High contrast near-infrared polarized reflectance images of demineralization on tooth buccal and occlusal surfaces at lambda = 1310-nm

Abstract

Background and objectives: Sound enamel manifests peak transparency in the near-IR (NIR) at 1310-nm, therefore the near-IR is ideally suited for high contrast imaging of dental caries. The purpose of this study was to acquire images of early demineralized enamel on the buccal and occlusal surfaces of extracted human teeth using NIR reflectance imaging and compare the contrast of those images with the contrast of images taken using other methods.

Materials and methods: Fifteen human molars were used in this in vitro study. Teeth were painted with a clear acid-resistant varnish, leaving two 2 mm x 2 mm windows on the buccal and occlusal surfaces of each tooth for demineralization. Artificial lesions were produced in the exposed windows after a 2-day exposure to a demineralizing solution at pH 4.5. Lesions were imaged using NIR transillumination, NIR and visible light reflectance, and fluorescence imaging methods. Crossed polarizers were used where appropriate to improve contrast. Polarization sensitive optical coherence tomography (PS-OCT) was also used to non-destructively assess the depth and severity of demineralization in each sample window.

Results: NIR reflectance imaging had the highest image contrast for both the buccal and occlusal groups and it was significantly higher contrast than visible light reflectance (P < 0.05).

Conclusion: The results of the study suggest that NIR reflectance imaging is a promising new method for acquiring high contrast images of early demineralization on tooth surfaces.

Fig. 1

Imaging configurations used in this study. (top) Orientation of the imaging cameras, light sources and tooth samples for visible and NIR reflectance measurements along with laser induced fluorescence. (bottom) Position of the InGaAs imaging camera, light source and tooth samples for the two NIR transillumination measurement systems used for imaging the buccal and occlusal surfaces.

Fig. 2

Buccal lesion images are shown for for one sample. (A) visible reflectance w/crossed polarizers, (B) NIR reflectance w/crossed polarizers, (C) fluorescence, and (D) NIR transillumination w/crossed polarizers. Note the position of the fiducial marks cut by the laser at the midway point of each 2×2 mm window. These marks demarcate the position of the line profiles used for calculating the image contrast on each sample.

Fig. 3

Occlusal lesion images are shown for one sample. (A) visible reflectance w/crossed polarizers, (B) NIR reflectance w/crossed polarizers, (C) fluorescence, and (D) NIR transillumination.

Fig. 4

The mean±s.d contrast values of the buccal surfaces for the various imaging methods. Bars containing the same color are not significantly different (p>0.05).

Fig. 5

The mean±s.d contrast values of the occlusal surfaces for the various imaging methods. Bars containing the same color are not significantly different (p>0.05).