Quantitative Evaluation of Caries and Calculus with Ultrahigh-Resolution Optical Coherence Tomography

Abstract

Dental caries on the crown's surface is caused by the interaction of bacteria and carbohydrates, which then gradually alter the tooth's structure. In addition, calculus is the root of periodontal disease. Optical coherence tomography (OCT) has been considered to be a promising tool for identifying dental caries; however, diagnosing dental caries in the early stage still remains challenging. In this study, we proposed an ultrahigh-resolution OCT (UHR-OCT) system with axial and transverse resolutions of 2.6 and 1.8 μm for differentiating the early-stage dental caries and calculus. The same teeth were also scanned by a conventional spectral-domain OCT (SD-OCT) system with an axial resolution of 7 μm. The results indicated that early-stage carious structures such as small cavities can be observed using UHR-OCT; however, the SD-OCT system with a lower resolution had difficulty identifying it. Moreover, the estimated surface roughness and the scattering coefficient of enamel were proposed for quantitatively differentiating the different stages of caries. Furthermore, the thickness of the calculus can be estimated from the UHR-OCT results. The results have demonstrated that UHR-OCT can detect caries and calculus in their early stages, showing that the proposed method for the quantitative evaluation of caries and calculus is potentially promising.

Keywords: calculus; caries; optical coherence tomography; roughness; scattering.

Figure 1

Schematic of our developed UHR-SD-OCT system. SCL, supercontinuum laser; FB, filter box; ND, neutral density filter; BS, beam splitter; I, iris diaphragm; GS, galvanometer; O, objective; L1–L4, lenses; M: mirror; SMF, single-mode fiber; G, transmission grating; C, line-scan camera; DAQ, data acquisition board.

Figure 2

Flow chart of the process of acquiring the surface roughness profile.

Figure 3

Interferogram acquired by the line-scan camera (a) before and (b) after the processes of DC removal, resampling, and window shaping. (c) Axial PSF measured in air and (d) OCT en-face image of the USAF resolution target. (e) Magnified image of the region indicated by the red square in (d). The upper left and lower right yellow squares in (e) are elements 1 and 2 in group 8 of the USAF resolution target.

Figure 4

OCT results of a normal tooth obtained by (a–c) the conventional-resolution OCT system and (d–g) the UHR-OCT system, and OCT results of a tooth with caries obtained from (h–j) the conventional-resolution OCT system and (k–n) the UHR-OCT system. (a,d,h,k) The en-face images at the tooth’s surfaces, (b,e,f,i,l,m) the cross-sectional images, and (c,g,j,n) the en-face images at the depths indicated by the white lines in (b), (f), (i), and (m), respectively. The yellow and red arrows indicate the enamel–dentin boundary and cavities, respectively. The red arrows in (a,d) indicate the small damage on the tooth surface. EN, enamel; DE, dentin.

Figure 5

OCT results of the tooth with caries obtained from (a–c) the convectional-resolution OCT and (d–h) the UHR-OCT systems including (a,d) 3D, (b,c,e,f) 2D, and (g,h) en-face images at the depths indicated by the solid white and white-dashed lines in (f). The white arrows indicate the tooth’s surface. The 3D imaging areas of conventional-resolution OCT and UHR-OCT systems are 5 × 5 and 1 × 1 mm², respectively.

Figure 6

Three-dimensional UHR-OCT images of (a) the normal tooth; (b) early dental caries, stage 1; and (c) early dental caries, stage 2. Statistical results of the estimated surface roughness of carious teeth in the different early stages obtained by (d) the conventional-resolution OCT system and (e) the UHR-OCT system. The “×” symbols in each statistical result represent the upper and lower limits of the confidence interval at a 95% confidence level. The 3D imaging areas of the conventional-resolution OCT and UHR-OCT systems are 5 × 5 and 1 × 1 mm², respectively.

Figure 7

Scattering coefficients estimated from teeth at different stages using (a) the conventional-resolution OCT system and (b) the UHR-OCT system.

Figure 8

OCT results of the tooth with calculus tooth obtained using (a,b) the conventional-resolution OCT system and (c–e) the UHR-OCT systems including (a,c) 3D and (b,d,e) 2D images. (f) The distribution of thickness of the calculus. The white arrows indicate the tooth’s surface. The 3D imaging areas of the conventional-resolution OCT and UHR-OCT systems are 5 × 5 and 1 × 1 mm², respectively.