Thermal Imaging of Root Caries In Vivo

Abstract

Improved methods are needed to assess the structure and activity of lesions on root surfaces in order to improve clinical decision making. Conventional visual and tactile methods for assessing lesion activity are not reliable, and the clinician is often unable to evaluate if the lesion is progressing or has remineralized. An important marker of an arrested lesion is a highly mineralized surface zone that forms when mineral is deposited in the outer layer of the lesion. In vitro studies have shown that a mineralized surface zone influences the kinetics of water evaporation and the surface temperature while drying. Temperature changes can be monitored by measuring the thermal emission with thermal imaging. Studies have also shown that the depth and severity of demineralization and the thickness of the highly mineralized transparent surface zone on arrested lesions can be measured nondestructively with optical coherence tomography (OCT). Thermal imaging at 8-µm to 13-µm wavelengths was completed on 30 test subjects with a suspected active root caries lesion by monitoring thermal emission from the tooth surfaces during 30 s of air drying. Lesions were also evaluated using cross-polarization OCT (CP-OCT) during lesion dehydration to identify transparent surface zones indicative of arrested lesions and determine if shrinkage occurred during drying. The overall thermal emission recorded during drying was significantly different (P < 0.001) when comparing sound tooth surfaces, lesion areas identified as arrested, and lesion areas identified as active, demonstrating that thermal imaging is a promising approach for the clinical assessment of lesion activity on root surfaces. Ten of the lesions in this study had distinct areas with transparent surface zones that were visible in CP-OCT images. Shrinkage was detected with CP-OCT during drying for 12 lesions. This study confirms that these novel approaches for assessing lesion activity on root surfaces can be implemented in vivo.

Keywords: active lesions; arrested lesions; caries diagnosis; lesion activity; lesion shrinkage; optical coherence tomography.

Figure 1.

The thermal imaging handpiece (A) consists of a 3-dimensional printed autoclavable attachment with an air channel to focus air across the tooth surface and a right-angle aluminum mirror. This is attached to an additional lens and a small thermal camera. (B) A photo of the handpiece being used clinically.

Figure 2.

Visible, thermal, and cross-polarization optical coherence tomography (CP-OCT) images of a shallow active root lesion and a large area of exposed sound dentin. (A) Color image of the tooth; the black arrow shows the path of the CP-OCT image (C), and the colored markers show the respective positions of the thermal emission profiles (D), where the green triangle is in the area of sound-exposed dentin (D), the red circle is in the active lesion area (L), and the blue square is in sound enamel (E). Four thermal images extracted at 1-, 5-, 10-, and 15-s time intervals during drying are shown (B). Intensity increases from black to white in the grayscale false color images for both the CP-OCT and thermal images. A CP-OCT scan (C) was taken at the position of the arrow (A) with the position of the 3 markers on the arrow. The white dashed arrow points to the position of the cementum-enamel junction (CEJ), the yellow solid arrow the gingiva, and the light blue dotted arrow the dentinal-enamel junction (DEJ). The DEJ can be seen clearly in the image as the lower line beneath the surface on the right side of the image. Some of the enamel has broken off near the CEJ, and the cementum layer has been worn away. The lesion is visible as a bright region at the position of the red circle. The dimensions of the CP-OCT image are 2 mm depth (y-axis) and 6-mm lateral position (x-axis). The thermal emission from the tooth surface during drying is shown (D) for the 3 positions indicated (A).

Figure 3.

Visible, thermal, and cross-polarization optical coherence tomography (CP-OCT) images of a large root lesion with both active and arrested areas. The lower half of the lesion appears distinctly lighter and glossy in the visible image, and the CP-OCT image shows that a transparent surface zone is present on that part of the lesion, suggesting that it is arrested. (A) Color image of the tooth; the arrow shows the path of the CP-OCT image (C), and the colored markers show the respective positions of the thermal emission profiles (D), where the green triangle is in the arrested lesion area (R), the red circle is in the active lesion area (A), and the blue square is in a sound area (S). Four thermal images extracted at 0-, 5-, 15-, and 30-s time intervals during drying are shown (B). A CP-OCT scan is shown (C), taken at the position of the arrow (A), with the position of the 3 dots marked on the arrow. The white dashed arrow points to the position of the cementum-enamel junction (CEJ), the yellow solid arrow the gingiva, and the light blue dotted arrow the dentinal-enamel junction (DEJ). A large part of the enamel has broken off near the CEJ, and there is significant loss of cementum and dentin. A distinct surface zone is visible (orange dotted arrow) on the lower portion of the lesion. The dimensions of the CP-OCT image are 2 mm (axial depth in air) and 6 mm lateral position. The thermal emission from the tooth surface during drying is shown (D) for the 3 positions indicated (A).

Figure 4.

Visible, thermal, and cross-polarization optical coherence tomography (CP-OCT) images of a root lesion that has spread extensively under the adjoining enamel. (A) Color image of the tooth; the 2 arrows show the path of the CP-OCT images (C), and the colored markers show the respective positions of the thermal emission profiles (D), where the red circle is in the lesion area (L) and the blue square is in sound enamel (E). Two thermal images extracted at 0- and 30-s time intervals during drying are shown (B). Two CP-OCT scans labeled 1 and 2 are shown (C), taken at the position of the arrows (A) with the position of the 2 markers on the arrow. The white dashed arrow points to the position of the cementum-enamel junction (CEJ), the yellow solid arrow the gingiva (G), and the light blue arrow the dentinal-enamel junction (DEJ). In the first CP-OCT image, the enamel (E) is intact and none of the root surface is exposed, and the large and deep lesion extends under the enamel (red dotted arrow). A surface zone is present on the exposed lesion surface (orange dotted arrow). The second CP-OCT scan shows that the enamel has broken off and there is a large area of exposed demineralized dentin (D) and that the lesion has spread well into and under the adjacent enamel. The dimensions of the CP-OCT images are 3 mm depth (y-axis) and 6 mm lateral position (x-axis). The thermal emission from the tooth surface during drying is shown (D) for the 3 positions indicated (A).

Figure 5.

(Top) The mean ± SD of ΔQ for the n = 10 samples for regions that had sound (S), active (A), and arrested (R) lesion zones. All groups are significantly different, P < 0.001. (Bottom) The mean ± SD of ΔQ for lesion (L) and sound (S) areas for all 25 of the teeth that yielded acceptable thermal images. All groups are significantly different, P < 0.0001.