Introduction: In vivo and in vitro studies have shown that high contrast images of tooth demineralization can be acquired in the near-infrared at several wavelengths. The purpose of this study is to compare the lesion contrast in reflectance at near-infrared wavelengths coincident with high water absorption with those in the visible, the near-infrared at 1300nm, fluorescence, and polarization sensitive optical coherence tomography (PS-OCT) measurements for early lesions on buccal surfaces.
Methods: Bovine incisors (n=40) were used in this in vitro study. Teeth were sectioned so that they were greater than 8mm in length, 2mm in width, and had at least 1mm of remaining enamel and then mounted on resin blocks. A CO2 laser was used to produce small incisions separating the four 2x2mm windows. Artificial lesions were created in two ways: (1) samples (n=20) were exposed for 4 days to pH 4.8 and pH 4.9; (2) samples (n=20) were exposed for 8 days to pH cycling between pH 4.7 and pH 7.0. Lesions were imaged using near-IR reflectance at three wavelengths, 1300, 1460, 1550nm, using an InGaAs camera. Visible light reflectance, fluorescence at 405nm excitation and greater than 500nm detection, and polarization sensitive optical coherence tomography were also used for comparison. Crossed polarizers were used for reflectance measurements to reduce interference from specular reflection.
Results: The contrast of both types of lesions were significantly higher (P < 0.05) for near-IR reflectance imaging at 1460 and 1550nm than it was for near-IR reflectance imaging at 1300nm, visible reflectance imaging, and fluorescence.
Conclusion: The markedly higher contrast at 1460 and 1550nm wavelengths, coincident with higher water absorption, suggest that these wavelengths are better suited than 1300nm for imaging early demineralization on tooth surfaces.