Dynamic SWIR imaging near the 1950-nm water absorption band for caries lesion diagnosis

Abstract

Significance: It is not sufficient to detect caries lesions on tooth surfaces; it is also necessary to measure the activity of the lesions to determine if intervention is needed. Changes in the reflectivity of lesion areas during dehydration with forced air at short wavelength infrared (SWIR) wavelengths can be used to assess lesion activity since these changes represent the evaporation dynamics of water from the lesion.

Aim: The aim of this study is to develop new optical methods for assessing lesion activity on tooth surfaces utilizing the strong water absorption band near 1950-nm.

Approach: The time-resolved reflectivity of 20 active and arrested caries lesions on the surfaces of human extracted teeth was monitored at 1300 to 2000 nm using broadband light sources and an extended range InGaAs camera during drying with air.

Results: Multiple parameters representing the rate of change of the lesion reflectivity correlated with the presence of a highly mineralized outer surface zone indicative of lesion arrest measured with x-ray microtomography (microCT). Performance at 1950-nm was higher than for other wavelengths.

Conclusions: This study demonstrates that SWIR imaging near 1950-nm has great potential for the assessment of lesion activity.

Keywords: dental caries; lesion activity; short wavelength infrared imaging.

Fig. 1

Color (visible) and microCT images of two extracted teeth with suspected (a) arrested and (b) active interproximal lesions. The lesions are enclosed by the solid and dashed circles.

Fig. 2

Schematic of the experimental setup showing (a) tungsten-halogen light source with bandpass filters, collimating lens and polarizer; (b) polarized 1950-nm fiber optic light source; (c) Xenics extended range InGaAs camera with lens and polarizer; (d) air nozzle; and (e) tooth samples mounted on XYZ stage. Light sources A and B were positioned on the same side for these measurements.

Fig. 3

(a) Visible image of a tooth with active (red arrow) and arrested (green arrow) lesion areas. (b) MicroCT image of the area aligned with the blue dotted line in (a). SWIR images taken at (c) 1950 nm and (d) 1300 nm before dehydration. SWIR images at 1950 nm acquired after (e) 10; (f) 20; (g) 30; and (h) 60 s of dehydration.

Fig. 4

Dehydration curves at each wavelength for active (a) and arrested (b) lesion areas. The wavelengths listed from top to bottom are 1950—black, 1460—green, 1675—red, 1535—blue, and 1300—gray. The air was turned on at 0 s to initiate dehydration. Best fits of Eq. 1 are plotted for each curve.

Fig. 5

Plots of the $\text{mean}\pm \mathrm{sd}$ of (a) delay; (b) $\%I_{\text{fin}}$; (c) OGR; and (d) ΔI for the active ($n=10$) and arrested ($n=10$) lesion areas. The OGR, $\%I_{\text{fin}}$, and delay were significantly different ($P<0.05$) for active versus arrested lesions.

Fig. 6

3D scatter plot of the $\%I_{\text{fin}}$, OGR, and delay. Red circles are arrested lesions, and blue diamonds are active lesions.