Automated analysis of lesion depth and integrated reflectivity in PS-OCT scans of tooth demineralization

Abstract

Background and objectives: Several studies have demonstrated that polarization sensitive optical coherence tomography (PS-OCT) can be used to nondestructively measure the severity of subsurface demineralization in enamel and dentin, track lesion progression over time and measure remineralization. The purpose of this study was to develop methods for the automated assessment of the depth and severity of demineralization in PS-OCT scans.

Materials and methods: Subsurface caries-like lesions of increasing depth and severity were produced in adjoining windows on 10 bovine enamel samples via exposure to demineralization for periods of 1-4 days. PS-OCT scans were acquired for each sample and analyzed using various methods to calculate the lesion depth and severity. Edge detection algorithms were most successful for measurement of the lesion depth for improved assessment of lesion severity.

Results: Edge-finding algorithms were able to detect significant differences (P<0.05) in the lesion depth and severity between each of the periods of demineralization and sound enamel. The lesion depth and mineral loss were also measured with polarized light microscopy and transverse microradiography after sectioning the teeth for comparison.

Conclusions: This study demonstrates that the depth and severity of early lesions can be calculated automatically for rapid analysis of PS-OCT images.

Fig. 1

A PS-OCT b-scan of the reflectivity in the orthogonal polarization (⊥-axis) to the incident linearly polarized light is shown taken along the long axis of the bovine enamel sample. The areas labeled (D0) are sound while (D1–D4) represent the 1-day periods in which that respective area was exposed to demineralization. Laser incisions separate each area (group). A red–white–blue color table was used in which strong reflectivity is in red and low reflectivity is blue.

Fig. 2

A: A gray scale PS-OCT b-scan of the reflectivity in the orthogonal polarization (⊥) to the incident linearly polarized light is shown taken along the long axis of the bovine enamel sample. The areas labeled (D0) are sound while (1–4) represent the 1-day periods in which that respective area was exposed to demineralization. B: Polarized light micrograph (PLM) of a thin section (200-μm) thick cut along the long axis of the same sample. C: Transverse microradiograph (TMR) of same thin section.

Fig. 3

Sequential steps in the imaging processing of each PS-OCT b-scan. A: Raw grayscale image in the orthogonal polarization (⊥). B: Image adjusted with a threshold filter. C: Image after anisotropic diffusion, a filtration process designed to locate features. D: Edges located using an edge locator function. E: Found edges of D overlaid on A.

Fig. 4

A: A gray scale PS-OCT b-scan of the reflectivity in the orthogonal polarization (⊥) for one of the groups that exhibited excessive erosion. B: The same image showing the assigned edges after application of the edge detection algorithm. The green dotted line shows the original position of the enamel surface.

Fig. 5

The mean ± standard deviation of the lesion depth determined using: (A) PS-OCT with the edge finding algorithm and (B) from polarized light micrographs. All groups are significantly different P <0.05.

Fig. 6

The mean 3standard deviation of the integrated reflectivity (ΔR) determined using PS-OCT using a fixed-depth algorithm (A) and the edge finding algorithm (B). All groups are significantly different P <0.05.

Fig. 7

The mean ± standard deviation of the integrated mineral loss (ΔZ) determined from transverse microradiographs. Groups with same color are statistically similar P >0.05.