Optical coherence tomography (OCT) constructs images through the wave interference that occurs when backscattered light from a sample is coupled with a reference light. OCT visualizes differences in the tissue's optical properties, which includes the effects of both optical absorption and scattering. In particular, swept-source OCT (SS-OCT) can construct images through the ultrahigh-speed scanning of the time-encoded wavelength of a near-infrared laser. The purpose of this study was to apply this technology in the practice of dentistry for imaging teeth and composite restorations, which may facilitate the clinical diagnosis of caries and tooth cracks, as well as the evaluation of existing restorations in the future.
The SS-OCT images obtained by intraoral scanning that involved sound or slightly demineralized enamel up to the deep dentin caries were examined and compared with those of the radiographs. In SS-OCT, sound enamel is almost transparent at the SS-OCT wavelength range or upper near-infrared region around 1,300nm. In caries lesions, the signal generally increases and the demineralized region appears brighter on the grayscale SS-OCT images, because of the formation of numerous submicron-size defects resulting from demineralization in carious lesions. If the caries penetrates into dentin, the penetration depth of the bright zone observed in SS-OCT extends beyond the DEJ level. In some cases, lateral expansion of the caries lesion creates micro gaps along the DEJ, where strong reflection occurs.
Cracked teeth have been a diagnostic challenge because of the difficulty in locating crack lines on incomplete tooth fractures. Since a crack has an unpredictable prognosis, including extraction, accurate diagnosis regarding the size and localization of the crack is required to determine the most appropriate treatment technique. We examined SS-OCT as a diagnostic tool for tooth cracks, and this methodology proved capable of providing clear imaging of tooth cracks, including information on penetration depth.
Since OCT is an optical imaging modality, its principal disadvantage is that the light attenuation from scattering by tissue limits the image penetration depth. Also, images obtained in OCT are significantly influenced by the optical properties of biological structures, e.g. the refractive index, which determines light refraction and light speed. However, OCT is a promising imaging technology in dentistry, because it performs real-time cross-sectional imaging of the tissue microstructure without the need for radiation dosing.
