Evaluating interproximal and occlusal lesion severity with a dual SWIR transillumination/reflectance probe

Abstract

We have developed a clinical probe capable of acquiring simultaneous, multispectral short wavelength infrared (SWIR) reflectance and occlusal transillumination images of lesions on tooth proximal and occlusal surfaces to reduce the potential of false positives and enhance diagnosis. The dual probe was 3D printed and the imaging system uses an InGaAs camera and broadband light sources at 1310 nm for occlusal transillumination and 1600 nm for cross-polarization reflectance measurements. In this study a mathematical model to estimate the penetration depth of "hidden" occlusal lesions from the SWIR images was developed. We compared the model's estimated lesion depth on 18 extracted teeth with lesions against microCT measurements. Although the model estimated depth deviates from that measured in microCT at higher depths, there is a good linear correlation (R² = 0.93) between the estimated depth from SWIR images and the measured depth using microCT. SWIR occlusal transillumination images at 1300 nm also provide information about interproximal lesion penetration depth which can be directly viewed from the occlusal surface. SWIR occlusal transillumination and reflectance depth measurements on 49 natural interproximal lesions were compared with microCT measurements. There was significant correlation between the depths measured with SWIR occlusal transillumination (R² = 0.81) and reflectance (R² = 0.19) compared with the depths measured with microCT.

Keywords: SWIR imaging; dental caries; interproximal lesions; occlusal lesions.

Fig. 1.

Models of occlusal (A) and interproximal (B) lesions viewed from the occlusal surface. White areas represent sound enamel and gray areas are lesion areas, respectively I₀ is the incident light intensity, I_S is the backscattered light from sound areas. I_L1 is the backscattered light from lesion areas at the tooth surface. I₁ is the incident light and I₂ the backscattered light from the subsurface lesion areas. I_L2 is light emitted from the tooth surface above subsurface lesion areas. Z is the distance of subsurface lesion areas below the tooth surface.

Fig 2.

(A) SWIR reflectance image of tooth occlusal surface showing positions of surface (I_L1) and subsurface lesion intensities (I_L2). (C) MicroCT 3D rendering of tooth’s occlusal surface. Horizontal dashed lines show positions of cross sections in (B&D). The depth of the subsurface lesion z_actual is shown in (B).

Fig. 3.

Estimating the depth from the occlusal surface of an interproximal lesion. (A) SWIR reflectance image of the tooth occlusal surface using a 2^nd occlusal lesion for I_L1 and the subsurface interproximal lesion intensity for I_L2. (C) MicroCT surface rendering, the dashed line shows the position of the extracted slice shown in (B) that shows the distance between the tooth surface and the interproximal lesion (z_actual). (D) MicroCT cross-section showing the occlusal surface lesion in the central fissure.

Fig. 4.

(A) color, (C) SWIR reflectance, (F) SWIR occlusal transillumination, and (D) MicroCT surface rendering with extracted slices in (B) and (E) showing the interproximal lesion and the depth and width measurements as shown by the two rulers in (B, C &F).

Fig. 5.

Model lesion depth estimated from SWIR reflectance vs. actual depth measured in microCT for “hidden” occlusal lesions and interproximal lesions with occlusal surface lesions as a reference point. The red dotted line represents matching depths for SWIR and microCT. There is high correlation of the lesion depth model with microCT (blue dotted line) R²=0.93, P<0.05. The SWIR model underestimates the lesion depth for larger lesion depths.

Fig. 6.

Interproximal lesion depth measured from the proximal surface in transillumination mode versus microCT. (top) Lesion depth measured with microCT and (bottom) cutoff at DEJ in microCT. There is high correlation with microCT (blue dotted lines) R²=0.81 and 0.82 (P<0.05).

Fig. 7.

Interproximal lesion depth measured from the proximal surface in reflectance mode versus microCT measurement. (top) Actual lesion depth in microCT and (bottom) cutoff at the DEJ in microCT. There is correlation of the lesion depth with microCT (green dotted lines) R²=0.19 and 0.25 (P<0.05).

Fig. 8.

Interproximal lesion width measured from the proximal surface using reflectance and transillumination versus microCT. There is correlation of the lesion width measured with transillumination with microCT (blue dotted line) R²=0.24, P<0.05 but not for reflectance (green dotted line) R²=0.05, P>0.05.