B) the integrated reflectivity and (C) the transparent surface layer thickness.