Infrared imaging confirms the role of the transparent surface zone in arresting dental caries

Abstract

Several studies have demonstrated the potential of short wavelength infrared (SWIR) reflectance, thermal imaging and optical coherence tomography for the nondestructive assessment of the activity of caries lesions. The purpose of this study was to test the hypothesis that the activity of arrested caries lesions on the coronal surfaces of extracted teeth would be changed by reducing the thickness of the highly mineralized transparent surface layer, which was measured using polarization sensitive optical coherence tomography (PS-OCT). The lesion activity was assessed using SWIR reflectance and thermal imaging during forced air drying of the lesion before and after mechanical removal of a surface layer ~ 50-μm thick covering the lesion. Both the intensity change in SWIR reflectance images at 1500-1750-nm wavelengths after drying the lesions and the change in thermal emission measured with a thermal camera at 8-13-μm wavelengths increased significantly (P<0.05) after reducing the thickness of the mineralized surface layer in the lesions indicating the permeability of the lesion to fluids increased. These results provide further evidence that the presence of a highly mineralized outer surface layer is a key indicator of lesion arrest.

Keywords: SWIR imaging; lesion activity; optical coherence tomography; thermal imaging.

Fig 1.

2D projection images of one of the lesions before and after removal of the surface layer derived from the 3D PS-OCT data. (a) Color (visible light) image of the lesion, (b, e) 2D projection images of the lesion depth before and after removal of the layer, (c, f) 2D projection images of the integrated reflectivity over the lesion depth (ΔR) before and after removal, and (d, g) 2D projection images of the transparent surface layer thickness before and after removal.

Fig 2.

(a) SWIR reflectance intensity profiles from sound and lesion areas of each tooth before and after removal of the surface layer (ground sample) for the lesion shown in Fig. 1. (b) Mean ± S.D. of ΔI measurement of the samples. Bars of the same color are statistically similar, P > 0.05 (n=5).

Figure 3.

(a) Temperature profiles from sound and lesion areas of each tooth before and after removal of the surface layer (ground sample) for the lesion shown in Fig. 1. (b) Mean ± S.D. of ΔQ measurement of the enamel lesion samples. Dashed line in (a) represents the initial temperature and ΔQ is the area under the dashed line enclosed by the time-temperature curve. Bars of the same color are statistically similar, P > 0.05 (n=5).