To improve dental health, researchers at the University of California, San Francisco evaluated optical imaging techniques for their efficacy in the identification of secondary tooth decay, which can occur even if a tooth has already been filled. The researchers compared two techniques — shortwave infrared (SWIR) radiation reflectance and thermal imaging — with measurements obtained with optical coherence tomography (OCT) and micro-computed tomography (MicroCT). The results demonstrated the potential of SWIR reflectivity and OCT imaging methods for the clinical monitoring of secondary dental caries (i.e., dental cavity) lesions. The researchers considered cavities and methods of treatment as the impetus for the research. Treatment for cavities, known commonly as fillings, often fails over time. The restorative materials used to fill dental lesions do not always bond well to the surrounding healthy tooth structure. Microscopic leaks can form, which enable fluids and bacteria to penetrate the restoration and form a secondary caries that grows around the previously restored cavity. “Dentists now spend more time replacing failed restorations than placing new ones due to the maladaptation of bonding materials to tooth structure,” said research team member Nai-Yuan N. Chang. Chang and his team investigated whether SWIR and thermal imaging could be combined with air drying the tooth to accurately diagnose a secondary cavity, and which technique could perform the task better. The researchers used forced air drying as a means of detection because cavities are more porous, and therefore hold more water, than healthy teeth. With SWIR imaging, the researchers detected active lesions by observing changes in the SWIR reflectivity as the tooth dried. When using thermal imaging, they identified cavities by measuring the difference in temperature change during air drying in the active lesions compared to healthy teeth. When combined with air drying, researchers used shortwave-infrared (SWIR) imaging to detect active dental cavities. This is possible because active cavities are porous and hold more water, which affects the infrared measurements around the affected area as the tooth dries. Courtesy of Chang et al., doi: 10.1117/1.JBO.28.9.094801. The researchers examined 63 extracted human teeth with a total of 109 suspected secondary lesions. They measured the thickness of the transparent surface layer at the lesion interfaces, which is indicative of lesion activity, using OCT. They further confirmed lesion severity and structure using MicroCT. OCT and MicroCT measurements of lesion structure, depth, and severity were correlated with fluid loss rates measured with the SWIR reflectance and the thermal imaging methods to determine whether SWIR and thermal imaging were indeed useful for detecting active lesions. Overall, SWIR performed better than thermal imaging in the assessment of secondary caries lesions on tooth coronal surfaces, although both methods performed best on smooth surface lesions. SWIR imaging at 1950 nm was advantageous for differentiating composite restorations and lesions from sound tooth structure. Despite the complex geometry and topography in a few lesions, SWIR reflectance imaging during dehydration was able to assess the permeability of the tooth with relatively close correlation to OCT. Thermal imaging performed well in identifying crevices between composite material and tooth structure, but at times it was masked by the complex topography of the tooth. SWIR imaging did not appear as susceptible to such interference, owing to the ability of SWIR to differentiate composite materials, tooth structures, and lesions with high contrast. The SWIR permeability measurements were well correlated with OCT measurements of the thickness of the transparent surface layer of the lesions. Increasing transparent surface layer thickness led to decreased permeability of lesions and potentially indicated full lesion arrest when it reached a thickness equal to or greater than 70 μm. According to the researchers, nondestructive SWIR reflectance and OCT imaging could provide, for the first time, a way to detect secondary cavities while they form. The findings of this study could lead to new approaches to diagnostic imaging in dentistry. “The traditional methods relying on tactile sensation via a dental explorer and visual inspection based on texture and color are highly subjective and unreliable,” Chang said. “However, there is currently no established dental imaging technology that can provide diagnostic information with high specificity and sensitivity when assessing dental decay activity.” Beyond improved diagnostics, the work supports the development of easily operable clinical devices, Chang said. The research was published in Journal of Biomedical Optics (www.doi.org/10.1117/1.JBO.28.9.094801).